An optical system and a method for chromatic confocal spectral-domain optical coherence tomography with a plurality of wavelets or for chromatic confocal two-beam interferometry

Abstract

The present invention relates to an optical system and a method for chromatic confocal spectral-domain optical coherence tomography or for chromatic confocal two-beam interferometry. The optical system comprises a multispectral light source unit configured to generate a plurality of spatially coherent multispectral light beams with spectra having a respective centroid wavenumbers, an achromatic focusing unit configured such that in at least a paraxial region around a common point located on an optical axis at the output of the achromatic focusing unit, the wavefronts of the respective spatially coherent multispectral light beams have different radii of curvatures, a chromatic confocal dual-beam interferometer having an non-zero optical path difference arranged downstream of the multispectral light source unit and a detection unit for detecting light returned from the dual-beam interferometer. The reference arm of the double-beam interferometer is configured to be achromatic and the object arm to have chromatic depth slitting of foci. In the reference arm, a plurality of depth separated reference light spots are formed. In the object arm, a plurality of at least partially colinear and overlapping multispectral stretches of diffraction limited light spots extended in a depth direction of the object arm is formed. The light reflected by a current sharply focused diffraction limited object light spot of each of the multispectral stretches of diffraction limited light spots is combined with a respective reference light spot, is confocally discriminated by a confocal discrimination unit of the detection unit and subsequently detected by at least one spectrometer, evaluation dual-beam interferometer or fast detector detects the confocally discriminated light.

Description

[0001] Applicant: Universitat Stuttgart

[0002] "An optical system and a method for chromatic confocal spectral-domain optical coherence tomography with a plurality of wavelets or for chromatic confocal two- beam interferometry”

[0003] Description

[0004] The present invention relates to an optical system and a method for chromatic confocal spectral-domain optical coherence tomography (OCT) with a plurality of wavelets or for chromatic confocal two-beam interferometry.

[0005] Methods for obtaining information about object shape and position without moving mechanical parts are known. An interferometric approach with spectral analysis is described for example in the landmark paper entitled "Space and time variables in optics and holography: recent experimental aspects" from J.-C. Vienot, J.-P. Goedgebuer und A. Lacourt, published in Applied Optics, Vol. 16, S. 454-461 (1977)

[0001] .

[0006] Various interferometric approaches for obtaining object shape and position information without moving mechanical parts are known.

[0007] A white-light interferometry (short coherence interferometry) approach employing spectral analysis of the interfering light at the output of an interferometer is described for example in G. Hege, „Speckleverfahren zur Abstandsmessung", dissertation, published in „Berichte aus dem Institut fur Technische Optik (ITO)“. Vol. 4. 1984, S. 20-25 [2], However, this approach cannot achieve a high lateral resolution at a larger depth extension of the measured object.

[0008] DE 43 09 056 A1 [3] describes another interferometric method, in which the light is spectrally analyzed at the output of an interferometer. Also with this approach, a high lateral resolution cannot be achieved for a larger depth extension of the measuring object.

[0009] The paper "Dispersive interferometric profilometer" by J. Schwider and L. Zhou published in Opt. Lett., Vol. 19. No 13, 1994 [4] describes an interferometric system, which combines a Fizeau interferometer and a spectrometer. The interference signal is spectrally split by a diffractive grating so that Mueller fringes, also known as Tolansky fringes, can be observed in the receiver plane. However, the depth measurement range is limited by the high numerical aperture of the microscopic objective.

[0010] The paper "Analysis of the wavelength-to-depth encoded interference microscopy for three-dimensional imaging” by G. Li and Y. Fainman published in the journal Optical Engineering 41 (6), pages 1281 to 1288 of June 2002 [5] describes an interferometric system with a chromatic depth splitting of the focus employing a diffractive lens positioned in the object beam path in combination with a tunable laser. In this approach, there is a relationship between focus position z_k (focal depth) and the wavenumber k.

[0011] DE 10 32 18 95 [6] describes a chromatic-confocal Fourier domain OCT approach with a phase evaluation. In this system, a spectral refractive power variable element is introduced in the Fourier plane of the associated imaging system, which generates the desired chromatic longitudinal splitting or depth splitting of the object beam in the object arm of the interferometer. In this approach, there is a relationship between focus position z_k (focal depth) and the wavenumber k.

[0012] WO 2006 / 042696 A1 [7] describes a common-path arrangement for chromatic- confocal spectral interferometry.

[0013] Further chromatic confocal spectral interferometry (CCSI) approaches are described in the publications E. Papastathopoulos, K. Kdrner and W. Osten, "Chromatically dispersed interferometry with wavelet analysis", Optics Letters 31 , pages 589-591 [8] and E. Papastathopoulos, K. Kdrner and W. Osten, "Chromatic Confocal Spectral Interferometry (CCSI) Proc. SPIE 6292 [9] and E. Papastathopoulos, K. Kdrner and W. Osten, "Chromatic Confocal Spectral Interferometry," Appl. Opt. Opt. 45, No. 32, pp. 8244- 8252

[0010] ,

[0014] The publication Liang-Chia Chen, Yi-Shiuan Chen, Yi-Wei Chang, Shyh-Tsong Lin, Sheng Lih Yeh "Spectrally-resolved chromatic confocal interferometry for one-shot nano-scale surface profilometry with several tens of micrometric depth range", published in Proc, of SPIE, Vol. 8759, 87592C1 to -C6

[0011] provides measurement results using a chromatic confocal spectral interferometer (CCSI) with a spectral wavelet.

[0015] DE 102010 046907 A1

[0012] describes a spectral interferometer with a chromatic depth splitting employing a Schwarzschild objective and a power compensated diffractive- optical element. The dispersion of the substrate of the diffractive-optical element is compensated for in the reference beam path. In conjunction with the Schwarzschild objective, the influence of dispersion can be reduced to the greatest extent possible, even in the case of unequal illumination of the pupil of the Schwarzschild objective due to an inclined surface of the target. Generally, considerable chromatic aberrations can still be observed in the peripheral region of the pupil, even in the case of well-corrected microscope objectives. The chromatic aberrations can become dominant in the case of an inclined surface of the measurement object. With a Schwarzschild objective as a mirror objective these effects generally do not occur.

[0016] A drawback of the known prior art chromatic confocal spectral interferometry techniques and spectral-domain optical coherence topography is that there is an undesirable trade-off between the spectral range given by the light source and the spectral range remaining in the detected signal due to confocal discrimination. Accordingly, there exists a trade-off between the depth measurement range and the depth resolution, since only light components of those foci-wavenumbers, which are sharply focused on or at the target and can thus pass the confocal aperture, essentially contribute to the detectable signal. However, with a greater axial spread of the foci (e.g., by using variable refractive power optical elements), the remaining spectral range in the detected signal becomes increasingly narrower. As generally known, the spectral bandwidth in the evaluated signal determines according to the principles of Fourier optics the axial resolution of the sensor, which tends to be inversely proportional to the spectral bandwidth. With a narrow remaining spectral range, the frequency of the wavelet, which contains the optical path difference and thus also the information about the depth position of the measured object, can be determined only comparatively roughly by means of a Fourier transform. The phase increase over the wavenumber can also only be determined comparatively roughly because of the truncated data mass. This greatly reduces the possibility of a highly accurate determination of the depth position of an object point, especially if the measuring object has a non- continuous surface in the sense of interferometry.

[0017] US 2014 / 0300904 A1

[0013] describes an approach for the generation of two spectral wavelets with exactly one light source. Two different optical paths with different refractive powers are formed by a beam splitter in the object beam path of the interferometer, i.e., within the interferometer. The optical paths are subsequently reunited in the interferometer by a beam unification unit. Thus, there are two separate optical paths (object beam paths) in the interferometer for the light which has already come into optical contact with the target, i.e., the object light. The difference in optical path lengths of the two optical paths for the object light must be adjusted very precisely so that defined phase slopes result. However, in US 2014 / 0300904 A1 the two optical paths for the object light each have comparatively large optical path lengths, including in different optical media. Accordingly, temperature influences and mechanical influences on the optical components in the two optical paths in the object beam path of the interferometer can lead to very undesirable and difficult to detect optical path differences after a final calibration of the interferometer. Such undesired optical path differences can lead to phase changes between the two spectral wavelets and a change the slope of the phase lines, i.e., can undesirably influence the phase slopes. This can falsify the evaluation and thus increase the measurement uncertainty of the method.

[0018] In contrast, WO 2019 / 120470 A1

[0014] describes an interferometer with a subsequent spectrometer, wherein two spectral wavelets are generated with a bifocal or multifocal diffractive-optical element (DOE) using only one reference light beam and only one beam splitter in the. However, the use of a bifocal or multifocal DOE may cause intensity and contrast losses in the signal wavelets.

[0019] The publication R. Schmitt, N. Kdnig, E. Manfrin de Araujo, "Surface profile analysis using a fiber-optic low coherence interferometer"

[0015] , published in Proc, of SPIE, vol. 7389, 738914-2, describes a fiber-coupled interferometer for distance measurement, in which the evaluation of the interference is performed with a downstream Michelson interferometer with inclined mirror surface under evaluation of a spatial interferogram. This is an alternative to the evaluation with a spectrometer, which is usually fiber- coupled.

[0020] It is an object of the invention to provide an improved method and systems for chromatic confocal spectral interferometry (CCS I) and / or a spectral domain optical coherence tomography (OCT) that overcome or alleviate one or more of the drawbacks of prior art systems and methods for obtaining depth information of a measured object.

[0021] A further object is to provide a compact and / or robust optical system based on chromatic confocal spectral interferometry (CCSI) and in particular chromatic confocal two-beam interferometry and / or or on spectral domain OCT, with a high depth measurement resolution and / or a low uncertainty of measurement. Further, it is an object to provide a sensor, in particular a single-shot sensor, that implements such optical system, that is suitable for commercial exploitation.

[0022] To address one or more of the above technical problems, there are provided a method and an optical system for chromatic-confocal spectral interferometry (CCSI) (including chromatic-confocal two-beam interferometry with the generation of a spatial interferogram, in particular also in single-shot operation mode) and for spectral domain optical coherence tomography (SD-OCT) as defined in the appended independent claims. The method and the system employ a plurality (i.e. , at least two) wavelets or at least one spatial interferogram for the detection a distance, depth, profile, shape, waviness and / or roughness or for optical path length measurements in or on technical or biological measured objects. The measured object may be a biological or a non- biological object and may have any form, including, but not limited to objects in a layer form.

[0023] In particular, a first aspect relates to an optical system for chromatic confocal spectral- domain optical coherence tomography (in particular a spectral-domain OCT with a plurality of wavelets) or for chromatic confocal two-beam interferometry. The optical system comprises: a multispectral light source unit configured to generate a plurality of spatially coherent multispectral light beams, wherein each of the multispectral light beams is formed by multispectral light having a spectrum with a centroid wavelength lambda and a corresponding centroid wavenumber, an achromatic focusing unit having an optical axis at its output, wherein the achromatic focusing unit is configured such that the multispectral light beams generated from the multispectral light source unit and passed through the achromatic focusing unit have at least approximately spherical wavefronts in at least a paraxial region around a common point located on the optical axis at the output of the achromatic focusing unit, wherein the wavefronts have different radii of curvatures; a chromatic confocal two-beam interferometer having an optical path difference different from zero arranged downstream of the multispectral light source unit, wherein the chromatic confocal two-beam interferometer comprises: an achromatic reference arm having an optical axis, wherein at least one end reflector is arranged in the reference arm; an object arm, wherein a chromatic depth splitting unit is arranged in the object arm, a beam splitting unit configured to split each of the multispectral light beams which has passed through the achromatic focusing unit into a pair of a multispectral reference light beam in the reference arm and a multispectral object beam in the object arm; a detection unit configured to detect interfering light reflected by the at least one end reflector in the achromatic reference arm and multispectral light reflected from a measured object arranged in the object arm, said detection unit comprising a confocal discriminator unit and at least one spectrometer, evaluation dual-beam interferometer or fast detector, wherein the optical system is configured such that: in the reference arm, each of the multispectral reference light beams forms a respective substantially diffraction limited reference light spot, and wherein the reference light spots formed by the different multispectral reference light beams are spatially separated and lie on a straight line, which is substantially coincident with the optical axis of the reference arm; and in the object arm, each of the multispectral object beams is chromatically split by the chromatic depth splitting unit and forms a respective multispectral stretch of diffraction limited light spots extended in a depth direction of the object arm, wherein the multispectral stretches of diffraction limited light spots are at least approximately collinear and at least partially overlap in the depth direction, thereby forming an overlapping region, a current diffraction limited object light spot of each of the multispectral stretches of diffraction limited light spots, the current diffraction limited object light spot having a current wavenumber, is focused at a current measurement point of the measured object in the overlapping region; the confocal discriminator unit confocally discriminates combined light from the reference arm after reflection on the at least one end reflector and light from the object arm after reflection on the measured object, and the at least one spectrometer, evaluation dual-beam interferometer or fast detector is configured to detect the confocally discriminated light.

[0024] According to a second aspect, there is provided a method for chromatic confocal spectral-domain optical coherence tomography (in particular a spectral-domain OCT with a plurality of wavelets) or for chromatic confocal two-beam interferometry, wherein the method may be carried out with the help of the system for chromatic confocal spectral-domain optical coherence tomography or for chromatic confocal two-beam interferometry according to the first aspect and modifications and examples thereof. The method may include: providing an optical system for chromatic confocal spectral-domain optical coherence tomography or for chromatic confocal two-beam interferometry according to the first aspect and modifications and examples thereof; generating, by the multispectral light source unit a plurality of spatially coherent multispectral light beams, wherein each of the multispectral light beams is formed by multispectral light having a spectrum with a centroid wavelength lambda and a corresponding centroid wavenumber, forming, by the achromatic focusing unit, the multispectral light beams emitted from the multispectral light source unit, such that after passing through the achromatic focusing unit, the multispectral light beams have at least approximately spherical wavefronts in at least a paraxial region around a common point located on the optical axis at the output of the achromatic focusing unit, wherein the wavefronts have different radii of curvatures; splitting, by the beam splitting unit of the chromatic confocal two-beam interferometer, each of the multispectral light beams which has passed through the achromatic focusing unit into a pair of a multispectral reference light beam in the reference arm and a multispectral object beam in the object arm, wherein in the reference arm, each of the multispectral reference light beams forms a respective substantially diffraction limited reference light spot, and wherein the reference light spots formed by the different multispectral reference light beams are spatially separated and lie on a straight line, which is substantially coincident with the optical axis of the reference arm; and wherein in the object arm, each of the multispectral object beams is chromatically split by the chromatic depth splitting unit and forms a respective multispectral stretch of diffraction limited light spots extended in a depth direction of the object arm, wherein the multispectral stretches of diffraction limited light spots are at least approximately collinear and at least partially overlap in the depth direction, thereby forming an overlapping region, and wherein a current diffraction limited object light spot of each of the multispectral stretches of diffraction limited light spots, the current diffraction limited object light spot having a current wavenumber, is focused at a current measurement point of the measured object in the overlapping region; confocally discriminating, by the confocal discriminator unit, (recombined) light from the reference arm after reflection on the at least one end reflector and light from the object arm after reflection on the measured object, and detecting, by the at least spectrometer, evaluation dual-beam interferometer or fast detector of the detection unit of the optical system, the confocally discriminated light.

[0025] Since the current wavenumbers correspond to the depth position of the current measurement point of the measured object, it is thus possible to obtain information about the depth position of the current measurement point from the detected signal.

[0026] Multispectral light source unit

[0027] A “multispectral light source unit" in the context of the present disclosure refers to any light source unit that is capable of simultaneously or sequentially emitting light (multispectral light) having a plurality of wavelengths or wavelength ranges. Thus, the term “multispectral light source” as used in the context of the present description encompasses multispectral light sources, wherein multispectral light is generated simultaneously and quasi-multispectral light sources, wherein multispectral light is generated by rapidly tuning or sweeping a light source in the spectral domain to generate in real-time multispectral light. In other words, in quasi-multispectral light sources the multispectral light is generated time serially, rather than simultaneously.

[0028] The multispectral light unit may for example comprise a plurality of individual light sources (primary light source), for example point light sources emitting light with different spectra, or one light source (primary light source), for example a broadband light source emitting light with a given spectrum. The light emitted from the primary light source or sources may be combined and / or split by employing beam splitters, fibers (such as single-mode fibers), mirrors (such as plane, concave, convex mirrors or mirror assemblies such as rosette mirrors), y-fiber couplers and / or other optical elements to thereby form a plurality of multispectral spatially coherent light beams. Light

[0029] The term “light” is used in the context of the present disclosure as a synonym for electromagnetic radiation from the deep UV region to the terahertz region.

[0030] A light spectrum / light spectra may be given in the wavenumber domain (k domain) or in the wavelength (lambda) domain. The wavenumber “k” is related to the wavelength of light lambda by k=2Pi / lambda. A spectral representation in the k-domain may be advantageous for further signal processing, e.g., using a fast Fourier transform (FFT), since methods for evaluating spectral wavelets, such as for example FFT-based methods (FFT: Fast Fourier Transform) are typically applied to data in the wavenumber domain (k-domain). This also applies to the representation of the wavelets, since in spectral interferometry, as a rule, the representation in k-domain can result in at least approximate straight lines for the phase dependency phi(k) on the wavenumber k. It is, of course, possible to use a spectral representation in the wavelength domain, e.g., because of the better comprehensibility. For example, known equations of wave optics, such as the equations for the wave optical depth of focus (shortly depth of focus) typically refer to the wavelength, rather than the wavenumber. It is of course, possible, to perform a conversion from a wavenumber domain to wavelength domain and vice versa.

[0031] Multispectral light beam

[0032] A multispectral light beam (multispectral light bundle) within the context of the present disclosure refers to a light beam (light bundle) formed of light having a given spectrum (as opposed to monochromatic light). . The spectrum of each multispectral light beam may have a centroid wavelength (or centroid wavenumber). The centroid wavelength / wavenumber of a spectral range corresponds to the “center of gravity” of the wavelengths / wavenumbers in the respective spectral range. The spectrum need not be a broad spectrum, but may also be a relatively narrow spectrum of e.g., between about 1 % to 40% of the centroid wavelength (or centroid wavenumber), further for example between about 2% to 20% of the centroid wavelength (or centroid wavenumber).

[0033] Spatially coherent

[0034] The term “spatially coherent light beam (light bundle)” is used within the context of the present disclosure to denote a light beam (light bundle) that can form an at least approximately diffraction-limited light spot. The term “spatially coherent” does not require a strict spatial coherence and includes “at least approximately spatially coherent”. That means, the wavefront deviation from an ideal spheric or plane shape in the light bundle is equal to or brlow the half of the centroid wavelength.

[0035] Chromatic / Achromatic

[0036] The term “chromatic” as used within the context of the present disclosure means that within the spectral range in which the optical system operates there is a wavelength (wavenumber) dependency of the power (refractive power and / or diffractive power) of an optical element, an optical path in an interferometer arm, etc.

[0037] The term “achromatic” as used within the context of the present disclosure means that within the spectral range in which the optical system operates there is substantially no wavelength (wavenumber) dependency of the power (refractive power and / or diffractive power) of an optical element, an optical path in an interferometer arm, etc.

[0038] The term “achromatic” does not require a strict achromacy in the respective wavelength spectrum in which the optical system operates, it is sufficient that the respective optical component, arm or path is at least approximately achromatic. That means the optical path difference of all wavelengths in the spectrum used in the interferometer is equal to or below the half of the centroid wavelength of the light bundle.

[0039] Achromatic focusing unit

[0040] The achromatic focusing unit is configured to generate at its output a plurality of spatially coherent multispectral light beams from the light emitted from the multispectral light source. The achromatic focusing unit may comprise one or more achromatic lenses. The achromatic focusing unit may be a part of the multispectral light source unit and may be arranged at the output of the multispectral light source unit. In this arrangement, the multispectral light source unit emits converging light beams. It is, however, also possible to arrange the achromatic focusing unit downstream (i.e. , after) the output of the multispectral light source unit, wherein the achromatic focusing unit may be either completely upstream of the chromatic confocal two-beam interferometer or may be at least partially integrated in an illumination beam path chromatic confocal two-beam interferometer. Generally, the achromatic focusing unit may be arranged before the beam splitting (i.e. upstream of the beam splitting unit) of the dual-beam interferometer.

[0041] In an example, the optical axis of the multispectral light source unit coincides with the optical axis OAS of the achromatic focusing unit and / or the optical axis of an illumination path of the dual-beam interferometer.

[0042] The wavefronts of the multispectral light beams formed at the output of the achromatic focusing unit need not be strictly spherical, but may be at least approximately spherical in at least a paraxial region around a common point at the output of the achromatic focusing unit. In other words, the wavefronts may exhibit some level of asphericity, for example to compensate, for dispersion, or other optical errors of optical components present in the system. The wavefronts of the multispectral spatially coherent light beams at a common point at the output of the achromatic focusing unit exhibit different curvatures. Thus, a plurality of effective point light sources from which a respective plurality of spatially coherent multispectral light beams “emerge” are formed with the help of the achromatic focusing unit (together with the multispectral light source unit), wherein the effective point light sources are spatially separated in a direction along the optical axis at the output the achromatic focusing unit (i.e. that are depth-separated) are formed.

[0043] The extent of separation depends on the application-specific aimed for or desired depth-separation for a specific application. The wavefront curvatures may, for example, be selected such that the foci of the light bundles with the shortest and the longest wavelength in the object arm, respectively, are separated by, for example up to 1000, more specifically up to 500, further specifically up to 200 or up to 500 centroid wavelengths of the light bundles. In an example, the wavefront curvatures may be different from each other by at least 3pm to 1 mm, more specifically by 10 pm to 500 pm, further specifically by 20 pm to 300 pm, depending on their centroid wavelengths.

[0044] Upstream / Downstream

[0045] If not stated otherwise, the terms “upstream” and “downstream” are with respect to a direction of propagation of light, for example the direction of propagation of light in the illumination path i.e., from the multispectral light source, through the achromatic focusing unit, into the chromatic confocal two-beam interferometer and up to the end reflector(s) in the reference arm and the measured object in the object arm. If a first optical element, unit or module is arranged “upstream” from a second optical element, unit or module, it means that it is arranged before the second optical element, unit or module in a direction of propagation of light.

[0046] Dual-beam interferometer

[0047] The term “dual-beam” interferometer is used as a synonym for two-beam, dual-path or two-path interferometer. The term “chromatic-confocal” is used to refer to a confocal type dual-beam interferometer, in which there is a chromatic splitting of foci in the object arm of the dual-beam interferometer. The chromatic-confocal dual-beam interferometer may be a Michelson-type, a Linnik-type, a Mirau-type interferometer or any other chromatic-confocal dual-beam interferometer.

[0048] The chromatic-confocal dual-beam interferometer comprises a beam splitting unit, an achromatic reference arm with at least one end reflector and an object arm with at least one chromatic depth splitting unit.

[0049] In the chromatic-confocal dual-beam interferometer, a part of the light emerging from the effective multispectral point light sources is focused (with the help of the achromatic focusing unit and any further optical components between it and an end reflector) to thereby form a corresponding plurality of depth-separated multispectral and thereby sharply imaged light spots (reference light spots) in the reference arm of the chromatic- confocal dual-beam interferometer. Another part of the light emerging from the effective point light sources is focused and chromatically split by the achromatic focusing unit, the chromatic depth splitting unit any further optical components to form a respective plurality of stretches (chains) of sharply focused light spots in the object. Sharply focused in the context of the present disclosure refers to focusing the light in a respective spot, which has a lateral extension about or below the one given by the diffraction limit. The diffraction limit is generally dependent on the numerical aperture of the lenses involved.

[0050] Beam splitting unit

[0051] The beam splitting unit of the dual-beam interferometer is configured to split the incoming light from the multispectral spatially coherent light beams (and thus the multispectral light “emerging” from the plurality of effective multispectral light spots formed upstream of the beam splitting unit) into two parts: a first entering the reference arm and a second part entering the object arm of the dual-beam interferometer. In the return path, the beam splitting unit combines the light returned from the reference arm (after a reflection on the at least one end reflector) and the object arm (after a reflection on the measured object). The beam splitting unit of the dual-beam interferometer may be any suitable beam splitting unit comprising at least one beam splitting layer. The beam splitting layer may be an achromatic or a color beam splitting layer (for example if the effective point light sources have non-overlapping spectra). In case of clearly overlapping spectra, achromatic beam splitting layers may be suitable.

[0052] The multispectral light source unit and / or the detection unit may also comprise at least one at least one beam splitting layer. The multispectral light source and the detection unit may share a part or all of the beam splitting layers. Reference arm

[0053] The reference arm of the dual-beam interferometer may comprise one or more subarms. In other words, the reference arm may be a single strand (single) reference arm or a split reference arm having two or more sub-arms, wherein after reflection on the respective end reflectors arranged in each of the sub-arms, the reference light beams from each of the sub-arms are united (e.g., by a respective beam splitter), so that after unification, their main rays are colinear and substantially coincide with the optical axis OAR of the split reference arm. Accordingly, after reference beam unification, the effective reference light spots formed by the respective reference light beams in the sub-arms of the split reference arm generally lie on a straight line, which is substantially coincident with the optical axis (OAR) of the split reference arm. Further, the effective reference light spots are spatially separated from each other. An extension of this arrangement to three or more sub-arms and / or four and more reference light beams is also possible.

[0054] The reference arm is configured such that is substantially achromatic within the relevant spectral range. Each reference light beam (formed from a part of the light from a respective one of the multispectral spatially coherent light beams that falls on the beam splitting unit) forms exactly one multispectral focus, so that the number of reference light spots corresponds to the number of effective light spots separated in the optical depth.

[0055] The light in the reference arm R may be reflected by an end reflector or a plurality of end reflectors, wherein the geometrical path length in the reference arm is substantially the same for every main ray of the respective multispectral reference light beams.

[0056] End reflector

[0057] In the reference arm, at least one end reflector is arranged. The end reflector may consist of or may comprise at least one mirror or mirror segment, in particular at least one spherically curved mirror or spherically curved mirror segment. A spherically curved mirror has a respective osculating sphere which is the imaginary sphere encompassing and extending from the respective spherically curved mirror. An osculating sphere of a spherically curved mirror has thus the same radius of curvature as the mirror and the same center point. The spherically curved mirror may be a convex or a concave mirror. A combination of various mirrors is also possible. The mirror(s) may be centered on the optical axis of the reference arm, or in case of a split reference arm comprising a plurality of sub-arms, on an optical axis of one of the sub-arms. Further, in case of split reference arms, each reference arm has its own end reflector, wherein the end reflectors of the different reference arms may have the same or different optical structure.

[0058] Object arm

[0059] In the object arm of the dual-beam interferometer a measured object (target) is arranged. Further, the object arm comprising a chromatic depth-splitting unit and optionally further optical elements, such as for example dispersion compensation optical elements and / or optical elements that at least partially compensate a focusing power of the chromatic depth splitting units.

[0060] In an example, there is no lateral splitting of the beam path in the object arm of the chromatic-confocal dual-beam interferometer. In other words, the object arm may be single-stranded. Further, in an example, also the reference arm may be singlestranded.

[0061] Chromatic depth splitting unit

[0062] Further, in the object arm, a chromatic depth splitting unit is arranged. The chromatic depth splitting unit may comprise one or more elements configured to chromatically split the incoming light. The chromatic depth splitting unit may have a focusing power, which may be positive or negative. In an example, the chromatic depth splitting may be realized by at least one diffraction-optical element (DOE). With the help of the achromatic focusing unit, the chromatic depth splitting unit and optionally further optical components, a plurality of stretches (chains) of light spots are formed from each of the multispectral spatially coherent light beams emitted from the multispectral light source unit (one stretch of light spot formed from a corresponding one of multispectral spatially coherent light beams). Within the context of the present description the stretches of object light spots (object light spots) formed in the object arm are also referred to as (extended) multispectral object light spots.

[0063] Thus, in the dual-beam spectral interferometer the depth spacing in the object arm 0 for the optically conjugated light spots (object light spots) of the effective light spots (formed at the output of the achromatic focusing unit) may be reduced for their centroid wavenumbers by a predetermined chromatic depth splitting. Ideally, the depth separation of the light spots with different centroid numbers is thereby made zero and a confocal point cPS is formed from the respective light spots. However, this is an ideal case that may be difficult to realize in a real optical system. In general, it is sufficient if the depth separation of the object light spots with two different centroid numbers is equal or smaller than a predetermined value.

[0064] Light spot

[0065] Further, within the context of the present disclosure, a light spot(s) may be also referred to for the sake of simplicity as “focus”, “foci”. A light spot, in particular a reference light spot, may be a real or a virtual light spot. Each light spot may be regarded as an effective light emitting spot emitting multispectral reference light having spectrum with a respective centroid wavenumber / wavelength.

[0066] Sharpy imaged light spots within the context of the present disclosure refers to light spots that are imaged such that their lateral extension substantially corresponds to the lateral diffraction limit given by the numerical aperture of the imaging system, i.e. is about or lower than the lateral diffraction limit. Such light spots may be also referred to as “diffraction limited” light spots or “microscopic” light spots. Allowable deviations from the diffraction limit after imaging of a light spot, defined by the Strehl ratio, may be equal to or greater than 0.1 , more specifically equal or greater than 0.4, further specifically equal or greater than 0.8 or 0.99.

[0067] Chromatic depth splitting of light spots

[0068] The terms ..chromatic depth splitting of light spots" and “chromatic depth splitting" are used as synonyms for a chromatic depth splitting of foci, i.e., forming of a foci that are separated from each other in a depth direction (z-direction), the foci being formed by light having different centroid wavelengths / centroid wavenumbers. The foci may be separated such as to not overlap with each other. For the description of the chromatic depth splitting a value “dzc” may be used, which describes the z-range of the chromatic depth splitting of light spots in the object arm of the two-beam interferometer wherein the z-coordinate is the optical depth direction.

[0069] Detection unit

[0070] The detection unit comprises a confocal discrimination unit comprising at least one confocal discriminator (such as an aperture, an end of a single-mode fiber, etc.). The confocal discrimination unit is configured to confocally discriminate light from the reference arm after reflection on the at least one end reflector and light from the object arm after reflection on the measured object. Thereby, focused light reflected from the current measurement point is allowed to pass and be detected and out of focused light from other measurement points is substantially blocked.

[0071] Further, the detection unit comprises a detector unit comprising one or more of a spectrometer, evaluation dual-beam interferometer or a line detector are configured to detect the confocally discriminated light. For example, in case the optical system is for a chromatic confocal spectral-domain optical coherence tomography with a plurality of wavelets, the detection unit may comprise at least one spectrometer. In case the optical system is for chromatic confocal two-beam interferometry, the detection unit may comprise at least one evaluation dual-beam interferometer, with the help of which a spatial Interferogramm is detected, such as a tilted-wave dual-beam interferometer. In case of a quasi-multispectral light sources (such as for example if at least one tunable laser or swept light source is employed in the multispectral light source unit), the detection unit may comprise at least one fast detector, such as for example a fast line detector or a fast two-dimensional detector. The fast detector may for example be a fast line CCD camera or a fast two-dimensional CCD camera. The fast detector may be in particular a detector having a read-out time that matches the speed of spectral sweeping or scanning in a quasi-multispectral light source.

[0072] The detected wavelets, spatial interferograms or signals from the fast (e.g., line) detectors may be processed by known methods to obtain depth information of the measurement point.

[0073] Glass

[0074] The term “glass” is used within the context of the present description to refer to any optical material (in particular any substantially transparent optical material) that may be used to form optical elements. The term “glass” thus encompasses, but is not limited to, organic and inorganic glass materials.

[0075] The methods and systems for chromatic confocal two-beam spectral interferometry and / or a spectral domain OCT according to the above described aspects and / or examples thereof provide for one or more of the following advantages:

[0076] In particular, it is possible to detect optical data from a wider spectral range as compared to known chromatic confocal spectral interferometry (CCS I) approaches or known known chromatic confocal interferometry approaches with a downstream dualbeam evaluation interferometer, thus enabling a more accurate phase evaluation. For example, the detection of a broader spectral wavelet as an overlap of several single narrow wavelets or the detection of at least two or more narrow spectral wavelets separated in the spectrum by a simple optical arrangement is possible. Several single narrow wavelets may improve the phase evaluation and can also be advantageous for the evaluation of the spectral wavelets by means of a Fast Fourier Transformation (FFT).

[0077] In an example, the at least two or more wavelets may be generated simultaneously or quasi-simultaneously, thus achieving a significant enlargement of the primary optical data mass in the wavenumber domain.

[0078] Further, it is possible to obtain a plurality of spectral wavelets (comprising at least two spectral wavelets) with substantially the same phase increase in the wavenumber domain. This simplifies the signal evaluation and / or significantly improve the depth measurement resolution.

[0079] In examples, the harmful influence of dispersions in the interferometric beam path(s) can be reduced as much as possible, in order to generate spectral signals in the form of wavelets in the wavenumber domain with a minimum of nonlinearities in the signal and a comparatively large spectral bandwidth. This may in particularly be achieved by making the optical path lengths for light of different wavelengths in the two-beam interferometer at least approximately equal.

[0080] Further, in an implementation as a single-shot point sensor, it is possible to minimize the spatial extent, especially in the transverse axis, so as to realize a compact, slim design. Still further the proposed method and system enable a comparatively vibration- resistant interferometer arrangement.

[0081] In examples a single object light beam and a corresponding single spectral wavelet may be generated from a primary light source or a secondary (effective) light source (for example obtained by beam splitting light from a primary light source). In examples, a spatial interferogram may be generated by employing chromatic confocal two-beam interferometry, wherein the spatial interferogram may be further evaluated to obtain depth information by using known methods.

[0082] The optical system and the respective method according to the above aspects may be used for various measurements, including the detection or measurement of a distance, depth, profile, shape, waviness and / or roughness or for optical path length measurements in or on non-biological or biological measured objects.

[0083] In an example, in the optical system and method according to the above aspect, the centroid wavelengths or the centroid wavenumbers of the multispectral spatially coherent light beams emitted by the multispectral light source unit may be different from each other.

[0084] For example, the centroid wavelengths of the multispectral spatially coherent light beams (and therefore of the respective light spots generated by them) may differ from each other by at least 10 nm, in case of multispectral light in the visible spectrum. In case of multispectral light in the near IR spectrum, the centroid wavelengths of the light spots may differ from each other by at least 20 nm.

[0085] With this arrangement, the light energy of the light source unit may be efficiently used, which enables the use of the optical system with a comparatively low light intensity. It is thus possible, to carry out measurements on targets that can be easily damaged by high intensity light.

[0086] In the optical systems according to the above aspect and examples thereof, the optical path difference (xp) of the chromatic-confocal dual-beam interferometer may be equal to or greater than ten times and equal to or less than ten thousand times one of the centroid wavelengths of the multispectral light beams generated by the multispectral light source, for example a central (middle) one or the largest one of the centroid wavelengths.

[0087] Further, preferably, the optical path difference may be substantially the same for all wavelengths of the light spectra of the multispectral light beams entering the chromatic- confocal dual-beam interferometer. This may be realized by, for example, employing at least one dispersion compensation element configured compensate for or reduce any undesired wavelength difference, introduced for example by the dispersion of the refractive parts of the employed optical elements. The at least one dispersion compensation element may be arranged for example in the reference arm and / or the object arm. It is also possible to design the multispectral light source unit and / or the achromatic focusing unit such as to pre-compensate for a dispersion occurring in one of more of the optical elements arranged downstream of the multispectral light source unit and / or achromatic focusing unit. Thus, it is possible to obtain detected signals with a minimum of non-linearities in over a large spectral range.

[0088] The optical system of the above aspect and examples thereof may be configured such that in an ideal case, those diffraction limited object light spots of each of the multispectral stretches of diffraction limited light spots that have the centroid wavenumbers coincide in a confocal point in the object arm. This may contribute to detecting wavelets that are in-phase and may improve the processing of the detected light to obtain a more accurate depth information.

[0089] Such ideal case may, however, be difficult to realize in a real-life optical system, for example due to the imaging and dispersion properties of the optical elements, difficult alignment of the optical elements, etc. Generally, in a real-life optical system, it is sufficient if those diffraction limited object light spots of each of the multispectral stretches of diffraction limited light spots having the centroid wavenumbers are positioned in a depth range (i.e. in a range or area along the depth direction of the object arm), which is at most 50% or at most 30% or at most 10% of the overlapping region.

[0090] In the optical system and method according to the above aspects and examples thereof, the at least one end reflector may have various configurations. For example, the at least one end reflector may comprises or consists of at least a first spherically curved mirror or mirror segment. The at least a first spherically curved mirror or mirror segment may be arranged such that a center point of the first spherically curved mirror or mirror segment lies on the optical axis of the reference arm or, in case of a split reference arm with two or more sub-arms, on an optical axis of a sub-arm of the split reference arm. A first reference light spot of the plurality of reference light spots may be formed at the center point of the first spherically curved mirror by a first reference light beam of the plurality of reference light beams.

[0091] Depending on whether the first spherically curved mirror is convex or concave, the light spot (first reference light spot) formed in the center point of the first spherically curved mirror (first center point) may be a real or a virtual light spot. In case of a spherically curved convex mirror, the reference light spot formed at a center point of the spherically curved convex mirror is a virtual light spot formed by focusing / imaging the respective reference light beam in the central point of the first spherically convex spherically curved mirror. In case of a concave spherically curved mirror, the reference light spot is a real light spot that is formed by light of the respective reference light beam, which after a reflection on the concave spherically curved mirror is focused in the center point. In other words, the optical system may be configured such that a reference light beam is aimed at the center point of a spherically curved mirror comprised in the end reflector in the reference arm.

[0092] The shape of the mirrors (concave, convex) may depend on the sign of a focusing power of the chromatic depth splitting unit, which can, for example, be positive or negative. In addition or alternatively, the shape of the mirrors may depend on the positioning of the (effective) emitting light spots in the depth in front of the chromatic- confocal dual-beam interferometer.

[0093] Various combinations of spherically curved concave and / or convex mirrors are possible.

[0094] A second (and each subsequent) reference points may be formed at appropriate locations, depending on the realization of the reference arm (single or split), the end reflector (comprising a single or a plurality of spherically curved mirrors) and / or the number of reference light beams. For example, if an end reflector comprises a plurality of spherically curved mirrors, a real or virtual reference light spot may be formed at the center point of each of the mirrors constituting the end reflector, as described above. Alternatively or in addition, a reference light spot may be formed by focusing / imaging a reference light beam on a point (incident point) on a mirror of a plurality of mirrors constituting the at least one end reflector, which point may in particular be on the optical axis of the respective reference arm or sub-arm. Alternatively or in addition, a reference light spot may be formed by focusing a reference light beam at a point of intersection of an osculating sphere of one of a plurality of mirrors constituting the at least one end reflector with the optical axis of the reference arm or, in case of a split reference arm, an optical axis of a sub-arm of the split reference arm.

[0095] Below are examples of end reflectors and reference light spots.

[0096] In an example, the end reflector may comprise or consist of a single first spherically curved mirror. In this case, a second reference light spot of the plurality of reference light spot may be formed by a second reference light beam from the plurality of reference light beams at a point of incidence AR of a main ray of the second reference light beam on the single first spherically curved mirror or at a point of intersection GO of an osculating sphere of the first spherically curved mirror with the optical axis of the reference arm or, in case of a split reference arm, an optical axis of a sub-arm of the split reference arm. The point of incidence of a main ray of the second reference light beam on the single first spherically curved mirror may lay on the optical axis of the reference arm or, in case of a split reference arm, an optical axis of a sub-arm of the split reference arm.

[0097] In another example, the at least one end reflector may further comprise at least one second spherically curved mirror or mirror segment, in addition to the first spherically curved mirror or mirror segment. In this case, a second reference light spot of the plurality of reference light spots may be formed by a second reference light beam from the plurality of reference light beams at one of the following points: a point of incidence AR of the main ray of the second reference light beam on one of the plurality of spherically curved mirrors or mirror segments; or at a point of intersection GO of an osculating sphere of one of the spherically curved mirrors or mirror segments, with the optical axis of the reference arm or, in case of a split reference arm, an optical axis of a sub-arm of the split reference arm; or at a center point of the at least one second spherically curved mirror or mirror segment. The point of incidence AR may lie on the optical axis of the reference arm, or in case of a split reference arm with two or more reference sub-arms, on an optical axis of a sub-arm of the split reference arm.

[0098] Further optionally, a further reference point (e.g., a third reference point) may be formed by a third reference beam at one of the above indicated points.

[0099] In an example, the at least one end reflector may further comprise a plane mirror in addition to the first spherically curved mirror, wherein the first spherically curved mirror is arranged concentrically around the plane mirror- The plane mirror may, for example, be a miniaturized plane mirror arranged at an opening in the center of the first spherically curved mirror. The plane mirror may be centered on the optical axis of the reference arm, or in case of a split reference arm, on an optical axis of a reference sub-arm. In other words, the plane mirror may be arranged such that its normal is substantially coincident with the optical axis of the reference arm, or in case of a split reference arm, on an optical axis of a reference sub-arm. In this case, a second reference light spot of the plurality of reference light spots may be formed by a second reference light beam of the plurality of reference light beams at a point of incidence of a main ray of the second reference light beam on the plane mirror.

[0100] In some examples, the end reflector may comprise a plurality of concentrically arranged spherically curved mirrors, with an inner mirror and at least one outer mirror in a ring form, wherein the mirrors are centered on the optical axis of the reference arm or on an optical axis of a sub-arm, in case of a split reference arm. The concentrically arranged spherically curved mirrors may have different radii of curvature. Various combinations of convex and concave spherically curved mirrors are possible. For example, all mirrors may be convex, all mirrors may be concave, an inner mirror may be convex and the at least one outer mirror may be concave, an inner mirror may be concave and the at least one outer mirror may be convex, etc. Preferably, the osculating spheres of all mirrors contact in a single point of contact arranged on an optical axis in the reference arm. In some examples, the end reflector may be formed in a rosette shape / form with multiple spherically curved mirror segments with alternating different curvatures, whose osculating spheres meet at a point of contact BP on an optical axis in the reference arm. For example, there may be two groups of mirror segments, the mirror segments of the first group each having a first radius of curvature and the mirror segments of the second group each having a second radius of curvature, wherein the mirror segments of the first and the second group are arranged in an alternating matter (i.e. a mirror segment of the first group is arranged adjacent to a mirror of the second group and so on).

[0101] The end reflector may, for example, comprises a plurality of concentric spherically curved mirrors or a plurality of spherically curved mirror segments arranged in a rosette form, wherein the osculating spheres of the plurality of spherically curved mirrors or mirror segments contact each other in a single contact point BP. Optionally, the point of incidence AR of a main ray of the second reference light beam coincides with the contact point BP.

[0102] In the above described exemplary optical systems a depth distance t12R' the first reference light spot and the second reference light spot may be set equal to a radius of curvature rS 1 of the first spherically curved mirror, if the second reference light spot is positioned at a point of incidence AR or the second light spot is positioned at a point of intersection GO. In case the end reflector comprises a plurality of spherically curved mirrors, including a first spherically curved mirror with a radius of curvature rS1 and a second spherically curved mirrors with a radius of curvature rS2 which is different from rS1 , and the second reference light spots is positioned at the center point M2 of the osculating sphere SK2 of the second spherically curved mirror, the depth distance t12R' separating the two reference light spots and may be set equal to the absolute value of the difference rS1 - rS2 (i.e., t12R' = | rS1 - rS2 | ).

[0103] The invention is not limited to the above examples, but may include end reflectors having other mirror arrangements. In the optical system of the above aspect and examples thereof the at least one end reflector may be a fixed (unmovable) end reflector or a movable end reflector. For example, the at least one end reflector (such as for example the end reflector in a rosette form) may be rotatable. In addition or alternatively, the end reflector may be movable in a depth direction and / or in at least one direction orthogonal to the depth direction. This enables a precise calibration of the optical system.

[0104] In addition or alternatively, the optical system may comprise an array of end reflectors arranged in the reference arm. In case the reference arm is a split reference arm, there may be an array of end reflectors arranged in each of the sub-arms of the split reference arm or only in one of the sub-arms of the split reference arm. The array may be, for example, a linear array or a two-dimensional array.

[0105] The confocal discriminator unit of the optical system according to the above described aspect and examples thereof may comprises a plurality of confocal discriminators. Each of the confocal discriminators may be configured to confocally discriminate a combined light spot formed from light coming from a pair of spots originating from the same multispectral spatially coherent light beams: a reference light spot and a corresponding current object light spot, the reference light spot and the corresponding current object light spot being formed from light of a single one of the multispectral spatially coherent light beams.

[0106] It is also possible to combine light of multiple pairs of light spots (reference light spot and a corresponding object light spot), each pair of light spots originating from one multispectral spatially coherent light beam, before confocal discrimination. For example, the confocal discriminator unit may comprise a beam combining unit configured to combine light coming from the plurality of reference light spots and the corresponding current object light spots, and a confocal discriminator configured to confocally discriminate the combined light.

[0107] Other variations are also possible. For example, the beam combining unit need not combine light from all of the reference light spots and the corresponding object light spots, but of a subgroup of these light spots, which combine light is subsequently confocally discriminated by a respective confocal discriminator.

[0108] The optical system according to the above described aspect and examples thereof may have a chromatic depth splitting unit, which comprises or consists of a diffractive- optical element. The diffractive-optical element may, for example be formed as a zone lens. The diffractive-optical element may have, for example a focusing power, such as a negative or positive focusing power.

[0109] In an example, the object arm and / or the reference arm of the interferometer may further comprise at least one optical compensating element configured to overcompensate or undercompensate the focusing power of the diffractive-optical element.

[0110] The at least one optical compensating element may, for example, be one or more of the following elements:

[0111] - a curved back surface of a glass substrate of the diffractive-optical element, wherein the curved back surface is the surface opposite to a surface on which the diffractive structure of the diffractive-optical element is formed;

[0112] - a thin lens with at least approximately achromatic negative or positive refractive power arranged in the reference arm and / or in the object arm (i.e. by a thin diverging or converging lens).

[0113] The overcompensating or undercompensating of the focusing power of the diffractive- optical element is carried out such as to ensure that there is a desired (e.g., predetermined or predeterminable) optical path difference in the chromatic confocal two-beam interferometer that is different from zero.

[0114] The optical system according to the above described aspect and examples thereof may comprise a depth position computational unit configured to process the detected light to determine a depth position of the current measurement point. As described above, the wavenumbers (and respective wavelengths) of the diffraction limited light spots focused on the current measurement point depend on the depth position of the current measurement point. Thus, by a spectral analysis of the detected confocally discriminated light detected by the detection unit, information about the depth position of the current measurement point may be obtained. The method of the above described aspect and examples thereof, may thus further comprise processing, by a depth position computational unit, the detected light to obtain depth information of the current measurement point of the measured object. The processing of the detected light may be carried out by known methods for evaluating wavelets or spatial interferograms.

[0115] To obtain a depth information of a whole measured area of the measured object, the method of the above described aspect and examples thereof may be repeated for a plurality of measurement points in the measured area. In other words, a scan of a plurality of measurement points of the measured object may be carried out using the method according to the above described aspect and examples thereof. To enable a scan of multiple points, the optical system may comprise a scanning unit, that for example may be configured to move the measured object relative to the chromatically split focused light in the object arm.

[0116] The computational aspects described here (such as for example the processing of detected light) and the respective computational units can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. When appropriate, aspects of these systems and techniques can be implemented in a computer program product, for example tangibly embodied in a machine-readable storage device for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions by operating on input data and generating an output.

[0117] To provide for interaction with a user, a computer system can be used having a display device, such as a monitor or a LCD screen for displaying information to the user and a keyboard, a pointing device such as a mouse or a trackball, a touch-sensitive screen, or any other device by which the user may provide input to computer system. The computer system can be programmed to provide a graphical user interface through which the computer program(s) interact(s) with the user.

[0118] These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description of preferred embodiments and accompanying drawings. Other features and advantages of the subject-matter described herein will be apparent from the description and the drawings and from the claims. It should be understood that even though embodiments are separately described, single features thereof may be combined to additional embodiments.

[0119] Figure 1 shows schematically a first exemplary centered optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCS I).

[0120] Figure 2 shows schematically an exemplary multispectral light source unit and an unfolded reference arm.

[0121] Figure 3 shows schematically an exemplary unfolded reference arm.

[0122] Figure 4 shows schematically a second exemplary centered optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCS I).

[0123] Figure 5 shows schematically a third exemplary centered optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCS I).

[0124] Figure 6 shows exemplary cases corresponding to possible combinations of diffractive-optical elements with curved mirrors.

[0125] Figures 7 to 10 show schematically exemplary reference and object arms of a dualbeam interferometer.

[0126] Figure 11 shows schematically a fourth exemplary centered optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCS I).

[0127] Figures 12 and 13 shows schematically exemplary light source units.

[0128] Figure 14 shows a magnified view of a detail of Figure 11 .

[0129] Figure 15 shows schematically an exemplary three-channel detection unit.

[0130] Figure 16 shows schematically another exemplary detection unit.

[0131] Figure 17 shows schematically a further exemplary multispectral light source unit. Figure 18 shows schematically an exemplary end reflector.

[0132] Figure 19 shows schematically another exemplary end reflector.

[0133] Figure 20 shows schematically another exemplary reference arm.

[0134] Figure 21 shows schematically a multispectral light source unit.

[0135] Figure 22 shows an exemplary end reflector in the reference arm.

[0136] Figure 23 shows an exemplary a split reference beam arm.

[0137] Figures 24 and 25 show further exemplary end reflectors in the reference arm.

[0138] Figure 26 shows an exemplary multispectral light source unit.

[0139] Figure 27 shows a region of an exemplary object arm.

[0140] Figure 28 shows schematically an exemplary detection unit.

[0141] Figure 29 shows schematically six wavelets detected by a detection unit.

[0142] Figures 30 and 31 show further exemplary multispectral light source units.

[0143] Figure 32 shows schematically an exemplary split reference beam path in a chromatic- confocal Michelson interferometer.

[0144] Figure 33 shows schematically another exemplary detection unit.

[0145] Figure 34 shows schematically six wavelets detected by a detection unit.

[0146] Figures 35 and 36 show schematically further exemplary multispectral light source units.

[0147] Figures 1 to 10 show exemplary optical systems for spectral domain OCT or chromatic confocal spectral interferometry (CCSI). In these optical systems, there are two multispectral and spatially coherent light beams B1 and B2 with respectively different curvatures r1 pW and r2pW of the corresponding wavefronts WF1 and WF2 in a point W at the output of a light source unit. Further, the two light beams have different centroidal wavenumbers kS1 and kS2, respectively. For this purpose, a beam splitting layer is arranged in the light source unit and in the detection unit, which may be designed as a broadband or as a color splitter layer. The light beams B1 and B2 are coupled into a dual-beam interferometer with a reference arm and an object arm. Light spots (reference light spots) are formed in the reference arm of the dual-beam interferometer. The reference light spots are positioned in the center of a spherically curved mirror surface and / or on a mirror of an end reflector arranged in the reference arm. The end reflector can be formed, for example, as a lateral concentric mirror arrangement or as a mirror arrangement in a rosette arrangement or as a mirror arrangement with statistically laterally distributed mirror areas. At a point WR in the reference arm R, the reference light beams B1 R and B2R have two different radii of curvature r1 and r2 of their respective wavefronts WF1 R and WF2R. In the dual-beam interferometer, a single end reflector with two laterally distributed spherically curved mirrors, each with different radii of curvature rS1 and rS2, can also be arranged. The osculating spheres of the two spherically curved mirrors always touch at a point of contact BP.

[0148] Figures 11 to 20, 22, 23 and 25 show further exemplary optical systems for spectral domain OCT or chromatic confocal spectral interferometry (CCSI). In these optical systems, there are three multispectral and spatially coherent light beams B1 , B2 and B3 each with different curvatures of the respective wavefronts WF1 , WF2 and WF3 in a point W at the output of a light source unit. Thus, at point W, there are three multispectral spatially coherent light beams B1 , B2 and B3 with three mutually different radii of curvature r1 pW, r2pW and r3pW of the wavefronts, wherein the three light beams have different centroid wavenumbers kS1 , kS2 and kS3. For this purpose, two beam splitting layers are arranged in the light source unit and in a detection unit, which can be designed as broadband or color splitter layers. The light beams B1 , B2 and B3 are coupled into a dual-beam interferometer with a reference arm and an object arm. A single end reflector with three laterally distributed spherically curved mirrors, each with a different radius of curvature rS1 , rS2 and rS3 is arranged in the reference arm of the dual-beam interferometer. The osculating spheres of the three spherically curved mirrors touch at a point of contact BP. Light spots (reference light spots) are formed in the reference arm of the dual-beam interferometer, which are positioned in the center of a spherically curved mirror and / or on a mirror of the end reflector. The end reflector can also be formed as a lateral concentric mirror arrangement or as a rosette arrangement or as a mirror arrangement with statistically laterally distributed mirror areas. At a point WR in the reference arm R, the reference light beams B1 R to B3R have three mutually different radii of curvature r1 , r2 and r3 of their respective wavefronts WF1 R to WF3R. Figure 24 shows another exemplary optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCSI). In this system, there are four multispectral and spatially coherent light beams B1 to B4 with different curvatures of the respective wavefronts WF1 to WF4 in a point W at the exit of a light source unit. The radii of curvature r1 pW to r4pW of the wavefronts are different from each other and each of the four light beams has a different centroid wavenumber kS1 to kS4. For this purpose, three broadband or color splitter layers are arranged in the light source unit and in a detection unit. The light beams B1 , B2, B3 and B4 are coupled into a dualbeam interferometer with a reference arm and an object arm. In the dual-beam interferometer, there is a single end reflector with three laterally distributed spherically curved mirrors, each with a different radius of curvature rS1 to rS3. The osculating spheres of the spherically curved mirrors touch each other at a point of contact BP. Light spots are formed in the reference arm of the dual-beam interferometer, which are positioned in the center of a spherically curved mirror and / or on a mirror of the end reflector. The one end reflector can be also formed as a lateral concentric mirror arrangement or as a mirror arrangement in rosette form or as a mirror arrangement with statistically laterally distributed mirror regions. At a point WR in the reference arm R, the reference light beams B1 R to B4R have four radii of curvature r1 , r2, r3 and r4 of the four wavefronts WF1 R to WF4R which are different from each other.

[0149] Figures 26 to 31 , 33 show still further exemplary optical systems for spectral domain OCT or chromatic confocal spectral interferometry (CCSI). In these optical systems, there are six multispectral and spatially coherent light beams B1 to B6 each with different curvature of the wavefront in a point W at the output of a light source unit. In other words, there are six spatially coherent light beams B1 to B6 with mutually different radii of curvature r1 pW, r2pW, r3pW, r4pW, r5pW and r6pW of the wavefronts WF1 to WF6. The light beams B1 to B6 are coupled into a dual-beam interferometer with a reference arm and an object arm. At a point WR in the reference arm R of the dual-beam interferometer, the reference light beams B1 R to B6R have six radii of curvature r1 , r2, r3, r4, r5 and r of the six wavefronts WF1 R to WF6R, which are different from each other. In an example, the first three light beams B1 , B2 and B3 have the centroid wavenumber kS1 and the second three light beams B4, B5 and B6 have the centroid wavenumber kS2. In another example (see e.g., Fig. 30), the six light beams B1 , B2 to B6 all have the same centroid wavenumber kS by using a broadband free-beam laser.

[0150] Further, in this optical system, two reflectors having three different curvatures rSL1 , rSL2 and rSL3 in their reflection field and respective beam splitting layers for optical coupling are employed in the light source unit and / or the detection unit. Each beam splitting layer is configured as a broadband or as a color splitter layer. The two reflectors can be, for example, configured as concentric mirror arrangements or as mirror arrangements in rosette arrangements or as mirror arrangements with statistically distributed mirror areas.

[0151] Figure 32 shows yet another exemplary optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCSI). In this system, two end reflectors are arranged in the interferometer (not to be confused with the reflectors of the light source unit and the detection unit). The system comprises a dual-beam interferometer with a split reference arm having a first sub-arm (first partial reference arm) R1 and a second sub-arm (second partial reference arm) R2. One end reflector is formed in the reference sub-arm R1 and one in the reference arm R2. Light travelling to and from the end reflectors in the two reference sub-arms is optically coupled via a beam splitting layer, which is formed as a broadband or as a color splitter layer. The first end reflector is formed with three mirrors having the curvatures rS1 , rS2 and rS3. The second end reflector is formed with three mirrors having the curvatures rS4, rS5 and rS6. The two end reflectors in the reference arm can also be formed, for example, as a concentric mirror arrangement or as a mirror arrangement in a rosette form or as a mirror arrangement with statistically distributed mirror areas. The two end reflectors are arranged at the same optical depth to ensure equal optical paths. The contact point BP1 of the osculating sphere on the first end reflector and the contact point BP2 of the osculating sphere on the second end reflector are optically conjugated. Both contact points BP1 and BP2 lie on a circle K12 which has its center TR on the beam splitting layer and on the optical axes OAR1 and OAR2. Figure 32 shows yet another exemplary optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCSI).

[0152] More specifically:

[0153] Figure 1 shows schematically the optical setup of an exemplary centered optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCSI) with two collinear z-ranges dzc1_H and dzc2_H of light spots chromatically split in the depth of the object arm 0.

[0154] Two chromatically depth-split light spots S10k1 ' and S20k2' of the respective z-ranges or stretches of light spots are formed by respective multispectral light beams of light having centroid wavenumbers kS1 and kS2, respectively. The multispectral light beams are imaged onto the measured object 8, as shown in detail 1. 3 of Figure 1 , to form diffraction limited light chromatically depth split light spots S10k1 ' and S20k2', respectively. Each chromatically depth-split light spot S10k1 ', S20k2' is represented by a respective region dzc1_H, dzc2_H extending in a depth direction (i.e., in a direction substantially parallel to the direction of optical axis of the object arm 0 of the interferometer), each region dzc1_H, dzc2_H corresponding to the area over which the respective chromatically depth-split light spot stretches or extends. The coverage range UE in the object arm 0 determines the depth measurement range of the arrangement for the measured object 8.

[0155] The system for chromatic-confocal spectral-domain OCT or for chromatic-confocal spectral interferometry (CCSI) comprises a multispectral light source unit 100 having an optical axis OA, arranged upstream of a dual-beam interferometer 6. The dualbeam interferometer shown in Figure 1 is a chromatic-confocal Michelson interferometer with a diffractive-optical element 70 in the object arm 0 and a non-zero optical path difference for a measured object 8 which is positioned in the measurement range and on which chromatic depth-split light spots are incident, of which at least two light spots are sharply focused in a current measurement point, namely the chromatically depth-split light spots S1 Ok1 ' and S20k2'.

[0156] The multispectral light source unit 100 shown in Fig. 1 comprises an achromatic focusing unit (not shown in Fig. 1 ) and is configured to generate two at least approximately spatially coherent light beams B1 and B2 having wavefronts WF1 and WF2, respectively at its output. The multispectral light source unit 100 comprises two primary light sources (not shown in Fig. 1 ). The primary light sources may, for example, be two broadband superluminescent diodes (broadlighters) D-810-HP and M-D-890- HP1 of the company SUPERLUM emitting light in different spectral ranges. With the help of suitable optics, two spatially coherent light beams B1 and B2 are formed from the light emitted from the two broadband superluminescent diodes as described in connection with Figure 2. The main rays SB1 and SB2 of the spatially coherent light beams B1 and B2 coincide on the optical axis OAS at the output of the multispectral light source unit 100. The light of the spatially coherent light beams B1 and B2 is aimed at different light spots in depth, as described herein.

[0157] The wavefronts WF1 and WF2 have different radii of curvature r1 pW and r2pW in the paraxial region around the optical axis OAS, respectively. More specifically, the wavefronts WF1 and WF2 of the two spatially coherent light beams B1 and B2, viewed in the immediate, i.e. , paraxial vicinity of any point W lying on the optical axis OAS at the output of the light source unit 100 and positioned upstream of the chromatic- confocal Michelson interferometer s (i.e., before the input El of the chromatic confocal interferometer) exhibit different radii of curvature r1 pW and r2pW. The radii of curvature r1 pW and r2pW may differ significantly, for example by 3pm to 1 mm, more specifically by 10pm to 500pm, further specifically by 20pm to 300pm. The difference of the radii of curvature may depend on the centroid wavelength of the employed light and / or may be application-specific. In an example, the difference between the two radii of curvature r1 pW and r2pW in the paraxial vicinity of the point W may be 160 micrometers.

[0158] Further, the centroid wavenumber kS1 and the centroid wavenumber kS2 of the spectra of the spatially coherent light beams B1 and B2, respectively derived from the centroid wavelengths lambda_S1 = 890 nm and lambda_S2=810 nm of the two broadband superluminescent diodes mentioned above, are different from each other. This results in a spectral range delta_Lambda of 200 nm within the half-widths for the above-mentioned light sources.

[0159] For the respective centroid wavenumbers, it holds true that kS1 is smaller than kS2. The centroid wavenumber kS1 of the spatially coherent light beam B1 is determined from the centroid wavelength lambda_S1 of 890 nm according to equation 1 : kS1 = 2Pi / lambda_S1 = 2Pi / 890 nm = 7.06 gm”1(1 )

[0160] The centroid wavenumber kS2 of the spatially coherent light beam B2 is determined from the centroid wavelength lambda_S2 of 810 nm in the same way according to equation 2: kS2 = 2Pi / lambda_S2 = 2Pi / 810 nm = 7.76 gm”1. (2)

[0161] The half-width (i.e. , the full width at half maximum) HWB of the spectrum of the light beam B1 , specified in the wavelength range, is HWB1 =140 nm and the half -width of the spectrum of the light beam B2 is HWB2=100 nm. The spectra of the light of the two spatially coherent light beams B1 and B2 are thus clearly different from each other, as symbolically shown in details 1.1 and 1.2 of Figure 1 and have clearly different centroid wavenumbers kS1 and kS2 with the difference delta_kS12. Thereby in Figure 1 , detail 1.1 the centroid wavenumber kS1 with 7.06 / micrometer ( / zm-1) is the smaller centroid wavenumber.

[0162] Furthermore, in the light source unit 100, there is a partial overlap of the spectral regions of the light of the multispectral light beams B1 and B2. Thereby, it is known that the centroid position of a spectrum may somewhat differ depending on whether the calculation is performed in the wavenumber domain or in the wavelength domain. However, such difference is only minimal and, therefore, neglectable.

[0163] The light of the multispectral, spatially coherent light beams B1 and B2 enters the chromatic-confocal Michelson interferometers via the input El and meets there a beam splitting layer 60. The beam splitting layer 60 spits each of the light beams B1 and B2 thereby forming two at least approximately spatially coherent reference light beams B1 R and B2R in the reference arm R and (immediately after the beam splitting layer 60) two at least approximately spatially coherent object light beams B1 O and B2O in the object arm 0. The beam splitting layer 60 may be a part of a beam splitting unit (not shown in Figure 1 for the sake of simplicity), such as a glass beam splitter cube. The beam splitting unit may be configured such that path lengths in the beam splitting unit (such a as a beam splitter cube) for the split light, are at least approximately the same in the reference arm R and in the object arm O. Existing path length difference in the beam splitting unit may be compensated for, for example into the optical system of the light source unit 100 for forming the spatially coherent light beams B1 and B2, so that diffraction-limited light spots can be formed in the reference arm R.

[0164] Further, the light source unit 100 may be configured such as to compensate for a path difference for the two light beams B1 and B1 introduced by one or more optical elements arranged in the reference arm R and / or the object arm O. For example, the optics of the light source unit 100 may be configured to compensate for a path difference introduced by a diffractive-optical element(s) (DOE) 70 with positive power for chromatic splitting in the object art O of the interferometer 6. In addition or alternatively, the light source unit 100 may be configured such as to compensate for an optical path difference due a finite substrate thickness of optical layers or elements, such as for example the substrate thickness of a micro-profiled diffractive-optical element (DOE) 70 with positive focusing power and / or the glass thickness of a compensation plate 704 made of glass in the reference arm R.

[0165] Thus, for the multispectral, spatially coherent and focused reference light beams B1 R and B2R, the optical system may be configured such that there is at least approximately no longitudinal chromatic aberration in the light spots SI R'and S2R' in the reference arm R and the imaging of the light spots SI R'and S2R' is at least approximately diffraction limited. This may be realized, for example, by configuring the optical system such that the two wavefronts of the reference light beams B1 R and B1 R after the compensation plate 704 are at least approximately spherical (in the sense of spatial coherence due to the aberration correction of the entire imaging path). For example, a spherical aberration that occurs - at the beam splitting unit (such as for example a beam splitter cube) and at the compensation plate 704, which are not shown in detail in Fig. 1 - may be already compensated for (pre-compensated) in the optical system of the light source unit 100.

[0166] Accordingly, the wavefronts WF1 and WF2 in the non-paraxial vicinity of the point W in front of the chromatic-confocal Michelson interferometer 6 need not to be strictly spherical, but may exhibit some (small) level of asphericity.

[0167] The reference light beams B1 R and B1 R have radii of curvature r1 and r2 in the immediate, i.e., paraxial, vicinity of point WR located after compensation plate 704. Due to the already described aberration correction, including aberration correction for multispectral light, the light spots S1 R' and S2R' formed from the spatially coherent reference light beams B1 R and B1 R in the reference arm R can be regarded as multispectral, diffraction-limited light spots without longitudinal chromatic aberration. Such multispectral light spots may also be generally referred to as "white light spots", without a limitation that spectrum should correspond to that of a white light. In the example described in connection with Figure 1 the existing spectrum has a spectral width of approximately 300 nm, that is lower than the spectral width of the white light.

[0168] A prerequisite for the formation of at least approximately diffraction limited "white light spots", i.e., of multispectral light spots, is that for the entire imaging path, i.e., from the light generation in the light source unit 100, through the optical components of the light source 100 and via the beam splitting to the light spots S1 R' and S2R' in the reference arm R, the optical system is configured such that the spherical and sphero-chromatic aberrations and longitudinal chromatic aberrations are minimized. By minimizing the aberrations, it is possible to advantageously realize at least approximately diffractionlimited imaging of the two multispectral light spots SI R'and S2R' in the reference arm.

[0169] The first light spot S1 R' from the spatially coherent reference light beam B1 R is aimed at the center M1 of a convex spherically curved mirror 72 of an end reflector 711 in the reference arm R. The convex spherically curved mirror 72 is centered on the optical axis OAR. In other words, the spatially coherent reference light beam B1 R is imaged at the center M1 , said image forming the first light spot S1 R’. The convex spherically curved mirror 72 may be miniaturized convex, spherically curved mirror 72. In an example, the radius of a miniaturized mirror 72 may be 40 micrometers. In another example, the radius of a miniaturized mirror may be 80 micrometers. In a further example, the radius of a miniaturized mirror may be 160 micrometers. These value are exemplary and are especially suitable for visible or near infrared light.

[0170] The corresponding main ray SB1 R of the reference light beam B1 R impinges perpendicularly on the centered, convex, spherically curved convex mirror 72 at the point of incidence AR. Furthermore, all rays of the light beam B1 R, or respectively the normal of the wavefront of the light beam B1 R, thus strike the convex, spherically curved convex mirror 72 perpendicularly and are therefore reflected back into themselves.

[0171] The second light spot S2R' formed by the spatially coherent reference light beam B2R, which also passes the compensation plate 704, hits the surface of the convex, spherically curved mirror 72 at the point of incidence AR on the optical axis OAR, so that the corresponding main ray SRB2 also hits the point of incidence AR. The imaging of the light spot S2R' occurs substantially without longitudinal chromatic aberrations and also without any further spherical aberrations.

[0172] The radius rS of the convex, spherically curved mirror 72 is selected such that it is equal to the difference rS of the radii of curvature r1 and r2 (in this example rs=160 micrometers). In the example shown in Figure 1 , the numerical aperture NA, which is given by the light source unit, is 0.11 for the light beam B1 R and is only slightly different from the numerical aperture of the reference light beam B2R. The respective halfaperture angle alpha_1 of the reference light beam B1 R corresponding to the numerical aperture NA may be determined by the following equation 3, In case all optical components in the reference arm R are in air: NA=sin(alpha_1 ) (3)

[0173] For NA = 0.11 , the half aperture angle alpha_1 of 6.32°. For the centroid wavelength lambda_S1 of the light of the first light beam B1 with lambda_S1 =890 nm, this results in a wave-optical depth of focus dRS1 , derived from the Rayleigh criterion, in the reference arm R in air, that may be determined by equation 4: dRS1 = lambdaS1 / [2*square (NA)] (4)

[0174] In the specific example, dRS1 = 36.78 micrometers.

[0175] The respective wave-optical depth of focus range, which may be determined by 2dRS1 is equal to 73.55 micrometers. The depth of focus range 2dRS1 may be enlarged by at least a factor of 2 by the arrangement shown in Figure 1 , which will be described in the following.

[0176] The light of the two reference light beams B1 R and B2R propagates after reflection at the convex, spherically curved mirror 72 back in the direction of the beam splitting layer 60. For the two main rays SB1 R and SB2R, the optical path length in the reference arm R is exactly the same for light of a given wavelength. This is due to the fact that the main rays SB1 R and SB2R impinge together in the same point AR and are reflected back from the same point AR in the direction of the beam splitting layer 60. Since the main rays SB1 R and SB2R are a part of respective light beams with at least approximately spherically curved wavefronts, the optical path length in the reference arm for each light ray of the light beams B1 R and B2R is also at least approximately the same for one light wavelength. Even if the longitudinal chromatic aberrations are corrected, the optical path length in the reference arm R is still generally wavelengthdependent, due to the presence of refractive materials in the reference arm R which generally cause dispersion. An example of such refractive material that may cause dispersion is a compensation plate 704 arranged in the reference arm R that compensates for the thickness of a glass substrate of the diffractive-optical element 70 arranged in the object arm 0 (for carrying out depth splitting). A dispersion may also occur in case the optical thickness of the compensation plate 704 is compensated for in a beam splitter and / or the compensation plate 704 is not used as a separate optical element.

[0177] The optical system may be configured such that the dispersion due to the refractive materials used and their thicknesses in the reference arm R is at least approximately equal to the dispersion due to the refractive materials and their thicknesses in the object arm 0, so that there is at least an approximate compensation of the dispersion for the light coming from the two interferometer arms R and 0 during the detection.

[0178] Thus, although there may be a dispersion in both interferometer arms R and 0 (which is undesirable in itself but usually technically unavoidable), the dispersion is made at least approximately equal in both interferometer arms R and 0. Ultimately, the dispersion is typically hardly avoidable, also in case diffractive elements are used for depth splitting in the object arm 0, since such diffractive elements are typically formed on substrates of refractive material with a certain thickness, in order to ensure mechanical stability.

[0179] For example, the diffractive-optical element 70 in the object arm 0, see detail 1.3 of Figure 1 , has a substrate of glass with a certain thickness for reasons of its mechanical stability. The dispersion of the substrate of the diffractive-optical element 70 may be at least partially compensated for by employing for example the compensation plate 704 arranged in the reference arm R. Thus, the dispersion in the two interferometer arms R and 0 is compensated or at least the difference in dispersion in the two interferometer arms R and is made so small that its influence on the resulting detected optical signals (and the resulting measurement errors due to imperfect compensation of the dispersion during the measurement) can be neglected or can be corrected numerically. The measurement errors can arise in particular if, due to an uncompensated dispersion, the wavelets W1 and W2, shown in detail 1.7 of Fig. 1 are not in phase, but are for example in antiphase. In the object arm 0, the spatially coherent object light beams B10 and B20 both pass a diffractive-optical element 70 (such as a monofocal diffractive-optical element) with a positive focusing power. The diffractive-optical element 70 may be formed as a diffractive zone lens with positive focusing power, which is located at a certain distance from the measured object. In the example shown in Figure 1 , the diffractive-optical element 70 may be arranged approximately at a distance of 15 mm from the measured object 8. The diffractive-optical element 70 may be optimized for the first diffraction order for a wavelength that is approximately the average of the two centroid wavelengths of the light of the light beams B1 and B2. In the example shown in Figure 1 , the average wavelength for which the first diffraction order of the optical element 70 is optimized is around 850 nm.

[0180] The diffractive-optical element 70 has a substrate made of glass, wherein the back side of the substrate, i.e. , the side opposite to the side on which the diffractive pattern is formed, is weakly concavely curved. For example, the vertical deviation from a plane in the optically used area of a “weakly curved mirror” can be up to a tenth of the diameter of the used area of the back side of the substrate of the mirror.The center thickness of the substrate of the diffractive-optical element 70 made of glass may be advantageously compensated for in the optical system.

[0181] The focal length of the zone lens with positive focusing power, which represents the diffractive-optical element 70, may be set appropriately. In the example shown in Figure 1 the focal length is f'c890 = 225 mm for the centroid wavelength lambda_S1 of 890nm. Due to the concavely curved surface of the back side of the substrate of the diffractive zone lens 70, a partial compensation of the power of the diffractive-optical zone lens takes place. The compensation is incomplete, i.e., is partial, since in case of a complete compensation the frequency zero may occur in a spectral wavelet W1 or W2, which could complicate the signal evaluation. The partial compensation of the focusing power of the diffractive-optical element 70 may be employed, in order to limit the maximum optical path difference x_max occurring in the chromatic-confocal Michelson interferometer 6. The path difference x_max results in partiuclar from the depth position of the target 8 in the overlap area UE, where a sharp imaging of a measuring point of the target 8 takes place, as shown in detail 1.4 of Figure 1 .

[0182] Generally, a scanning of the resulting spectral wavelets W1 and W2 with a line sensor in a single-channel spectrometer 28 or in a spectrometer 29 (e.g., a spectrometer with a fast line sensor with e.g., 2000 sensor pixels), respectively, can be performed in very reliable compliance with the scanning theorem, if the optical path difference in the dualbeam interferometer does not exceed 0.5 mm. For this purpose, the spectral range of the single-channel spectrometer 28 or the spectrometer 29 may be designed for 700 nm to 1000 nm.

[0183] For the resulting optical path difference in the chromatic-confocal Michelson interferometer 6, only the range in which sharp light spots are formed on a measured object 8 in the object arm O need to be considered. Thus, comparatively low-frequency spectral wavelets W1 and W2 are formed during the detection of the interfering light, as shown in detail 1.7 of Figure 1. The evaluation of such wavelets is typically technically unproblematic with known line detectors with e.g. 2000 sampling points. Furthermore, the fine structure of the diffractive-optical element 70 may be configured such that residual spherical aberrations in the beam path, which may be caused for example by the substrate of the diffractive-optical element 70, are almost completely eliminated, which the skilled person can take into account in the design of the diffractive-optical element 70. Thus, an at least approximately diffraction -limited formation of chromatically split light spots in the object arm O for the addressed spectrum is realized.

[0184] The diffractive-optical element 70 is configured to diffract an incident convergent light beam of a strictly monochromatic light with a specific wavenumber / wavelength in a predetermined diffraction order (e.g., the first diffraction order) to thereby generate a single light spot from the diffracted light. In other words, a single light spot is formed by the diffractive-optical element 70 from an incident convergent light beam of monochromatic light of one specific wavenumber / wavelength.

[0185] Since the object light beams B1 O and B2O are multispectral, a plurality of light spots is formed with the help of the diffractive-optical element 70, each light spot corresponding to a wavenum ber / wavelength from the range of wavenum bers / wavelengths included in the respective multispectral light beam B1 O and B2O. In other words, the diffractive-optical element 70 forms a plurality of light spots for the multispectral light with the relatively broad spectrum of wavenum bers / wavelengths of the first object light beam B1 O and a plurality of light spots for the multispectral light having the spectrum of the wavenum bers / wavelengths of the second object light beam B2O.

[0186] The multispectral light of the focused object light beam B1 O thus undergoes a chromatic depth splitting of light spots over a z-range dzc1_H by the diffractive-optical element 70 in the object arm 0. This is realized by means of the object bundle OB_d diffracted in the first diffraction order.

[0187] In the depth direction z (i.e. , in the direction parallel to the optical axis of the object arm 0), a stretch or range of chromatic split of light spots dzc1_H results from the difference of the positions of the light spots S1 OHu' and S1 OHo' at the wavenumbers k1 Hu and k1 Ho of the spectrum of the first object light beam B10. The z-range dzc1_H may be defined as the depth separation in the object arm 0 of the two light spots S1 OHu' and S1 OHo' formed from light with wavenumbers k1 Hu and k1 Ho, respectively. The wavenumber k1 Hu denotes the lower wavenumber at the half value of the light intensity in the multispectral, spatially coherent light beam B1 , i.e. the wavenumber at the lower value of the half width HWB1 . The wavenumber k1 Ho denotes the upper wavenumber at the half value of light intensity in the multispectral, spatially coherent light beam B1 , i.e. the wavenumber at the upper value of half width HWB1 . The range dzc1_H is thus also dependent on the half-value width HWB1 of the light spectrum of the object light bundle B10, see in addition also detail 1.4 of Figure 1 . In the example shown in Figure 1 , the half-value width of the spectrum in the wavelength range of the light beam B1 is 140 nm according to manufacturer’s specifications. The z-range dzc1_H increases with increasing half-width HWB1.

[0188] The multispectral light of the convergent object light beam B2O also undergoes chromatic depth splitting of light spots over a z-range dzc2_H by the diffractive-optical element (DOE) 70 in the object arm. The z-range dzc2_H results from the difference in the positions of the light spots S2OHu' and S2OHo' at the wavenumbers k2Hu and k2Ho, wherein the wavenumber k2Hu denotes the lower wavenumber at the half value of the light intensity in the multispectral, spatially coherent light beam B2, i.e. the wavenumber at the lower value of the half width HWB1 , and the wavenumber k2Ho denotes the upper wavenumber at the half value of light intensity in the multispectral, spatially coherent light beam B2, i.e. the wavenumber at the upper value of half width HWB2. The z-range dzc2_H is dependent on the half-width HWB2 of the light spectrum of the object light beam B2O. In the example shown in Figure 1 , the half-width of the spectrum in the wavelength range of the light beam B2 is 100 nm according to the manufacturer's specifications. The range dzc2_H increases here with increasing halfwidth HWB2.

[0189] For the sake of better visualization, the z-ranges dzc1_H and dzc2_H of the chromatic depth splitting are drawn in Figure 1 out of scale (much too large). This also applies to the representation of the z-ranges of the chromatic depth splitting in all subsequent figures.

[0190] Without restriction to generality, the multispectral light of the convergent object light beam B1 O in the example shown in Figure 1 is generally of longer wavelengths than that of the convergent object light beam B2O and the half-width HWB2 is smaller than the half-width HWB1. Thus, the splitting effect of the diffractive-optical element 70 in the first diffraction order results in a smaller z-range dzc2_H as compared to the z- range dzc1_H. In the example shown in Figure 1 , the optical system with the diffractive- optical element 70 and the spectra of the light of the two object light beams B1 O and B2O are designed such that the larger z-range dzc1_H completely overlaps the smaller z-range dzc2_H. Thus, the smaller z-range dzc2_H corresponds to the overlap range UE in which a target 8 can be metrologically measured, see detail 1.4 of Figure 1. Other arrangements are also possible (such as partially overlapping dzc1_H and dzc2_H). In the overlap range UE there are pairs of coinciding light spots, each pair consisting of a chromatically split light spot of light of the object light beam B1 O and a chromatically split light spot of light of the object light beam B2O. However, the light of the coinciding light spots has always wavenumber k1 and k2, respectively, which are different from each other. For example, a pair of light spots S10k1 '-S20k2' exists in a single point cP012 on the measured object 8.

[0191] Ideally, the light spots S1 OkS1 ' and S20kS2' of the centroid wavenumbers kS1 and kS2 coincide in the confocal point cPS, see detail 1.4 of Figure 1 . In reality, as shown in detail 1.5 of Figure 1 , there is a small depth distance t120kS1 kS2' between the light spots S1 OkS1 ' and S20kS2' of the centroid wavenumbers kS1 and kS2, which determines the depth range dtc in which light spots with their centroid wavenumbers are located. Thus, in reality there is no “confocal point cPS”, due to the fact not always light sources whose centroid wavenumbers kS1 and kS2 exactly match the microprofiled diffractive-optical element 70 are available. In extreme cases, the depth range dtc can even be up to 50% of the coverage range UE. However, with technically good design and matching of the diffractive-optical element 70 to the centroid wavenumbers kS1 and kS2, the depth range dtc may be in the lower single-digit percentage range of the coverage region UE.

[0192] In order to be able to measure a target 8 with respect to its depth position in a point, the target 8 is arranged within the coverage region UE. Then exactly one pair of light spots S10k1 '-S20k2' with the wavenumber k1 for the light spot S10k1 ' and with the wavenumber k2 for the light spot S20k2' is sharply focused on this measured object 8 in the point cPO12 (current measurement point). The light with the wavenumber k1 originates from the object light beam B1 O and the light with the wavenumber k2 originates from the object light beam B2O. Thereby, the wavenumber k1 and the wavenumber k2 are within the spectral bandwidth HWB1 and HWB2 of their light sources.

[0193] From the at least approximately diffraction-limited light spots S1 Ok1 'and S20k2' an at least approximately spatially coherent light beam is reflected at the measured object 8 with sharp imaging of the same. The two reflected and spatially coherent light beams with the wavenumbers k1 and k2 are then coupled out of the dual-beam interferometer 6 via the beam splitting layer 60 in the direction of the detection unit 16 by reflection and thereby focused with the objective 79, so that sharply focused light spots S10k1" and S20k2" are formed after the objective 79. The light spot S1 Ok1 " is thereby sharply focused on the confocal aperture 261 and the light spot S20k2' is thereby sharply focused on the confocal aperture 262. The confocal apertures 261 and 262 are each arranged at a different optical depth, as shown in detail 1.6 of Figure 1 .

[0194] The detection unit 16 comprises the focusing objective 79, a beam splitter 20, downstream confocal apertures 261 and 262, a single-channel spectrometer 28 and a spectrometer 29 for detecting the confocally discriminated and interfering light from the reference arm R and the object arm 0, respectively.

[0195] The focusing objective 79 is arranged downstream of the chromatic-confocal dualbeam interferometer and forms microscopically fine light spots of interfering light on one or more confocal apertures in each case. The focusing objective 79 enables at least approximately diffraction-limited imaging of light spots in the detection beam path of multispectral light beams without chromatic depth aberrations, since these are already retracted in the return path of the light, during confocal discrimination. The optical design of the focusing objective 79 may also take into account any existing glass path lengths in the reference arm R and object arm 0.

[0196] The light is split on its way to the confocal apertures 261 and 262 by a beam splitting layer 20 (e.g., of a glass beam splitter cube not shown in Figure 1 ). Confocal discrimination takes place at each of the confocal apertures 261 and 262, respectively, and the transmitted, confocally discriminated light that originates from the light beam B1 enters the single-channel spectrometer 28 and the transmitted, confocally discriminated light that originates from the light beam B2 enters the single-channel spectrometer 29. In the process, light from the light beam B1 , which is in the spectral vicinity of the wavenumber k1 also partially passes through the confocal aperture 261 . Similarly, light from the light beam B2 in the spectral vicinity of the wavenumber k2 also passes through the confocal aperture 262. Thus, somewhat extended wavelets are formed in the spectral domain.

[0197] The single-channel spectrometer and the spectrometer 29 are arranged downstream of the confocal apertures 261 and 262. The single-channel spectrometers 28 and 29 each comprise a line scan camera, for example a line scan camera having about 2000 sampling points, represented by respective sensor pixels. Due to the power compensation of the diffractive-optical element 70 with positive power by the concavely curved substrate back surface 703 of the diffractive-optical element 70 with negative focusing power, a maximum occurring optical path difference x_max in the chromatic- confocal Michelson interferometer 6 of less than 0.53 mm may be realized in a spectral range of 700 nm to 1000 nm in the wavelength range. Thus, signal detection in compliance with the sampling theorem is possible with of the line scan cameras having about 2000 sampling points.

[0198] In the optical system shown in Figure 1 , the optical path lengths of the main rays SB10 and SB2O of the object light beams B1 O and B2O in the object arm O with the light spots S1 Ok1 'and S20k2' on the measured object 8 are smaller than the optical path lengths of the main rays SB1 R and SB2R in the reference arm R due to the focusing effect of the diffractive-optical element 70 with positive focusing power and the undercompensation of the concavely curved substrate back surface 703 of the diffractive-optical element 70 (which are exactly the same). Thus, an optical path difference xp exists in the chromatic-confocal Michelson interferometer 6, which is at least approximately the same for all wavelengths due to the precise compensation of dispersion in the chromatic-confocal Michelson interferometer 6, even though the optical path of light of different wavelengths may be different in each case due to the dispersion of the optical materials used in the chromatic-confocal Michelson interferometer 6. The optical path difference xp is twice the distance zp, which is the distance from the point cPO12 on the measured object 8 to a point AO, and thus depends at least approximately on the position of the target 8 in depth alone. The point AO is a point in the object arm O on the optical axis OAO for which the optical path difference is zero. At the point AO z=0 is valid. The optical path difference xp may be set - as already mentioned - at a maximum of 0.53 millimeters to enable the evaluation of the spectral wavelets W1 and W2 with the spectrometers 28 and 29 for the near-infrared spectral range from 700 nm to 1000 nm with about 2000 spectral sampling points. The magnitude of the optical path difference xp results from the interaction of the diffractive-optical element 70 with positive focusing power and the concavely curved substrate back surface 703 of the diffractive- optical element 70. As shown in details 1.3 and 1.4 of Figure 1 , the concavely curved substrate rear surface 703 with negative focusing power partially compensates for the positive focusing power of the diffractive-optical element 70, but not completely, so that for all positions of a measured object 8 in the depth range dzc2_H the magnitude of the optical path difference xp is always different from zero and is, for example, of the order of half a millimeter. In other words, there is an undercompensation of the focusing power for the centroid wavelength lambda_S1 of the diffractive-optical element 70 by the concavely curved substrate back surface 703 with negative focusing power shown in detail 1.4 of Figure 1 .

[0199] Thus, for near-infrared light, when single-channel spectrometers 28 and 29 are used, the scanning theorem can be reliably satisfied for the near-infrared spectral range from 700 nm to 1000 nm. The term "single channel" is used within the context of the present description to mean that only one spectrum with a plurality of spectral elements is recorded at a time.

[0200] As described above, due to the introduced undercompensation of the focusing power for the centroid wavelength lambda_S1 of the diffractive-optical element 70, there is always a non-zero optical path difference in the chromatic-confocal Michelson interferometer 6 with a diffractive-optical element for all depth positions of the target 8 within the coverage region UE. This allows an unambiguous evaluation of the depth position of the target 8 within the overlap region UE, since in this case none of the wavelets W1 and W2 has zero frequency in the spectral domain.

[0201] The light reflected from the reference light beams B1 R and B2R at the convex mirror 72 passes through the beam splitting layer 60 in transmission and is focused by the objective 79 so that the sharply focused multispectral light spots S1 R" and S2R" are formed after passing through the beam splitter 20. Thereby, the light spot S1 R" is sharply imaged onto the confocal aperture 261 by means of the light transmitting the beam splitting layer 20. In contrast, the light spot S2R" is sharply imaged onto the confocal aperture 262 by means of the light reflected from the beam splitting layer 20. Thus, both the light from the light spots S10k1" and S1 R" and the light from the light spots S20k2" and S2R" are superimposed during confocal discrimination, and interference of light occurs. In addition, however, some of the light in the spectral vicinity of the wavenumbers k1 and k2, each of which has only a weak defocus, still partially passes through the confocal discriminators 261 and 262, which are designed here as confocal apertures. The superimposed light of the focused light spot S10k1" and the weakly defocused light at the light spot S1 Ok1 ", which is in the spectral vicinity of the wavenumber k1 , and the multispectral light of the light spot S1 R" arrive for detection in the single-channel spectrometer 28. The superimposed light of the focused light spot S20k2" and the weakly defocused light at the light spot S20k2", which is in the spectral neighborhood of the wavenumber k2, and the multispectral light of the light spot S2R" arrive for detection in the single-channel spectrometer 29.

[0202] Due to the interference capability of the superimposed light and the existing optical path difference xp, two comparatively low-frequency spectral wavelets W1 and W2 are generated by interference, as shown in detail 1.7 of Figure 1. The light of light beam B1 forms the basis for the generation of wavelet W1 and the light of light beam B2 enables the generation of wavelet W2. The wavelets W1 and W2 are at least approximately in phase due to the nearly perfect compensation of dispersion in the chromatic-confocal Michelson interferometer 6 (which is the goal in the design of the optical system). It is, however, advantageous to use only the middle regions bm1 and bm2 of the respective wavelets W1 and W2 and not the edge regions of the wavelets W1 and W2 for computational evaluation. In particular, advantageously, the computational evaluation may be performed in regions where the phase response is at least approximately rectilinear, in order to minimize measurement errors. The wavelets W1 and W2 may be numerically brought together to a common spectral wavenumber axis in quasi-real time by a computational unit 30. The computational unit is configured to combine and optionally display two or more spectra, for example spectral wavelets, on a single k-axis of wavenumbers.

[0203] Based on the resulting data, a depth value zp for a measurement point cPO12 on the measured object 8 may be numerically determined by a depth position computational unit 31 . For this purpose, methods of the fast Fourier transform may be used, for example. An exemplary evaluation of two wavelets in phase has been described for example in WO 2019 / 120470 A1 and US 11 248 900 B2.

[0204] By the provision of the above system and measurement method, it is possible to enlarge the optical data massif by providing more than one wavelet, preferably in phase.

[0205] It shall be emphasized that in the schematic representation according to Figure 1 the light spots S1 OkS1 'and S20kS2' with the centroid wavelengths kS1 and kS2 of the two point light sources coincide at least approximately in a point cPS (see detail 1.4). However, this corresponds to an aimed for, ideal case, which may not always be achievable in reality, as shown in detail 1.5 of Figure 1 . In the idealized case according to detail 1.4, there is a complete coverage of the path dzc2_H by the path dzc1_H and there is a coverage region UE.

[0206] Various modifications of the above-described example are possible.

[0207] For example, a first modification of the above example (without figure) the interference light can also be optically merged after the confocal apertures 261 and 262, which in this first modification may be formed by single-mode fibers. The merged interference light may then enter a downstream evaluation Michelson interferometer with a slightly tilted plane mirror, so that a tilted-wave interferometer arrangement may be realized. Thus, a spatial short-coherence interferogram is formed. The spatial short-coherence interferogram may be reliably acquired with a fast line detector having for example 2000 sensor pixels and can be evaluated in real time with well-known methods for evaluating white-light interferograms. Thus, a highly accurate depth position of a measurement point of a measured object 8 (target) arranged in the object arm 0 can be obtained.

[0208] To be noted is, that in this case, the evaluation is not performed via two wavelets in the spectral domain, but via a spatial single-shot dual-beam interferogram in x-space, i.e. via the optical path difference. This is shown in details 16.1 and 16.2 of Figure 16. The phase information of the spatial short coherence interferogram can also be evaluated, which can provide a resolution for the depth position of the measured object 8 in a measurement point down to the single-digit nanometer range.

[0209] Figure 2 shows schematically the optical setup of an exemplary multispectral light source unit 100 and an unfolded reference arm R of a dual-beam interferometer, which may be used in the optical arrangement shown in Figure 1 .

[0210] The light source unit 100 comprises two point light sources 110 and 120, a coupling beam splitter 19 for beam unification of light from the point light sources 110 and 120, a collimator lens 51 and an objective lens 52. The collimator lens 51 and the objective lens constitute an achromatic focusing unit (also referred to as achromatic focusing optics).

[0211] The point light source 110 is a multispectral first point light source exhibiting a primary light emitting spot S1 o and emitting multispectral light with a spectrum having a centroid wavenumber kS1 and spectral half-width HWB1 in the wavenumber domain. In the example shown in Figure 2, the point light source comprises a continuum emitter and a single-mode fiber with a single fiber output. The position of the single-mode fiber output is finely adjustable in the three spatial coordinates by associated adjusting device (not shown in Figure 2).

[0212] In the examples shown in Figures 1 to 3, the centroid wavelength lambda_S1 of the emitted light is 890 nm, corresponding to a centroid wavenumber kS1 of 7.06 / zm-1, and the spectral half-width is 140 nm. However, this is only exemplary and the multispectral first point light source 110 can be configured to emit light with a different centroid wavelength lambda_S1 and / or different spectral half-widths.

[0213] The second multispectral point light source 120 is a multispectral light source exhibiting a primary light emitting spot S2o and emitting multispectral light having a spectrum with a centroid wavenumber kS2 and a spectral half-width HWB2 in the wavenumber domain. The centroid wavenumber kS2 (and the respective wavelength lambda_S2) of the second multispectral point light source 120 differ from the centroid wavelength kS1 (and the respective wavelength lambda_S1 ) of the first multispectral light source 110.

[0214] The second point light source 120 may comprise a continuum emitter and a singlemode fiber (light emitting fiber) having a single-mode fiber output. The position of the fiber output may be finely adjustable in the three spatial coordinates by associated adjustment device. In an example, the centroid wavelength lambda_S2 of the emitted light may be 810nm, corresponding to a centroid wavenumber kS2 of 7.76 / zm-1. The spectral half-width may be 100 nm. The multispectral second point light source 120 can be configured for other centroid wavenumbers kS2 and / or spectral half-widths.

[0215] In both the first and the second point light sources 110 and 120, the light emitted from the continuum emitter is coupled into a first end of the respective single-mode fiber and is emitted from a second end of the single-mode fiber. Thus, two (microscopic) multispectral light emitting light spots S1o and S2o are formed at the second end of the respective single-mode light emitting fiber, as shown in detail 2.1 of Figure 2 for the point light source 110 (the point light source 120 has the same construction).

[0216] The single-mode light-emitting fiber of the point light source 110 may have, for example a fiber core diameter DS1 of 5 micrometers. The single-mode light-emitting fiber of the point light source 110 may be a part of the broadlighter M-D-890-HP1 from SUPERLUM with a half-width in the wavelength range of 160 nm and a centroid wavelength lambda_S1 of 890 nm. This results in the centroid wavenumber kS1 with kS1 = 7.06 [irrT1. The single-mode light-emitting fiber of the point light source 120 may also be formed with a fiber core diameter of 5 micrometers. The single-mode light-emitting fiber of the point light source 120 may be a part of a commercially available broadlighter, such as the broadlighter D-810-HP from SUPERLUM emitting light with a spectrum having a half-width in the wavelength range of 100 nm and a centroid wavelength lambda_S2 of 810 nm. From centroid wavelength lambda_S2, the centroid wavenumber kS2 is obtained with kS2 = 7.76 / zm-1. Details 2.2 and 2.3 of Figure 2 symbolically represent the spectra of the two point light sources 110 and 120 in the wavenumber domain.

[0217] A multispectral, spatially coherent light beam is emitted from each of the two point light sources 110 and 120. One or both point light sources 110 and 120 may be movable (by employing suitable moving devices, not shown in the figure). For example, a moving device may be associated with the point light source 110, in order to be able to move the point light source 110, preferably, with high precision both in depth and in the two transverse coordinates, i.e. , x and y.

[0218] Via the coupling beam splitter 19, the light beam of the point light source 110, emanating from the microscopically fine, multispectral, light-emitting light spot S1 o, is coupled into the illumination beam path by reflection, so that an effective microscopically fine, multispectral, light-emitting light spot S1 is formed along the optical axis of the illumination beam path. The light from the point light source 120 passes through the coupling beam splitter 19. The microscopically fine, multispectral, light-emitting light spot S2o may thus also simultaneously be regarded as the effective microscopically fine, multispectral, light-emitting light spot S2. The effective light spot S1 of the point light source 110 and the effective light spot S2 of the second point light source 120 do not overlap but are located at a distance t12 (depth distance) from each other, as shown in detail 2.4 of Figure 1 . The effective divergent light means emanating from the effective light spots S1 and S2 are incident on the collimator lens 51 arranged downstream of the coupling beam splitter 19. The depth distance t12 may be calculated as described further below. The microscopically fine, multispectral, light-emitting light spots S1 and S2 lie on a straight line gS. In the case of a point sensor - as shown in Figure 1 - the straight line gS preferably coincides with the optical axis OA of the subsequent imaging system.

[0219] In the example shown in Figure 2, point light sources 110 and 120 are arranged such that they are equally spaced from the focal point F51 of the collimator lens 51 . This can be realized by a respective design of the collimator lens 51 . The collimator lens 51 is a lens which need not be field corrected. Preferably, the collimator lens is designed such that it is able to image a point on the optical axis in a diffraction-limited manner over a certain depth of range (for example a depth range of about 0.2 mm), which is symmetrical around the focal point F51 .

[0220] The main rays of the two light beams emanating from the two effective light emitting light spots S1 and S2 coincide at least approximately, and are also at least approximately collinear with the optical axis of the collimator lens 51 (which is comprised in the achromatic focusing unit). Thus, the main rays SB1 and SB2 are formed after the collimator lens 51. The collimator lens 51 may for example have a numerical aperture of NA=0.14 and a focal length of 22 mm. Downstream of the collimator lens 51 , a focusing lens 52 is arranged. The focusing lens 52 may for example have a numerical aperture of NA=0.11 and a focal length of 30 mm.

[0221] In the example shown in Figure 2, the real size of the two effective light spots S1 and S2 may be around 5 micrometers and thus the light beams emanating from the light spots S1 and S2 can be considered - with regard to the numerical aperture of the collimator lens 51 - as at least approximately spatially coherent. In particular, the light spots S1 and S2 in their real lateral extent of 5 micrometers are at least approximately in the order of magnitude of the diameter of the first ring of an Airy disk, which results from the numerical aperture of the collimator lens 51 of NA=0.14 for the wavelengths in the near-infrared spectral range used.

[0222] After the light source unit 100 (for example at the exit of the light source unit 100), the radii rpW1 and rpW2 of the wavefronts of the light beams B1 and B2 emitted from the light source 100 are considered at a point W in the paraxial region. The difference Diff(rpW) of radii rpW1 and rpW2 at point W of the unfolded reference arm R may be determined according to equation 5:

[0223] Diff(rpW) = r1 pW - r2pW (5)

[0224] The optical system is designed such that difference of the radii Diff(rpW) corresponds to the radius rS of the spherically curved convex mirror 72 arranged in the reference arm R. The convex mirror 72 is shown in Figure 2 not up to scale, but strongly enlarged for the sake of better visualization. The convex mirror 72 constitutes or is a part of an end reflector 711 in the reference arm R.

[0225] The difference of radii Diff(rpW) may be set, for example, to 160 micrometers.

[0226] The condition Diff(rpW) = rS follows from designing the optical system such that the light beam B1 with radius rl pWis aimed at the center point M1 of the spherically curved convex mirror 72 on the end reflector 711 , which is shown greatly enlarged for the sake of better visualization. The center point M1 also coincides with the first reference light spot S1 R'. In contrast, the light beam B2 with radius r2pW aims into the point of incidence AR on the convex mirror 72, which thus coincides with the second reference light spot S2R'. By using such optical arrangement, the light in the return beam path can essentially reach detection via confocal discrimination.

[0227] The spherically curved convex mirror 72 has, accordingly, a diameter 2rS of 2rS =320 micrometers. Such a convex mirror 72 can be easily manufactured by using known methods.

[0228] In the arrangement shown in Figure 2, the light spot S2R' is placed at point of incidence AR on the convex mirror 72 at the apex of the convex mirror 72. To enable a three- dimensional adjustment of the position of the light spot, the convex mirror 72 may be movable, preferably in the three spatial coordinates (x, y, z). The convex mirror 72 may be movable by a suitable moving device (such as a motor, etc.) known in the art. The precise adjustment of the difference of the radii according to equation 6:

[0229] Diff(rpW) = r1 pW - r2pW = r1 - r2 = rS = t12R' = 160 micrometers (6) i.e., to the radius rS of 160 micrometers, may be performed by a movable point light source 110 (moved for example by a suitable moving device such as motor, piezoelement, etc.). For this purpose, light spot S1 R' may be brought by adjustment to the center M1 of convex mirror 72 on the end reflector 711 , which the skilled person can easily control by analyzing the light reflected from convex mirror 72.

[0230] From the depth imaging scale alpha_dash of the imaging stage comprising the collimator lens 51 with the focal length f51 'of 22 mm and the focusing lens 52 with the focal length of 30 mm, the depth distance t12 may be calculated according to equation 7: t12 = t12R' * (f517 f52')2 = 160 pm * (22 mm / 30 mm)2 = 86.04 pm. (7)

[0231] Accordingly, the optical system is designed such that the two effective microscopically fine, light-emitting and multispectral light spots S1 and S2 are separated in front of the collimator objective 51 by a depth distance t12 of 86 micrometers.

[0232] As described above, for a given wavelength, the optical path length - at least in the paraxial region - is the same for the beams of the reference light beams B1 R and B2R. However, due to the dispersion of the glass material of the optical components arranged in the reference arm R, the optical path length differs from light wavelength to light wavelength. The dispersion in the reference arm R may be at least partially compensated for by designing the object arm O with the same optical path lengths in the same refractive material. In Figure 2, the glass block 602 symbolically represents the sum of the glass (and / or other refractive optical materials) path lengths in the reference arm R that may be compensated for by at least one dispersion compensation element. However, typically, the glass blocks 602 plays a negligible role in the ray- optical observation in the paraxial region. Detail 2.5 of Figure 2 shows the radii of the wavefronts which result at an arbitrarily selected reference point WR in the reference arm R and immediately in front of the centered, spherically curved convex mirror 72 on the end reflector 711 (i.e., with no other optical element between the point WR and the spherically curved convex mirror 72). At the spherically curved convex mirror 72, all the light from both reference light beams B1 R and B2R is reflected back, resulting in a good light utilization. Thereby, the wavefront of the reference light beam B2R experiences an inversion of the wavefront. In contrast, the wavefront of the reference light beam B1 R does not experience an inversion of the wavefront, which is shown in details A and B of detail 2.5 of Figure 2.

[0233] Figure 3 shows schematically (similar to Figure 2) an exemplary unfolded reference arm R, which may be used in the optical arrangement shown in Figure 1. Details 3.1 and 3.2 of Figure 2 show symbolically the spectra of the two point light sources 110 and 120 in the wavenumber domain.

[0234] The reference arm R shown in Figure 3 differs from the reference arm shown in Figure 2 in particular by the design of the end reflector 713. In contrast to Figure 2, the end reflector 713 in the reference arm R shown in Figure 3 is formed with two spherically curved convex and rotationally symmetrical mirrors 731 and 732, that are arranged concentrically to each other. The mirror 732 denotes the outer spherically curved, convex, centered mirror in a ring form, which is connected by means of the step 754 to the inner spherically curved convex and rotationally symmetrical mirror 731. This arrangement represents a second design possibility for an end reflector in the reference arm R.

[0235] Details 3.3 and 3.4 of Figure 3 show the end reflector 713, for the sake of better viewability, the details 3.3 and 3.4 are not up to scale, but are greatly enlarged. As shown in details 3.3 and 3.4, the optical arrangement of the end reflector 713 causes a geometrical division of the wavefront of the light beam B1 R. Due to the geometric division of the wavefront of the light bundle B1 R, the utilization of the light for the light bundle B1 R is somewhat worse than in Figure 2, because light rays from the bundle B1 R, which are reflected at the inner, spherically curved, convex, centered mirror 731 , are blocked by the later confocal discrimination and are therefore not detected. Only the light beams of the light bundle B1 R that strike the outer, spherically curved, ring- shaped mirror 732 reach the detection after the confocal discrimination. This is symbolically represented by the area gB1 .

[0236] In contrast, the light rays of the light beam B2R essentially strike the inner, spherically curved, convex, centered mirror 731 , are reflected within themselves, and subsequently arrive at the detector after confocal discrimination. This is represented by the area gB2. In detail 3.4, four arrows with a transverse bar at the arrowhead represent light rays which are not reflected back into themselves and are therefore blocked by the confocal discrimination, i.e. , cannot contribute to the detection.

[0237] The optical system shown in Figure 3 is designed such that the difference Diff(rpW) of radii rpW1 and rpW2 at point W of the unfolded reference arm R fulfills the equation 8:

[0238] Diff(rpW)=Diff(rS)=r1 pW-r2pW=rS1 -rS2=t12R'. (8)

[0239] Figure 4 shows schematically the optical setup of an exemplary optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCSI) comprising a multispectral light source unit 102 similar to the light source unit 101 described in connection with Figure 2. The light source unit 102 comprises a beam splitter 191 and a focusing lens (focusing objective) 5 arranged downstream of the beam splitter 191. The beam splitter 191 is a single-coupling beam splitter with a color splitter layer for beam unification in the light source unit 102. The focusing lens 5 may have a two-stage design and comprise a collimator lens 51 and a focusing lens 52, which are not shown individually in Figure 4.

[0240] The multispectral light source unit 102 is configured to generate two spatially coherent light beams B1 and B2 by employing multispectral point light sources 110 and 120,

[0241] The spectra of the multispectral point light sources 110 and 120 comprised in multispectral light source unit 102 do not overlap, as shown schematically in details 4.1 and 4.2 of Figure 4. The single-coupling beam splitter 191 with a color splitter layer is used to combine the light beams emanating from the point light sources 110 and 120. The point light source 110 has the shorter wavelength light and its microscopic multispectral light emitting light spot S1 is slightly farther away from the focusing lens 5 than the microscopic multispectral light emitting light spot S2. The multispectral light source unit 102 has an optical axis OAS at its output.

[0242] The optical system comprises further a dual-beam interferometer 6, which is a chromatic-confocal Michelson interferometer s with a diffractive-optical element 701 in the object arm O and a non-zero optical path difference for a measured object 8 which is positioned in the measurement range and on which depth-split light spots are incident, of which at least two light spots are sharply focused, namely the light spots S10k1 ' and S20k2'.

[0243] In the reference arm R of the dual-beam interferometer 6, an end reflector 712 comprising a spherically curved concave mirror 74 with a center opening is arranged. The concave mirror 74 (for example a miniaturized concave mirror) is centered on the optical axis OAR of the reference arm. The center point M1 of the concave mirror 74 lies on the optical axis OAR. Further, the end reflector 712 comprises a plane mirror 741 (for example a miniaturized plane mirror) arranged in the center opening of the concave mirror 74. The normal to the plane mirror 741 is parallel to the optical axis.

[0244] The plane mirror 741 is movable in a depth direction (i.e., in a direction parallel the optical axis OAR), so that the position of the plane mirror 741 can be adjusted. This may be realized by arranging the plane mirror 741 on a piezo actuator 742. The depth- adjustable piezo actuator 742, on which the miniaturized plane mirror 741 is arranged, may be controlled by a computer-aided control device to produce an "in phase situation" for the two wavelets W1 and W2 by making the geometrical paths of the two main rays SBR1 and SBR2 of the two reference beams B1 R and B2R in the reference arm R substantially equal. The first light spot S1 R' formed by the first reference beam B1 R is aimed at the center point M1 from the spherically curved concave mirror 74. The second light spot S2R' formed by the second reference beam B2R is located on the miniaturized plane mirror 741 . Detail 4.3 of Figure 4 shows an enlarged view of a portion of the reference arm R including the measured object 8 and also shows the two radii of curvature r1 and r2 of the two wavefronts of the two light beams B1 R and B2R at a point WR. The two light beams B1 R and B2R show a high spatial coherence.

[0245] The dual-beam interferometer s comprises a micro-profiled diffractive-optical element (DOE) with positive focusing power arranged in the object arm 0. In contrast to the arrangement according to Figure 1 , the diffractive-optical 701 has a planar substrate surface. In other words, the surface of the diffractive-optical 701 that is opposite to the surface on which the diffraction grating is formed, is a substantially planar surface.

[0246] The micro-profiled diffractive-optical 701 is formed as a zone lens, which is used in the first diffraction order and which focuses the light in the object arm 0. The diffractive- optical element 701 is optimized, at least approximately, for the light from the two light beams B1 and B2 in the region around the centroid wavenumbers kS1 and kS2. To compensate to a large extent the focusing effect of the diffractive-optical element 701 , a thin converging lens 705 is arranged in the reference arm R. “Thin” imeans in this context that the (geometrical) thickness of the lens is smaller than or equal to a tenth of the focal length of the lens. The thin converging lens 705 partially compensates the focusing power of the diffractive-optical element 701 with positive focusing power in the used spectral range. The thin converging lens has a lower focusing power than the power of the diffractive-optical element 701. Thus, there is an undercompensation of the focusing effect of the diffractive-optical element 701. The partial compensation of the refractive forces of the diffractive-optical element 701 by the thin converging lens 705 ultimately allows the generation of wavelets W1 and W2, see detail 4.4, which are of such low frequency that they can be well evaluated with rasterized detectors - such as line scan cameras - with e.g. as few as about 2000 pixels in a line.

[0247] The optical system comprises further a detection unit 161 comprising a focusing objective 79, a color splitter 201 for beam separation and a fiber-coupled singlechannel spectrometer 281. The single-channel spectrometer 281 is configured for operation in the wavelength range of 700 nm to 1000 nm and exhibits a fast line sensor with 2000 pixels. The term "single channel" means here that only one spectrum is recorded at a time in the wavelength range from 700 nm to 1000 nm, which comprises at least two spectral wavelets.

[0248] The light reflected from the reference light beams B1 R and B2R at the concave mirror 74 and at the miniaturized plane mirror 741 passes through the beam splitting layer 60 and is focused by the focusing lens 79 in the detection unit 161 , so that the sharply focused reference light spots S1 R" and S2R" are formed after passing through the color beam splitter 201 for beam separation. The light spot S1 R" is formed by the light transmitted through the color beam splitter 201 and the light spot S2R" is formed by the light reflected at the color beam splitter 201 . The light spots S1 Ok1 " and S1 R" and the light coming from the light spots S20k2" and S2R" are superimposed during confocal discrimination and interference occurs. In this process, the light spots S1 R" and S10k1" impinge on the input of the single-mode fiber 113 for confocal discrimination, which acts as a confocal discriminator in the detection beam path. The light spots S2R" and S20k2" impinge on the input of the single-mode fiber 123 for confocal discrimination, with the single-mode fiber 123 also acting as a confocal discriminator in the detection beam path. Via the single-mode fiber 113 and 123 and via the y-coupler 124 in the detection beam path, the confocally discriminated light enters the single-channel spectrometer 281 via the single-mode fiber 125. The singlechannel spectrometer 281 designed for the wavelength range from 700nm to 1000nm and has a fast line sensor with 2000 pixels.

[0249] The light from light beam B1 ultimately produces wavelet W1 and the light from light beam B2 ultimately produces wavelet W2. Wavelets W1 and W2 are at least approximately in phase due to the near perfect compensation of dispersion in the chromatic-confocal Michelson interferometer 6. The wavelets W1 and W2 are processed by a digital computing unit 31 to calculate the depth position to a depth value zk for the point cPO12 currently of the target 8 that is currently measured. Figure 5 shows schematically the optical setup of an exemplary optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCSI). The optical system comprises a multispectral light source unit 101 and a Mirau interferometer 61 (an exemplary dual-beam interferometer), into which the light from the multispectral light source unit 101 is coupled.

[0250] The light source unit 101 comprises two fiber-coupled point light sources 110 and 120 and a beam splitter 192, which in this case serves to combine the beams of light from the two point light sources 110 and 120 in a path to the Mirau interferometer 61 and to split the beams in the return path of the light from the Mirau interferometer 61 . In other words, the beam splitter 192 is designed for both beam coupling and decoupling.

[0251] In addition, the light source unit 101 has two single-mode fibers 111 and 121 , each having a Y-fiber coupler 112 and 122.

[0252] The two fiber-coupled point light sources 110 and 120 couple the emitted light into the single-mode fibers 111 and 121 , respectively. The coupled light then passes via the Y- fiber couplers 112 and 122, respectively, into the single-mode fibers 114 and 126 at the fiber ends of which the microscopically fine, multispectral light spots S1 o and S2o are formed. After coupling via the beam splitter 192, these form the respective effective microscopically fine, multispectral, light-emitting light spots S1 and S2. The two multispectral, light-emitting light spots are arranged on a straight line gS that substantially coincides with the optical axis OA of the subsequent imaging optics (including or consisting of the collimator lens 511 ). The spectral characteristics of the two point light sources 110 and 120 are similar to those in Figure 1 and are already explained there in the description of Figure 1 , see also details 5.1 and 5.2 of Figure 5.

[0253] With respect to the collimator lens 511 , the light spot S1 is positioned weakly extrafocal and the light spot S2 weakly intrafocal, each at equal distances from the focal point F511 of the collimator lens 511 . The depth distance t12 of the of the two light spots S1 and S2 may be selected appropriately. In the optical system shown in Figure 8, the depth distance t12=0.48 mm. The collimator lens 511 thus forms a weakly divergent light beam B1 and a weakly convergent light beam B2 from the light of the two light spots S1 and S2. In the example shown in Figure 5, the collimator lens 511 has a focal length f51 T of 50 mm and at least a numerical aperture of 0.08 and is designed for the near-infrared range of the light emitted from the point light sources 110 and 120. The multispectral light source unit 101 outputs the weakly divergent coherent light beam B1 and the convergent coherent light beam B2. The multispectral light source unit 101 has the optical axis OAS at its output.

[0254] The associated wavefronts of the light beams B1 and B2 have in magnitude at least approximately the same radii of curvature r1 pW and r2pW in the immediate, i.e., paraxial, vicinity of a point W at the entrance El of the dual-beam interferometer, see detail 5.3 of Fig. 5. As described above, the curved wavefronts with the radii of curvature r1 pW and r2pW are generated by the depth positioning of the collimator lens 51 relative to the two point light sources S1 and S2 at the depth distance t12.

[0255] In the example shown in Figure 5, the point light sources S1 and S2 are each positioned at the same depth distance - namely t12 / 2=0.24 mm - from the focal point F511 . The collimator lens 511 is a lens which does not have to be corrected with respect to a large field but only needs to produce diffraction-limited sharp images of only one point on the optical axis over a depth range of about 0.4 mm which is symmetrical with respect to the focal point F511 . The collimator lens 511 can be a suitable microscope objective, for example a commercially available microscopic objective with 5x magnification and design to infinity, which is sufficiently well corrected in depth on the optical axis.

[0256] The light exiting the collimator objective 511 enters the Mirau interferometer 61 via the input El. The Mirau interferometer 61 is a chromatic-confocal Mirau interferometer with a non-zero optical path difference for a measured object 8 which is positioned in the measuring range and on which depth-split light spots are incident, of which at least two light spots are sharply focused, namely the light spots S10k1 ' and S20k2'. The Mirau interferometer 61 comprises a microscope objective 53, a diffractive-optical element 70 arranged in the object arm 0 and a beam splitter plate 601 .

[0257] The microscope objective 53 may be designed for glass path lengths that result from the thickness of the beam splitter plate 601 and the thickness of the substrate of the diffractive-optical element 70. For example, the microscope objective 53 may have a numerical aperture NA = 0.3 and a focal length of f53' = 12.5 mm and may be designed for light in the near infrared range. A glass block 602 symbolically represents the sum of the glass path lengths in the reference arm R.

[0258] The diffractive-optical element 70 may be micro-profiled diffractive-optical element having a structured surface bearing a diffraction grating and a concave substrate surface 703 opposite to the structured surface. The DOE 70 may be formed as a zone lens used in the first diffraction order. The DOE 70 has a positive focusing power.

[0259] The diffractive-optical element 70 may be optimized at least approximately for the spectral range around the centroid wavenumbers kS1 and kS2 of the two point light sources S1 and S2. The power of the diffractive-optical element 70 may be largely, but not necessarily completely compensated for by the concave substrate surface 703. The amount of undercompensation may be such that there is still a sufficiently large optical path difference in the interferometer for the dzc1_H and dzc2_H regions of chromatic depth splitting of light spots, so that modulated wavelets can always be produced. Thus, for light of centroid wavenumbers kS1 and kS2, despite compensation by the concave back surface 703 of microprofiled diffractive-optical element 70, the overall effect of microprofiled diffractive-optical element 70 is still one of weak focusing.

[0260] The focal length of the DOE 70 may be selected appropriately and may be, for example, 180 mm for the centroid wavelength 890 nm. The concave substrate rear surface 703 of the diffractive-optical element 70 may have a negative focal length of minus 400 mm, which serves to partially compensate for the positive diffractive power of the DOE 70, in order to obtain wavelets with a sufficiently low frequency that can be evaluated well. The light focused by the microscope objective 53 impinges on the beam splitter plate 601 with the beam splitting layer 60, on which the first focused reference light beam B1 R and the second focused reference light beam B2R are respectively formed by reflection. The two reference light beams B1 R and B2R form the reference light spots S1 R' and S2R', respectively. The virtual reference light spot S1 R' is located in the center M1 of a small convex, spherically curved mirror 721 with the associated osculating circle SK1. Thereby, the convex, spherically curved mirror 721 is arranged on the front lens of the microscope objective 53. The light of this reference light beam B1 R is reflected back into itself at the convex, spherically curved mirror 721 . The light of the reference light beam B2R forms the sharply focused light spot S2R' directly on the convex, spherically curved mirror 721 and is also reflected. The convex, spherically curved mirror 721 and the radius rS1 are not drawn to scale, but are shown considerably enlarged for the sake of better viewability.

[0261] The optical arrangement with the collimator lens 511 and the microscope lens 53 with the focal lengths already mentioned above, results in a depth image scale of alpha_dash=0.0625. Thus, the depth distance t12 of 0.48 mm in the light source unit 101 results in the depth distance t12R' of 0.030 mm in the reference arm R for the light spots S1 R' and S2R'. The depth distance t12R' corresponds exactly to the radius of curvature rS1 of the convex, spherically curved mirror 721 . Detail 5.4 of Figure 5 shows the two radii r1 and r2 of the wavefronts WF1 R and WF2R of the reference light beams B1 R and B2R at point WR, whose difference in radii here is also 0.030 mm.

[0262] The wave-optical depth of focus T890nm for the wavelength lambda_S of 890nm is obtained for the numerical aperture NA of 0.3 in the reference arm R by equation 9:

[0263] T890nm = lambda_S I (NA2) = 890 nm I 0.09 = 9.9 microns. (9)

[0264] After reflection from the convex, spherically curved mirror 721 , the light from the two reference light beams B1 R and B2R propagates back toward the beam splitting layer 60, where re-reflection of the light occurs. For the two main rays SB1 R and SB2R, the optical path length in the reference arm for a given wavelength of light is the same. Due to the wavelength-dependent focusing power of the microprofiled DOE 70, which acts as a positive zone lens, a chromatic depth splitting in the z-range dzc1_H and dzc2_H occurs in the object arm 0 for the two object light beams B1 O and B2O, with the foci of the short-wave radiation being farthest away from the microprofiled DOE 70. The object bundle OB_d diffracted in the first diffraction order are produced. To be noted in this respect, is that only these are important, since light in higher diffraction orders is blocked from detection due to return-beam confocal discrimination.

[0265] The radiation reflected at a point cPO12 of a measured object (target) 8 of wavenumbers k1 and k2, and also partially the light in their spectral vicinity, re-enters the microscope objective 53 and the collimator objective 511 via the micro-profiled diffractive-optical element 70, also in the first diffraction order, through the beam splitter plate 601 with the beam splitting layer 60.

[0266] Downstream of the micro-profiled DOE 70, weak light beams in the zeroth diffraction order as well as in higher diffraction orders are also formed. Since these weak light beams are suppressed by the subsequent confocal discrimination, the light beams are not further regarded here.

[0267] For the light of the reference light beams B1 R and B2R reflected at the convex, spherically curved mirror 721 , which comes from the reference arm R, a reflection at the beam splitting layer 60 follows and the reflected light also enters the microscope objective 53 and the collimator objective 511. The light refocused by the collimator objective 511 , which originates from the first reference light beam B1 R, arrives sharply focused at the entrance of the single-mode fiber 114. The end of the single-mode fiber 114 acts as the first confocal discriminator 271. The light refocused by the collimator lens 511 , which originates from the second reference light beam B2R, arrives sharply focused at the input of the single-mode fiber 126. The end of the single-mode fiber 126 acts as the second confocal discriminator 272.

[0268] Thus, a superposition of the reference light with the light from the object arm O and interference occurs at the respective ends of the single-mode fibers 114 and 126.

[0269] The confocally discriminated light, which originates from the light of the first reference light beam B1 R, enters the single-mode fiber 129 via the single-mode fiber 114 and the Y-fiber coupler 112 and the single-mode fiber 115 and the Y-fiber coupler 128 together with the confocal discriminated light from the object arm 0, which originates from the first point light source 110 and has the wavenumber k1. From this Y-fiber coupler 128 and through the single-mode fiber 129, which extends from the y-fiber coupler 128 to the single-channel spectrometer 281 , the first light interferingly enters the fiber-coupled single-channel spectrometer 281 with a fast line sensor (not shown in Figure 5) for spectral detection, where a first spectral wavelet W1 is detected.

[0270] The confocal discriminated light, which originates from the light of the second reference light beam B2R, passes with the confocal discriminated light from the object arm 0, which ultimately originates from the second point light source 120 and has the wavenumber k2, via the single-mode fiber 126, the Y-fiber coupler 122, the singlemode fiber 127, which extends from the y-fiber coupler 122 to the y-fiber coupler 128, the Y-fiber coupler 128 into the single-mode fiber 129 and from there into the fiber- coupled single-channel spectrometer 281. There, spectral acquisition takes place, where the second spectral wavelet W2 is detected.

[0271] Thus, a continuous depth scanning of the object arm is performed with light spots of different wavelengths, respectively different wavenumbers in the coverage region UE in the object arm 0, since the point light sources 110 and 120 also represent continuum emitters.

[0272] The evaluation of the two wavelets W1 and W2 may be carried out by suitably programmed digital computing device 31 for calculating the depth position z. In this case, the evaluation can be carried out by evaluating the phase variation over the wavenumber, since the light intensity variation over the wavenumber depends on the optical path difference x in the dual-beam interferometer and thus also on the current depth position of a current measurement object point cPO12 of the measured object 8. The spectral range of the fiber-coupled single-channel spectrometer 281 may match the spectral range of the light sources and may, in the example shown in Figure 5, may be from 700 nm to 1000 nm. Detail 5.6 of Figure 5 shows that in reality there is a small depth distance t120kS1 kS2' between the light spots S1 OkS1 ' and S20kS2' of the centroid wavenumbers kS1 and kS2, which determines the depth range dtc in which light spots with their centroid wavenumbers are located. Thus, in reality there is no a “single” confocal point cPS, but a fairly good approximation to it, since the depth distance t120kS1 kS2' is only a few percent of the coverage region UE.

[0273] The diffractive-optical elements 70 and 701 are not limited to the above-described examples but may exhibit different power and / or may be used in combination with different curved mirrors.

[0274] Figure 6 shows four exemplary cases 1.1 , 1.2, 2.1 and 2.2 corresponding to possible different combinations of diffractive-optical element with curved mirrors depending on the sign of the power of a diffractive-optical element 70 and 701 respectively, on the shape of a spherically curved mirror - i.e. convex or concave shape - and on the depth position of the point light sources 110 and 120 for a dual-beam interferometer 6 and 61 respectively. Here always only one mirror in the reference arm R is considered.

[0275] In case 1.1 , if the multispectral light source comprises point light sources 110 and 120, the first point light source 110 is assumed to be the light source with the longer wavelength, which is arranged closer to the entrance El of the subsequent interferometer than the second point light source 120.

[0276] In case 1 .2, if the multispectral light source comprises point light sources 110 and 120, the first point light source 110 is assumed to be the light source with the shorter wavelength, which is arranged further away from the entrance El of the subsequent interferometer than the second point light source 120.

[0277] In case 2. 1 , if the multispectral light source comprises point light sources 110 and 120, the first point light source 110 is assumed to be the light source with the shorter wavelength that is arranged closer to the input El of the subsequent interferometer than the second point light source 120.

[0278] In case 2. 2, if the multispectral light source comprises point light sources 110 and 120, the first point light source 110 is assumed to be the light source with the longer- wavelength, which is arranged more distant from the input El of the subsequent interferometer than the second point light source 120.

[0279] If the multispectral light source comprises a free-beam laser, only the radii of curvature r1 and r2 of the wavefronts in the reference arm R at a point WR are considered, since in this case, there is no point light source.

[0280] A compensation of the focusing power of a diffractive-optical element 70 in the object arm O in the chromatic-confocal Michelson interferometer 6, for example by means of a concavely curved substrate back surface 703, can be completely omitted if a high- resolution line detector is used, for example a line detector with a number of detector elements in the order of 10000. However, the use of a high resolution line detector typically slows down the evaluation considerably, which is why a partial compensation of the focusing power of a diffractive-optical element 70 in the object arm O in the chromatic-confocal Michelson interferometer 6 (for example by means of a concavely curved substrate back surface 703) may be advantageous.

[0281] Figures 7 to 10 show schematically exemplary reference arms R and object arms O of a dual-beam interferometer, that may be used in the optical system described in connection with the preceding figures. The reference arms R and the object arms O are shown unfolded on the optical axis, respectively. Figures 7 to 10 show ideal cases, in which a confocal point cPS is formed in each case at the centroid wavenumbers kS1 and kS2.

[0282] A higher frequency wavelet next to a light spot symbolically represents a light spot with shorter wavelength light and a lower frequency wavelet next to a light spot symbolically represents a longer wavelength light spot. In all cases - for the sake of better viewability - the depth distances of the light spots are always shown extremely enlarged. In each Figure 7 to 10, the unfolded reference arm R is shown on the left and the unfolded object arm 0 on the right.

[0283] In Figures 7 to 10, the reference sign 702 denotes a micro-profiled diffractive-optical element (DOE) with negative diffractive power (formed as a zone lens) arranged in object arm 0, wherein the DOE 702 diverges light and is used in the first diffraction order. The reference sign 706 denotes the convex rear side of the diffractive-optical element 702, i.e., the side opposite to the structured diffractive surface of the diffractive-optical element. The positive refractive power of the convex back side 706 largely, but not completely, compensates for the negative power of the diffractive- optical element 702.

[0284] The reference sign 703 denotes a concavely curved substrate back surface of a diffractive-optical element 70. As described above, the negative refractive power of the concavely curved substrate back surface 703 largely, but not completely, compensates the positive power of the diffractive-optical element 70.

[0285] Figure 11 shows schematically an optical setup of an exemplary optical system for spectral domain OCT or chromatic confocal spectral interferometry (CCS I) with a multispectral light source unit 103 comprising three point light sources that may differ significantly with respect to their centroid wavenumbers kS1 , kS2, and kS3, respectively.

[0286] The multispectral light source unit 103 is similar to the light source unit 100 or 102, the main difference being the number of point light sources (three instead of two). The multispectral light source unit 103 may comprise three microscopic primary light emitting spots (not shown in Figure 11 ), each of them emitting multispectral light. After coupling into the beam path, these form respective effective, microscopic, multispectral, light-emitting light spots S1 , S2, and S3, which are spaced in depth and located on a straight line gS. The straight line gS generally coincides with the optical axis OA of the light source unit 103. The light source unit 103 generates three spatially coherent light beams B1 , B2, and B3 by means of the effective, microscopically fine, multispectral, light-emitting light spots S1 , S2, and S3. The multispectral light source unit 103 has the optical axis OAS at the output.

[0287] The three effective, microscopic, multispectral, light-emitting light spots S1 , S2, and S3 are spaced in a depth direction by a predetermined distance.

[0288] The centroid wavenumbers kS1 , kS2, and kS3 of the light from the light spots S1 , S2, and S3 are different from each other. Without restriction to generality, the light of light spot S1 has the smallest centroid wavenumber kS1 and light of light spot S3 having the largest centroid wavenumber kS3 of the three light spots S1 , S2 and S3.

[0289] The centroid wavenumbers kS2 and kS3 may correspond to the centroid wavenumbers kS1 and kS2 as described in connection with Figures 1 to 3. Thus, in the arrangement according to Figure 11 , the centroid wavenumber kS2 may be equal to 7.06 / zm-1and the centroid wavenumber kS3 may be equal to 7.76 / zm-1. The light spot S1 may, for example, emits light originating from the super broadlighter D-980- HP with a centroid wavelength of 980 nm. This results in a calculated centroid wavenumber kS1 of 6.41 / zm-1. In the light source unit 103, three primary at least approximately spatially coherent light beams Bo1 , Bo2 and Bo3 are produced, whose main rays coincide at least approximately.

[0290] The light source unit 103 comprises further a focusing lens (objective lens) 5. The focusing lens 5 may consist of or comprise a collimator objective 51 with a focal length f51' = 22 mm and a focusing objective 52 with a focal length f52' = 30 mm as also shown in the optical system according to Figure 2. The light spot S2 may be positioned in the focal point F51 of the collimator objective 51 , see detail 11.4 of Figure 11. The depth image scale for light spots in the vicinity of the focal point F51 is 0.5378 according to the laws of geometrical optics.

[0291] The at least approximately spatially coherent multispectral light beams B1 , B2 and B3 emerge from the light source unit 103 with the focusing lens 5.

[0292] The wavefronts of the three coherent light beams B1 , B2 and B3, considered in the immediate, i.e. paraxial, vicinity of any point W located at the output of the light source unit 103 on the optical axis OAS and / or at an input El of the chromatic-confocal Michelson interferometer 6 exhibit different radii of curvature r1 pW, r2pW and r3pW. The consideration of the radii of curvature r1 pW, r2pW and r3pW is carried out here only in the paraxial region, while neglecting non-paraxial optical paths.

[0293] From the light of the light beams B1 , B2 and B3, separated and diffraction-limited imaged light spots S1 R', S2R' and S3R' are formed in the depth of the reference arm R by the imaging effect of the objective 5, respectively. The light spots S1 R', S2R' and S3R' are each multispectral, wherein the respective spectra of each of the light spots S1 R', S2R' and S3R' are different.

[0294] The difference of the two radii of curvature r1 pW and r2pW in the paraxial vicinity of the point W at the output of the light source unit 103 may be set to 0.160 mm, and the difference of the two radii of curvature r2pW and r3pW may be set to 0.160 mm. Accordingly, in the reference arm R, the depth distance t12R' of the resulting light spots SI R'and S2R' is t12R'=0.160 mm and the depth distance t23R' of the resulting light spots S2R' and S3R' is also t23R'=0.160 mm. For this purpose, for a depth imaging scale alpha_dash = 0.5378 of the lens 5, the depth distance t12 and the depth distance t23 in the light source unit 103 are calculated to be 86.0 micrometers each. The precise approximation of the depth distances t12 and t23 only occurs as a result of a calibration of the optical system and will be described further on.

[0295] In the arrangement according to Figure 11 , the size of the depth distances t12R' and t23R' may be controlled by a measuring linear slide (linear gauge) with a collecting screen in the reference arm R on the basis of the illustrated light spots S1 R', S2R' and S3R' and may be precisely set by an adjustment in the light source unit 103. Deviations in the lower single-digit micrometer range are still permissible. As a result of this adjustment / calibration, the microscopically fine and multispectral effective light spots S1 and S2 as well as S2 and S3 are separated from each other in the light source unit 103 by the depth distances t12 and t23 of 0.086 mm each.

[0296] An end reflector 715 is arranged in the reference arm R. The end reflector 715 is shown in detail 11.5 of Figure 11 and again in Figure 14. The end reflector 715 has an inner spherically curved, convex, centered mirror 731 and an outer spherically curved, concave, centered mirror in ring form 752 as well as a step 755, said step 755 connecting the mirror 731 and the mirror 752.

[0297] A first osculating sphere (osculating sphere of the first mirror 731 ) comprises the first mirror 731 of the end reflector 715. A second osculating sphere (osculating sphere of the ring-shaped mirror) comprises the ring-shaped mirror 752 of the end reflector 715,

[0298] The osculating circle SK1 , which is shown as a dashed-line circle is a section of a first osculating sphere of the inner mirror 731 in the plane of the drawing. The osculating circle SK2, which is also shown as a dashed-line circle is a section of a second osculating sphere to the outer ring-form mirror 752 in the drawing plane. The osculating circles SK1 and SK2 touch each other in the drawing plane at the point of contact BP. There is exactly one point of contact BP of the two osculating circles SK1 and SK2 of the two osculating spheres SK1 and SK2 in the reference arm R in the drawing plane. In the exemplary system shown in Figure 11 , the point of contact BP coincides with the point of incidence AR for the main rays SB1 R, SB2R and SB3R.

[0299] In the optical system shown in Figure 11 , a plurality of light spots is formed in the reference arm R, the light spots being depth-separated. A first (virtual) light spot S1 R’ is formed in a position coinciding with the center point M1 of the osculating circle SK1 (see also Figure 14). A second light spot S2R' is formed in the incidence point AR, which is also the point of contact BP of the osculating circles SK1 and SK2. A third light spot S3R' is located in the center M2 of the osculating circle SK2.

[0300] The object arm 0 may have a similar structure as the object arm 0 of the optical system described in connection with Figure 1 , with the difference that there are three instead of two coherent bundles B1O, B2O and B3O, which are split chromatically in depth by a diffractive-optical element 70 with a positive focusing power (e.g., a microprofiled DOE).

[0301] Downstream of the micro-profiled DOE 70, three object light spots S1 OkS1 S20kS2' and S30kS3' with the centroid wavenumbers kS1 , kS2 and kS3 of the three effective light-emitting spots S1 , S2, S3 are formed. In the ideal case shown in Figure 11 , the three light spots S1 OkS1 ', S20kS2' and S30kS3' coincide at least approximately in a confocal point cPS. In this ideal case, there is a high degree of coverage UE of the three chromatic depth splitting ranges dzc1_H, dzc2_H and dzc3_H.

[0302] It is not necessary that all three light spots S1 OkS1 ', S20kS2' and S30kS3' coincide at least approximately in a confocal point cPS. As shown in detail 11.6 of Figure 11 , only the light spots S10kS1' and S20kS2' may coincide in a confocal point cPS and not the light spot S30kS3', resulting in a small depth range dtc, which is, however, still tolerable.

[0303] Thus, in reality, a single confocal point cPS is only approximately formed by three light spots S10kS1', S20kS2' and S30kS3' of the three centroid wavenumbers kS1 , kS2 and kS3. In other words, the confocal point cPS may exist only as an approximation, as shown for example in detail 11.6 of Figure 11 , where the light spot S30kS3' does not coincide exactly with the light spots S10kS1', S20kS2'. However, this generally does not pose a problem, in particularly if the depth range dtc in the object arm 0, in which the three light spots S1 OkST, S20kS2' and S30kS3' are located, is in the lower single-digit percentage range from the overlap range UE and for example does not exceed 10%. In some cases, a higher extension of the depth range is also tolerable.

[0304] The coverage region UE is determined in Figure 11 by the area dzc3 of the chromatic depth splitting of the light spots in the object arm 0 of the light beam B3O. Only light with wavenumbers within the spectral half-width HWB3 is considered.

[0305] In addition to many defocused light spots, also the focused light spots S10k1', S20k2' and S30k3' fall on a current measurement point of the measured object 8, to which the wavenumbers k1 , k2 and k3 are assigned here. The light spots S10kT, S20k2' and S30k3' are sharply focused because light with the corresponding wavenumbers k1 , k2 and k3 is sharply focused. The incident light is reflected at the object 8. For the returning reflected light the chromatic splitting is reversed at the diffractive-optical element 70 and after passing the beam splitting layer 60 the object light beams are superimposed with the light from the reference arm R. After passing the focusing objective 79 in the detection unit 162, which in this example is designed for three channels, confocal discrimination occurs at the confocal apertures 261 , 262 and 263, which are shown in Figure 11 only symbolically.

[0306] With a careful adjustment of the position of the confocal apertures 261 , 262 and 263, the superimposed light spots S1 R" and S1 Ok1 can pass at the confocal aperture 261 , the superimposed light spots S2R" and S20k2" can pass at the confocal aperture 262 and the superimposed light spots S3R" and S30k3" can pass at the confocal aperture 263. Defocused light is largely suppressed and not allowed for detection.

[0307] The structure of a detection unit 162 is described further below in connection with Figure 15.

[0308] Figure 12 shows schematically an exemplary light source unit 103, that may be used in the optical system shown in Figure 11. In the light source unit 103, three effective light spots S1 , S2 and S3 emitting into the illumination beam path are formed by employing a beam splitter group 22 with two beam splitting layers 221 and 222 arranged in parallel for splitting the incoming light beams into three collinear beam paths.

[0309] The multispectral light source unit comprises three integrated multispectral point light sources 110, 120 and 130, wherein the light emitted from each of the point light sources 110, 120 and 130 is coupled into a first end of a respective single-mode fiber 111 , 121 and 131 , respectively. The second, output end of each of the single-mode fibers 111 , 121 and 131 serves (forms) a respective primary light emitting spot So1 , So2 and So3. The three integrated point light sources 110, 120 and 130 may be microscopic, multispectral light sources. The spectra of the light sources 110, 120 and 130 are shown in details 12.1, 12.2 and 12.3 of Figure 12. The three light sources 110, 120 and 130 emit multispectral light with different wavenumbers kS1 , kS2 and kS3, respectively, wherein the centroid wavenumber kS1 is the smallest and the centroid wavenumber the greatest. In an example, kS1 = 6.41 / zm-1, kS2 = 7.06 / zm-1and kS3 = 7.76 / zm-1.

[0310] The multispectral light source unit 103 further comprises a first adjustable plane plate

[0311] 223, a second adjustable plane plate 224, a beam splitter group 22 and a focusing lens 5 arranged downstream of the beam splitter group 22.

[0312] The first adjustable plane plate has a thickness a1 and is attached to the output end of the single-mode fiber 121 , through which the light emitted from the output end of the single-mode fiber 121 passes before reaching a beam splitter group 22. The second adjustable plane plate 224 has also a thickness a1 and is attached to the output end of the single-mode fiber 131 , through which the light emitted from the output end of the single-mode fiber 131 passes before reaching a beam splitter group 22. Thus, light from point light source 120 is coupled in via single-mode fiber 121 through the first adjustable plane plate 223 of thickness a1 , and light from point light source 130 is coupled in via the single-mode fiber 131 through the second adjustable plane plate

[0313] 224, also of thickness a1 . The output end of the single-mode fiber 111 is directly connected to an entry surface of a first prism of the beam splitter group 22. In the arrangement, the optical connection of the ends of the single-mode fibers 111 , 121 , and 131 to glass, prevents strong back reflections at the fiber end, which are undesirable.

[0314] The beam splitter group 22 is formed as a three prism group and exhibits two parallel beam splitting layers 221 and 222 for combining the light emitted from the primary light emitting spots So1 , So2 and So3 and for coupling it into the illumination path.

[0315] The beam splitter group 22 comprises a first prism formed in a pyramid form, with a right triangular cross section in the plane of the drawing, wherein the length of the orthogonal sides of the triangular cross section is “a”. One of the faces of the first prism , which is opposite to the base face of the first prism, is connected to the end of the single-mode fiber 111.

[0316] The second prism of the beam splitter group 22 has two parallel faces, on which the first beam splitting layer 221 and the second beam splitting layer 222 are provided, respectively. The distance between the two parallel faces and thus the first beam splitting layer 221 and the second beam splitting layer 222 is “a”. The face of the second prism bearing the first beam splitting layer 221 , which is connected to the base face of the first prism. The face of the second prism bearing the second beam splitting layer 222 is connected to an inclined base face of the third prism.

[0317] The third prism of the beam splitter group 22 has a prism form having a first base face a peripheral surface, which extends from the first base face in a direction substantially orthogonal to the base face, and an inclined base face connected to the peripheral surface. The inclined base face is further connected to the face of the second prism bearing the second beam splitting layer 222. The cross-section of the third prism in the plane of the drawing has a trapezoidal form, having a first base side, two peripheral sides extending from the first base side in a direction orthogonal to the first base side and an inclined side connected to the peripheral sides. The length of one of the peripheral sides is set to “a” and the length of the second peripheral side is set to “2a- a1”, i.e., is shortened by the glass path length a1. The glass path length a1 may, for example be 1 mm and the length “a” may be 10 mm. Thus, for each of the three beam paths, glass path length is the same, which enables diffraction-limited imaging of the multispectral light spots S1 , S2, and S3.

[0318] After coupling into the beam path, three effective, microscopic, multispectral, lightemitting light spots S1 , S2, and S3 are formed that correspond to the light emitting spots So1 , So2 and So3. The light source unit 103 generates three spatially coherent light beams B1 , B2, and B3 emerging from the effective, microscopically fine, multispectral, light-emitting light spots S1 , S2, and S3, which are spaced in depth and located on a straight line gS. The straight line gS generally coincides with the optical axis OA of the light source unit 103. The multispectral light source unit 103 has the optical axis OAS at the output.

[0319] The main rays SB1 SB2 and SB3 of the spatially coherent light beams B1 , B2 and B3 coincide at least approximately at the output of the light source unit 103 on the optical axis OAS. The wavefronts of the spatially coherent light beams B1 , B2 and B3 have different curvatures, as described in connection with Figure 11 . The further beam path is the same as described in connection with Figure 11 to which a reference is made.

[0320] Figure 13 shows schematically another exemplary light source unit 104, that may be used in the optical system shown in Figure 11. The light source unit 104 may be configured for generating light of high spatial coherence using a broadband SLD system with 300nm HWB in the NIR.

[0321] The light source unit 104 comprises three point light sources 110, 120 and 130 and three single-mode fibers 111 , 121 and 131 that are similar to those described in connection with Figures 11 and 12. The three point light sources 110, 120, and 130 have different centroid wavenumbers kS1 , kS2, and kS3, where kS3 is the maximum wavenumber here and kS1 is the minimum wavenumber. The multispectral light source unit 104 has the optical axis OAS at the output. The spectra of the light sources 110, 120 and 130 are shown in details 13.1 , 13.2 and 13.3 of Figure 13.

[0322] Unlike the light source unit 103, the micro-optical light coupling of the light emitted from each of the point light sources 110, 120 and 130 is performed by microlenses for beam shaping attached at the fiber outputs of each of single-mode fibers 116, 117 and 118 and by two pellicle beam splitters 212 and 213, respectively. For the sake of better viewability, only microlens 211 of pellicle beam splitter 213 corresponding to the point light source 110 is shown in detail 13.4. The coupling of the light emitted from the light sources 120 and 130 is carried out in a similar manner.

[0323] Light from the fiber-coupled point light source 110 is coupled into the illumination beam path via the single-mode fibers 111 and 116, as well as via the y-fiber coupler 112 and via the first pellicle beam splitter 212. Thus, an effective multispectral light emitting spot S1 is formed. At the output of the single-mode fiber 116, a microlens 211 is arranged for beam shaping, as shown in detail 13.4. The microlens 211 may, for example, be attached to the output of the single-mode fiber 116. The single-mode fiber 116 also provides a confocal discriminator for the returning light.

[0324] The light from the point light source 120 is coupled in reflection via the single-mode fibers 121 and 117 and via the y-fiber coupler 122 and by means of the second pellicle beam splitter 213. Thus, an effective multispectral light emitting spot S2 is formed. Further, the single-mode fiber also represents a confocal discriminator for the returning light. A microlens, not shown in figure 13, is arranged at the output of the single-mode fiber 117 for beam shaping. For example, a microlens may be attached to the output of the single-mode fiber 117.

[0325] The light from the point light source 130 is coupled into the illumination beam path in transmission via the single-mode fibers 131 and 118 and via the y-fiber coupler 132 and via the pellicle beam splitter 213. Thus, an effective multispectral light emitting spot S3 is formed. Here, too, a microlens, not shown in figure 13, is arranged at the output of the single-mode fiber 118 for beam shaping. For example, the microlens may be attached to the output of the single-mode fiber 118. The single-mode fiber 118 also represents a confocal discriminator for the returning light.

[0326] Thus, three effective light emitting spots S1 , S2 and S3 exist in the illumination beam path of the multispectral light unit 104. The effective light emitting spots S1 , S2 and S3 generate respective spatially coherent multispectral light beams B1 , B2 and B3 at the output of the light source unit 104. The light from the light beams B1 , B2 and B3 enters the chromatic-confocal Michelson interferometer 6 via the input El thereof. The further optical path of the light through the Michelson interferometer 6 and the returned light from the Michelson interferometer 6 are similar to those described above in connection with the preceding figures and in particular Figure 11 . In particular, in the optical arrangement shown in Figure 13, the returned light from the chromatic-confocal Michelson interferometer 6 is coupled out via the input El after the return light from the reference arm R and the object arm 0 has been combined. More specifically, the light reflected by an end reflector 715 in the reference arm R with its mirrors 731 and 752 is combined with the light reflected by the measured object 8 in the object arm 0 and outputted via the input El, as described in connection with Figure 11.

[0327] The light currently sharply focused on the measured object 8 - i.e., the light of the current wavenumbers k1 , k2 and k3 - impinges sharply focused on the ends of the single-mode fibers 116, 117 and 118, is confocally discriminated there and passes as transmitted light via the y-fiber couplers 112, 122 and 132 and the y-fiber couplers 128 and 133 via the single-mode fiber 129 into a fiber-coupled single-channel spectrometer 281 . The single-channel spectrometer 281 may be optimized for the spectral range of the emitted light, for example for the wavelength range from 700 nm to 1000 nm. The single-channel spectrometer 281 comprise at least one fast line sensor, for example a fast line sensor having about 2000 pixels. Detail 13.5 of Figure 13 shows the three resulting wavelets when an opaque measured object 8 is optically probed in the coverage region UE.

[0328] Figure 14 shows a magnified view of detail 11.5 of Figure 11 with the end reflector 715. As shown in Figure 14, there is exactly one point of contact BP of the osculating spheres of the two spherically curved mirrors 731 and 752 on the optical axis OAR, represented by the osculating circles SK1 and SK2 in the drawing plane.

[0329] The radii of the osculating circles SK1 and SK2 may be set to be slightly different from each other. For example, the radius rS1 of the osculating circle SK1 may be rS1 =0.160 mm and the radius rS2 of the osculating circle SK2 may be rS2=0.144 mm. Detail 14.1 of Figure 14 shows the three differently curved wavefronts WFR1 , WFR2 and WFR3 of the light beams B1 R, B2R and B3R at a point WR on the optical axis OAR of the reference arm R. The point WR is positioned before the end reflector 715, without an optical element being present between point WR and the end reflector 715. Detail 14.2 of Figure 14 shows a magnified view of the light spots S1 OkS1 S20kS2' and S30kS3' formed in the object space, i.e. , on the measured object 8. As shown in detail 14. 2, the light spots S1 OkS1 S20kS2' and S30kS3' of the three centroid wavenumbers kS1 , kS2 and kS3 do not meet exactly in a single point. Thus, between the light spots S1 OkS1 'and S20kS2' there is the depth distance t120kS1 kS2', between the light spots S20kS2' and S30kS3' there is the depth distance t230kS2kS3' and between the light spots S1 OkS1 'and S30kS3' there is the depth distance t130kS2kS3'. Accordingly, there is a small depth range dtc in which all light spots SI OkST, S20kS2' and S30kS3' are located. The depth range dtc is shown enlarged in detail 14.2 and may be only a few percent of the coverage region UE. If, in ideal case, dtc is equal to zero, there a confocal point cPS in the strict sense. Otherwise, in a non-ideal case, a confocal point cPS is approximated by a region dtc extended in the depth direction.

[0330] Figure 15 shows schematically the optical setup of a three-channel detection unit that may be used in the optical systems described above, such as the optical system described in connection with Figure 11.

[0331] The detection unit 162 comprises a focusing objective 79 for three channels, a beam splitter group 44 and a fiber-coupled single-channel spectrometer 281 for detecting three wavelets W1 , W2 and W3. The wavelets W1 , W2 and W3 are shown in detail 15

[0332] The detection unit 162 comprises further, a first single-mode fiber 451 , a second singlemode fiber 452, a third single-mode fiber 453, a first y-coupler 454, a second y-coupler 455, a first adjustable plane-parallel plate 461 with the attached single-mode fiber 452 in the detection beam path, a second adjustable plane-parallel plate 462 with the attached single-mode fiber 453 in the detection beam path, a first confocal discriminator 471 formed as a single-mode fiber end on a glass surface, a second confocal discriminator 472 formed as a single-mode fiber end on a glass surface and a third confocal discriminator 473 formed as a single-mode fiber end on a glass surface. The beam splitter group 44 comprises two parallel arranged beam splitting layers 441 and 442 for splitting the beam paths and may have a structure similar to the beam splitter group 22 described in connection with Figure 12.

[0333] Further, as described in connection with Fig. 12, some of the optical elements may also serve to couple in light from point like sources 110, 120 and 123 into the illumination path. Accordingly, some of the optical components may be shared by the detection unit 162 and a multispectral light source unit.

[0334] After passing the focusing objective 79, arranged in the detection unit 162, splitting occurs in the beam splitter group 44 with two parallel arranged beam splitting layers 441 and 442 for confocal discrimination of the light of the light beams Bt1 , Bt2 and Bt3 at the confocal apertures 261 , 262 and 263, which are shown in Figure 15 only symbolically. Each of the confocal apertures 261 , 262 and 263 acts as a confocal discriminator. With careful adjustment of the confocal apertures 261 , 262 and 263, the superimposed light spots S1 R" and S1 Ok1 can pass at the confocal aperture 261 , the superimposed light spots S2R" and S20k2 can pass at the confocal aperture 262, and the superimposed light spots S3R" and S30k3 can pass at the confocal aperture 263.

[0335] The determination of the depth position zp based on the detected wavelets may be carried out by a suitably programmed depth position computational unit 31 . In particular, for a targeted measurement point of the measured object 8, equation 10 holds true: xp=tan(alpha*) = delta_phi I delta_k, (10) wherein: xp denotes the optical path difference; and k denotes the wavenumber calculated according to 2Pi / Lambda; and alpha* denotes the angle of the rise of the phase phi, when the phase phi as a function of the wavenumber k is approximated as a straight line; and delta_phi / delta k denotes the first derivative of phi, wherein delta_phi is an increment of the phase phi - depending on the wavenumber k - and delta k_k is an increment in the wavenumber k domain.

[0336] (see also the publication [8] with respect to the determination of the measured depth).

[0337] The measured depth zp may be determined from the optical path difference xp according to equation 11 : xp = 2zp (11 )

[0338] In particular, the value of the depth position zp depends on the angle alpha* and in particular on the slope tan(alpha*) of the phase line. The slope tan(alpha*) of the phase line varies with the depth position zp, which is shown symbolically in detail 15.

[0339] Further, the evaluation of the z-position of the measured object 8 can also be performed by via a calculation of the carrier frequency of the spectral wavelets W1 , W2, W3.

[0340] Figure 16 shows an exemplary detection unit 163 that may also be used in the above described optical systems, and in particular in the optical system of Fig. 11 .

[0341] In the example shown in Figure 16, instead of a single-channel spectrometer 281 for the evaluation of the optical interference signals, a miniaturized, fiber-coupled, adapted unbalanced evaluation Michelson interferometer 282 with a tilt is used. The other optical components of the detection unit 163 are similar to the respective optical components of the detection unit 162 described in connection with Figure 15.

[0342] The tilt is realized by a weak tilt of one of the interferometer mirrors. The evaluation Michelson interferometer 282 generates a spatial interferogram rl on a fast detector line such as a CMOS line scan camera 283. The term “tilt” as used here means that there are mutually inclined at least approximately plane wavefronts at the output of the evaluation Michelson interferometer 282, the interference of which is detected as a spatial interferogram rl. In an arrangement according to Figure 11 , the evaluation Michelson interferometer 282 can be used in the detection beam path instead of a single-channel spectrometer 281 .

[0343] The unbalanced evaluation Michelson interferometer 282 generates, when a measured object is located in the object arm 0, a spatial interferogram rl on the line detector (such as a fast CMOS line camera 283). The spatial interferogram rl is used for the determination of the optical path difference xp in the interferometer 6, or 61 at a measurement point on the measured object 8. For this purpose, the evaluation Michelson interferometer 282 with a tilt is configured to at least approximately a mean (average) optical path difference x_mMI on the optical axis 0AM. Thus, the spatial interferogram rl is formed approximately in the center on the fast CMOS line scan camera 283.

[0344] The mean optical path difference x_mMI in the evaluation Michelson interferometer 282 preferably deviates only slightly from the optical path difference xp in the Michelson interferometer 6, which typically results when the measured object 8 is measured in the overlap region UE. The deviation of the two optical path differences x_mMI and xp may be at most in the upper single-digit percentage range of the optical path difference x_mMI and can typically be on the order of 10 to 100 wavelengths of the largest centroid wavelength of the light source unit 103. The deviation in the two optical path difference may be also partly determined based on the pixel count of the CMOS line scan camera 283 (or other line detector).

[0345] The depth position zp of a measuring point on the measured object 8 may be determined based on the optical path difference xp according to equation 11 (xp = 2zp). The depth position zp may, for example, be determined after a calibration via the position of the spatial interferogram rl on the CMOS line scan camera 283.

[0346] For a fast evaluation of a spatial interferogram rl, i.e. , its exact position on the CMOS line scan camera 283, from which the depth position zp of a measuring point on the measured object 8 can be determined, a plurality of approaches are known in the art. The known approaches enable a determination of the position of a spatial interferogram rl on the fast CMOS line scan camera 283 with a high resolution. Based on the determined position, the depth position zp of a measuring point on the measured object 8 can be determined with high resolution, for example with a resolution in a single-digit nanometer range and possibly even below.

[0347] The above described compensation of the focusing power of a diffractive-optical element in the object arm 0, for example by means of a concavely curved substrate rear surface 703, by a thin converging lens 705, in the reference arm R, etc. can be completely omitted if in the evaluation interferometer 282 the optical path difference is set so large that a spatial interferogram rl can be detected approximately in the center of the line detector, for example formed as a fast CMOS line camera 283. In this case, no high-resolution line detector is required, but a line detector (for example a CMOS line scan camera 283) with up to 2000 pixels is sufficient.

[0348] Figure 17 shows schematically another exemplary multispectral light source unit for generation of three spatially coherent light beams B1 , B2 and B3, each having a different centroid wavenumber kS1 , kS2 and kS3, respectively. The multispectral light source unit 104a has the optical axis OAS at the output. The multispectral light source unit 104a may be regarded as a modification of the multispectral light source unit 104 and may be used in the previously described optical systems, in particular in the optical system described in connection with Figure 11 .

[0349] The multispectral light source unit 104a comprises a beam splitter group 23 in a prism form with two edge color splitter layers 195 and 196. The beam splitter group 23 is formed as a three-part cemented prism 24 with the two edge color divider layers 195 and 196. The three-part prism 24 is a prism, known for example from 3-chip camera technology for color splitting into three spectral channels. The edge color-splitting layers 195 and 196 are designed respectively for the centroid wavelengths lambda_S1 , lambda_S2 and lambda_S3. The use of such a prism arrangement with the edge color splitter layers 195 and 196 improves the light efficiency.

[0350] The multispectral light source unit 104a comprises further a broadband SLD system 140 with a half-width HWB of 300nm in the NIR from 700nm to 1000nm, from which the focused light is coupled into an input end of a single-mode fiber 141. The light output from an output end of the single-mode fiber 141 is collimated by a collimator lens 512, thereby forming a collimated light beam. The collimated light beam passes through an isolator 142 and a beam splitter cube 193 and enters the beam splitter group 23. By means of the two edge color splitter layers 195 and 196, the collimated beam passed through the beam splitter cube 193 is spectrally split into three light beams B1 , B2 and B3, the light of which has the centroid wavenumbers k1 S, kS2 and kS3, respectively. The centroid wavenumber k1 S of light beam B1 is the smallest and the centroid wavenumber kS3 of light beam B3 is the largest centroid wavenumber.

[0351] The light transmitted at the edge color splitter layer 195 and reflected at the edge color splitter layer 196 arrives at a weakly convexly curved mirror 197 and is reflected by it, thereby forming a diverging light beam, which leaves the beam splitter group 23, passes the beam splitter cube 193 in reflection and emerges from it as a weakly diverging light bundle B1.

[0352] The light transmitted at the edge color splitter layer 195 and at the edge color splitter layer 196 reaches a plane mirror 198 and is reflected by it, thereby forming a light beam, which leaves the beam splitter group 23, passes the beam splitter cube 193 in reflection and emerges from it as a collimated light beam B2.

[0353] The light reflected at the edge color splitter layer 195 arrives at a weakly concave curved mirror 199, is reflected by it thereby forming a light beam, which leaves the beam splitter group 23, passes the beam splitter cube 193 in reflection and emerges from it as a weakly focused light bundle B3.

[0354] Thus, each wavefront of the three light beams B1 , B2 and B3 has a different radius of curvature. The light of the three light beams B1 , B2 and B3 impinges on a focusing lens 52 of the light source unit 104a. The focusing lens 52 focuses the three light beams B1 , B2 and B3 with the main rays SB1 , SB2 and SB3, wherein the main rays SB1 , SB2 and SB3 generally coincide with the optical axis OAS at the output of the multispectral light source unit 104a. At the point W at the output of the multispectral light source unit 104a, the different radii of curvature of the three wavefronts of the three light beams B1 , B2 and B3 in the paraxial region are r1 pW, r2pW and r3pW, respectively.

[0355] The light of the three light beams B1 , B2 and B3 enters the chromatic-confocal Michelson interferometer 6 via an input El. The chromatic-confocal Michelson interferometer 6 is constructed as a spectral interferometer with a diffractive-optical element 70 in the object arm 0 and has a non-zero optical path difference for a measured object 8 and is shown here only symbolically. The chromatic-confocal Michelson interferometer 6 may be a chromatic-confocal Michelson interferometer 6 as described above in connection with the optical systems of the preceding figures.

[0356] In the reference arm R of the chromatic-confocal Michelson interferometer 6 an end reflector 715 is arranged, which is shown in detail in Figure 14. As shown in Figure 15, the end reflector 715 has an inner spherically curved convex and thereby rotationally symmetrical mirror 731 and an outer spherically curved concave and thereby rotationally symmetrical mirror in ring form 752.

[0357] Figure 17 shows the light spots S1 R', S2R' and S3R' formed in the reference arm R, corresponding to the three light beams B1 , B2 and B3. The entire optical path in the chromatic-confocal Michelson interferometer 6 is shown for example in Figure 11, to which a reference is made. The interfering light leaves the chromatic-confocal Michelson interferometer 6 via an output X and enters the detection unit 162 with the focusing objective 79 for three channels not shown here with the fiber-coupled singlechannel spectrometer 281 not shown here. By means of the digital computing means 31 , the calculation of the depth position zp of the measured object 8 is performed.

[0358] Various further modifications of the above described multispectral light source units are possible.

[0359] For example, in a first modification of the multispectral light source unit 140a (without a figure), instead of the broadband SLD system 140a, a free-beam laser in the nearinfrared spectral range may be employed in combination with the beam splitter group 23. The parallel light beam emitted from the free beam laser is directly directed into the beam splitter group 23 via the isolator 142, resulting in the beam path already described in Figure 17.

[0360] In a second modification of the multispectral light source unit 140a (without a figure), focused light emitted from a broadband SLD system 140 (for example a broadband SLD system with a half-width HWB of 300 nm in the NIR from 700 nm to 1000 nm) is coupled into the single-mode fiber 141. By means of the collimator lens 512, a light beam is formed. However, the light beam is not collimated, but weakly focused. Further, instead of the three mirrors 197, 198 and 199 with different curvature, three plane mirrors are arranged in the beam splitter group 23 at different distances (in a depth direction). Thus, a separation in depth of effective light spots S1 , S2 and S3 can also be achieved.

[0361] Figure 18 shows schematically an exemplary end reflector 713 that may be used in the above described optical systems, and in particular in the optical system shown in Figure 11. The end reflector 713 may, for example, be similar to the end reflector described in connection with Figure 3. In Figure 18, the angles of the beams are greatly enlarged for a better viewability.

[0362] The end reflector 713 comprises two spherically curved convex and rotationally symmetrical mirrors 731 and 732, that are arranged concentrically to each other. The outer mirror 732 has a ring form and is connected by the step 754 to the inner spherically curved convex and rotationally symmetrical mirror 731 .

[0363] The osculating spheres, represented in the drawing plane by the osculating circles SK1 and SK2 of the convex mirrors 732 and 731 , respectively, touch at a single point of contact BP. The osculating spheres with the osculating circles SK1 and SK2 are the (imaginary) spheres encompassing and extending from the respective spherically curved mirrors 732 (with the respective osculating circle SK1 ) and 731 (with the respective osculating circle SK2), respectively. The contact point BP lies on the optical axis OAR of the reference arm.

[0364] The centers the osculating circle circles SK1 and SK2, and thus of the spherically curved mirrors 732 and 731 , are M1 and M2 respectively. The centers M1 and M2 lie on the optical axis OAR and are arranged at a distance t12R’ from each other.

[0365] The osculating circle SK1 of the convex mirror 732 has a radius rS1 = t13R’, which is greater than the radius rS2 = t23R’ of the osculating circle SK1 of the convex mirror 731.

[0366] In the example shown in Figure 18, the distances t13R’, t23R’ and t12R’ are set such as to fulfill the following conditions: t13R’ = rS1 = r1 - r2; t12R’ = r1 - r2; and t23R‘ = rS2 = r2 - r3, wherein: r1 denotes the radius of curvature of the wavefront of a first multispectral reference light beam B1 R in the reference arm R at a point WR immediately in front of an end reflector, without any other optical component between the point WR and the end reflector; r2 denotes the radius of curvature of the wavefront of a second multispectral reference light beam B2R in the reference arm R immediately at the point WR; and r3 denotes the radius of curvature of the wavefront of a third multispectral reference light beam B3R in the reference arm R at the point WR.

[0367] Figure 18 shows further the light spots S1 R’, S2R’ and S3R’ formed by the reference light beams B1 R, B2R and B3R, respectively. A first virtual light spot S1 R’ formed by the reference beam B1 R coincides with the center M1 of the outer convex mirror 732. A second virtual light spot S2R’ formed by the reference beam B2R coincides with the center M2 of the inner convex mirror 731 . A third (real) light spot S3R’ is formed at the contact point BP by focusing the third reference beam B3R at a point AR on the surface of the inner convex mirror 731 , wherein the point AR lies on the optical axis OAR. The point AR is thus also a point of incidence of the third reference beam AR. Further, the point AR coincides with the contact point BP:

[0368] As shown in Figure 18, the reference light beams B1 R, B2R and B3R are reflected in themselves by the end reflector 713. The main rays SB1 R, SB2R and SB3R of the reference light beams B1 R, B2R and B3R, respectively, coincide with the optical axis OAR of the reference arm R.

[0369] Figure 19 shows another exemplary end reflector 716, which may be used in the above described optical systems, and in particular in the optical system shown in Figure 11 . In Figure 19, the angles of the beams are shown greatly enlarged for a better viewability.

[0370] The end reflector 716 is formed as a mirror group in the shape of a cup. The end reflector 716 is formed by two mirrors 732 and 751 : an inner, spherically curved, concave and rotationally symmetrical mirror 751 centered on the optical axis OAR and an outer convex rotationally symmetrical mirror 732 in a ring form centered on the optical axis OAR, wherein the inner concave mirror and the outer convex mirror are connected with a step 756. The osculating spheres of the spherically curved mirrors 732 and 751 touch in a single point of contact BP, which lines on the optical axis OAR. The respective osculating circles (which are cross-sections of the respective osculating spheres in the plane of the drawing) are SK1 for the outer convex mirror 732 and SK2 for the inner concave mirror 756.

[0371] The centers the osculating circle circles SK1 and SK2, and thus of the spherically curved mirrors 732 and 751 , are M1 and M2 respectively. The centers M1 and M2 lie on the optical axis OAR and are arranged at a distance t13R’ from each other.

[0372] The osculating circle SK1 of the convex mirror 732 has a radius rS1 = t12R’, which is greater than the radius rS2 = t23R’ of the osculating circle SK1 of the concave mirror 751.

[0373] In the example shown in Figure 19, the distances t13R’, t23R’ and t12R’ are set such as to fulfill the following conditions: t13R’ = r1 — r3; t12R’ = rS1 = r1 - r2; and t23R‘ = rS2 = r2 - r3, wherein: r1 , r2 and r3 denote the radius of curvature of the wavefront of a first, second and third multispectral reference light beams B1 R, B2R and B3R, respectively, in the reference arm R at a point WR immediately in front of an end reflector, without any other optical component between the point WR and the end reflector.

[0374] Figure 19 shows further the reference light spots S1 R’, S2R’ and S3R’ formed by the reference light beams B1 R, B2R and B3R, respectively.

[0375] A first virtual light spot S1 R’ formed by the reference beam B1 R coincides with the center M1 of the outer convex mirror 732. A second (real) light spot S2R’ is formed at the contact point BP by focusing the second reference beam B2R at a point AR on the surface of the inner concave mirror 751 , wherein the point AR lies on the optical axis OAR. The point AR is thus also a point of incidence of the third reference beam AR. Further, the point AR coincides with the contact point BP. A third (real) light spot S3R’ is formed at the center M2 of the inner concave mirror 751 by the light of the third reference light beam B3R reflected by the inner concave mirror 751.

[0376] As shown in Figure 19, the reference light beams B1 R, B2R and B3R are reflected in themselves by the end reflector 716. The main rays SB1 R, SB2R and SB3R of the reference light beams B1 R, B2R and B3R, respectively, coincide with the optical axis OAR of the reference arm R. Detail 19 of Figure 19 shows a magnified view of the light spots S1 OkS1 S20kS2' and S30kS3' formed in the object space, i.e. , on the measured object 8. As shown in detail 19, the light spots S1 OkS1 ', S20kS2' and S30kS3' of the three centroid wavenumbers kS1 , kS2 and kS3 do not meet exactly in a single point. There is a small depth range dtc in which all light spots SI OkST, S20kS2' and S30kS3' are located. The depth range dtc is shown enlarged in detail 19 and may be only a few percent of the coverage region UE. If, in ideal case, dtc is equal to zero, there a confocal point cPS in the strict sense. Otherwise, in a non-ideal case, a confocal point cPS is approximated by a region dtc extended in the depth direction.

[0377] Figure 20 shows schematically a reference arm R comprising an assembly 719 of end reflectors 713 with parallel optical axes in the form of a linear array. The end reflectors 713 may the previously described end reflectors. The assembly 719 may be used in an optical arrangement enabling a single-shot measurement of the measured object 8 at many measurement points of the measured object arranged along a line. The respective multispectral light source unit may be respectively configured as an array of individual multispectral light source sub-units with parallel optical axes. Further, the detection unit may be accordingly designed as a multi-channel optical arrangement.

[0378] In an example without a figure, the end reflectors may be formed with parallel optical axes in the form of a two-dimensional, i.e., planar array. This enables an arrangement for a single-shot measurement of the measured object with a plurality of measuring points in an area of the measured object 8. To enable such measurement, the multispectral light source unit may be formed as a two-dimensional array of individual multispectral light sub-units. Further, the detection unit may be formed as a multichannel array.

[0379] Figure 21 shows schematically a multispectral light source unit 105 for generating light of high spatial coherence with a plurality of “n” triads of effective microscopic multispectral light emitting light spots, wherein the triads are arranged in an array. For the sake of better visualization, only the effective light spots S1 , S2, and S3 of one triad (the n-th triad) group are shown. Here, n is an integer that may, for example, be less than 1000.

[0380] The multispectral light source unit 105 is capable of generating light of high spatial coherence having a plurality of arranged triples of effective microscopic multispectral light emitting light spots S1 , S2 and S3.

[0381] The light spots may be formed by a plurality of groups of three multispectral point light sources each, which are optically separated in a depth direction. The light spots in each group have different centroid wavenumbers kS1 , kS2, and kS3. Any of the above described light sources configured to generate three effective microscopic multispectral light emitting light spots S1 , S2 and S3 may be employed.

[0382] For example, to generate the effective microscopic multispectral light emitting light spots S1 , S2 and S3, the light from three point light sources (not shown) with different spectra may be coupled into a common beam path via beam splitters (not shown). As an example for the coupling of three point light sources, reference is made to Figure 12 and the respective description.

[0383] The three point light sources of each group have, as described above, different centroid wavenumbers kS1 , kS2 and kS3. The imaging of all effective light spots into a spectral interferometer may be performed by means of a telecentric two-stage imaging stage with the focusing objective 5 as the first objective of the same. For this purpose, the telecentric imaging stage may also be field corrected.

[0384] In another example without a figure a multi-channel optical system may comprise an array of “n” triads of effective microscopic multispectral light emitting light spots as shown in Figure 21 in combination with an array of end reflectors 714 as described in connection with Figure 22 and a multi-channel detection system with a plurality of line spectrometers operating in parallel. The multi-channel detection system may, for example, comprise three or more spatially resolving spectrometers. The spatially resolving spectrometers can also be referred to as multichannel spectrometers. Thus, an optical system configured for a measurement of many measuring points arranged in an area of the measured object 8 may be realized in connection with a multispectral light source unit designed areally and a detection unit designed as a multichannel arrangement for the detection of light spots in a whole area.

[0385] Figure 22 shows an exemplary end reflector 714 in the reference arm R. The end reflector 714 may be used in the optical systems described above.

[0386] The end reflector 714 has an inner spherically curved concave centered mirror 751 and an outer spherically curved concave centered mirror 752, wherein the inner concave mirror 751 and the outer concave mirror 752 are connected by a step 757. The outer concave mirror 752 is in the form of a ring enclosing the inner spherically curved concave mirror 751 . The osculating sphere of the inner concave mirror 751 and the outer concave mirror 752 touch in each other in a single point of contact BP (tangential point) which lies on the optical axis OAR of the reference arm.

[0387] The osculating spheres of the two mirror surfaces 751 and 752 are represented by the osculating circles SK1 and SK2, which are the cross sections of the osculating spheres in the plane of the drawings. The end reflector 714 can reflect the three reference light beams B1 R, B2R and B3R in the reference arm R to form light spots S1 R’, S2R’ and S3R’, respectively. The first light spot S1 R’ is formed by the light of the first reference light beam B1 R, which is reflected from the outer concave mirror 752 and focused in the center point M1 of the outer concave mirror 752. The second light spot S2R’ is formed by the light of the second reference light beam B2R, which is reflected from the inner concave mirror 751 and focused in the center point M2 of the inner concave mirror 752. The third light spot S3R’ is formed by the light of the third reference light beam B3R, which is focused in a point AR on the inner concave mirror 751 lying on the optical axis OAR. The point AR coincides with the contact point BP. The radiuses of curvatures of the mirrors 251 and 252 are set such as the following conditions are fulfilled: t13R’ = rS1 = r3 - r1 ; t23R’ = rS2 = r3 - r2; and t12R’ = r2 - r1 , wherein: r1 , r2 and r3 denote the radius of curvature of the wavefront of a first, second and third multispectral reference light beams B1 R, B2R and B3R, respectively, in the reference arm R at a point WR immediately in front of an end reflector, without any other optical component between the point WR and the end reflector. rS 1 denotes the radius of curvature of the outer concave mirror 757; rS2 denotes the radius of curvature of the inner concave mirror 751 (rS 1 > rS2); and t12R’ denotes the distance between the center points M1 and M2.

[0388] The reference BD1 R, BD2R denotes the portion of the reference beam B1 R that passes through a confocal discriminator in the detection path and contributes to the detected signal.

[0389] Figure 23 shows an exemplary a split reference beam arm (reference beam path) with two reference sub-arms R1 and R2. The split reference beam arm / path may be used in one of the optical systems described above.

[0390] The split reference arm comprises a color splitter layer 785, a first end reflector 782 in the first reference sub-arm R1 comprising or consisting of a spherically curved concave mirror 781 and a second end reflector 784 arranged in the second reference sub-arm R2 comprising or consisting of a spherically curved concave mirror 783.

[0391] The color splitter layer 785 may be a color splitter layer with an “edge” characteristic, i.e., light of one spectral range is transmitted, and light of the other spectral range is reflected. The transmission area and the reflecting ranges are separated by a steep flank. The steep flank may extend in a spectral range of equal or less of 10% the wavelength at 50% transmission of the light. By using the color splitter layer 785, the two reference sub-arms R1 and R2 are optically coupled. Two light beams B1 R and B2R formed of light with shorter wavelengths pass through the color splitter layer 785 by transmission into the first reference sub-arm R1 and are reflected there at the concave, spherically curved mirror 781 of the end reflector 782. One light beam B3R formed of light with a wavelength longer than the wavelengths of the light beams B1 R and B2R is reflected by the color splitter layer 785 into the reference sub-arm R2 and is reflected there at the concave, spherically curved mirror 783 of the end reflector 784.

[0392] The spherically curved concave mirror 781 in the first reference arm R1 has a radius of curvature rS1 . The spherically curved concave mirror 783 in the second reference arm R2 has a radius of curvature rS2. In the example shown, the two concave, spherically curved mirrors 781 and 783 have different radiuses of curvature rS1 and rS2.

[0393] The first reference light beam B1 R1 propagating in the first reference sub-arm R1 is reflected by the first concave mirror 781 and focused after reflection in the center point M1 of the first concave mirror 781 . Thereby a first light spot S1 R’ is formed at the center point M1.

[0394] The second reference light beam B2R1 propagating in the first reference sub-arm R1 is focused in a point of incidence AR on the first concave mirror 781 , wherein the point of incidence AR1 lies on the optical axis 0AR1 of the first reference sub-arm R1 . Thereby, a second light spot S2R’ is formed at the point of incidence AR1 . The point of incidence AR1 is also the point at which the main rays SB1 R1 and B2R1 of the first reference light beam B1 R1 and the second reference light bam B2R1 , respectively hit the first concave mirror 781 .

[0395] The third reference light beam B3R2 propagating in the second reference sub-arm R2 is focused in the center point M2 of the second concave mirror 783 lying on the optical axis 0AR2 of the second reference sub-arm R2. Thereby, a third light spot S3R’ is formed at the center point M2. The main ray SB3R2 of the second reference beam B3R2 propagating in the second reference sub-arm R2 hits the second concave mirror at a point of incidence AR2 lying on the optical axis 0AR2 of the second reference sub-arm R2.

[0396] The points of incidence AR1 and AR2 are ideally optically conjugate, which is illustrated in Figure 23 by the circle K12 on which the two points of incidence AR1 and AR2 lie. Thus, the optical path for the light in the two reference arms R1 and R2 is the same for the main rays SB1 R1 , SB2R1 and SB3R2. In a real case, an approximation to this ideal case of equality of the optical paths, which are represented by the distances TRAR1 and TRAR2, in the two reference arms R1 and R2 requires careful adjustment.

[0397] Figure 24 shows an exemplary end reflector 717 in the reference arm R. The end reflector 717 is configured to reflect four light beams B1 R, B2R, B3R and B4R propagating in the reference arm and to generate four light spots S1 R’, S2R’, S3R’ and S4R’, respectively. The end reflector 717 may be used in one of the above described optical systems.

[0398] The end reflector 717 has three spherically curved concave and rotationally symmetrical mirrors 751 , 752 and 753: an inner spherically curved concave centered mirror 751 , a first outer ring-shaped spherically curved concave centered mirror 752, and a second outer ring-shaped spherically curved concave centered mirror 753. The three spherically curved concave mirrors 751 , 752 and 753 are arranged concentrically and centered on the optical axis OAR of the reference arm. The inner concave mirror 751 is connected to the first outer ring-shaped concave centered mirror 752 by the step 757 and the first outer ring- concave mirror 752 is connected to the second outer ring- shaped concave mirror 753 by the step 758.

[0399] The osculating spheres of the three spherically curved mirrors 751 , 752 and 752 with the osculating circles SK3, SK2 and SK1 , respectively, touch each other in one common point BP, which is the only common point. At the point BP the tangents to the three spherically curved mirrors coincide. The point BP lies on the optical axis OAR of the reference arm. The point BP coincides with the point of incidence AR of the main rays SB1 R, SB2R, SB3R and SB4R of the four reference light beams B1 R, B2R, B3R and B4R in the reference arm, respectively.

[0400] With the above arrangement of the reference arm with the end reflector 717, the following light spots S1 R’, S2R’, S3R’ and S4R’ are formed:

[0401] - Light spot S1 R’ at the center M1 of the second outer concave mirror 753, formed by the first reference light beam B1 R reflected by the second outer concave mirror 753 and focused in the center M1 of the second outer concave mirror 753;

[0402] - Light spot S2R’ at the center M2 of the first outer concave mirror M2, formed by the second reference beam B2R reflected by the first outer concave mirror 752 and focused in the center M2 of the first outer concave mirror 752;

[0403] - Light spot S3R’ at the center M3 of the inner concave mirror 751 , formed by the third reference beam B3R reflected by the inner concave mirror 751 and focused in the center M3 of the inner concave mirror 751 ;

[0404] - Light spot S4R’ formed at the point of incidence AR (which coincides with the contact point BP), formed by the fourth reference beam B4R focused at the point of incidence AR on the inner concave mirror 751 .

[0405] The four spots are spatially separated from each other in the depth direction, wherein the following conditions apply: t14R’ = rS1 = r4 - r1 ; t24R’ = rS2 = r4 - r2; t34R‘ = rS3 = r4 -r3; r4 = rS1 + r1 , wherein: r1 , r2 , r3 and r4 denote the radius of curvature of the wavefront of a first, second, third and fourth multispectral reference light beams B1 R, B2R, B3R and B4R, respectively, in the reference arm R at a point WR immediately in front of an end reflector, without any other optical component between the point WR and the end reflector; rS 1 denotes the radius of curvature of the second outer concave mirror 753; rS2 denotes the radius of curvature of the first outer concave mirror 752; rS2 denotes the radius of curvature of the inner concave mirror 751 ; and rS1 > rS2 > rS3.

[0406] The object arm 0 of the dual-beam interferometer may be also configured such that four light spots SI OkST, S20kS2’, S30kS3’ and S40kS4’ with the centroid wavenumbers kS1 , kS2, kS3 and kS4, respectively. Detail 24 of Figure 24 shows the conditions in the object arm 0 with the optical axis OAO in the object arm. As described above, ideally, all four light spots SI OkST, S20kS2’, S30kS3’ and S40kS4’ coincide in a single confocal point cPS. However, in a real optical arrangement, there is no confocal point cPS, but a depth region dtc. Within this region dtc the somewhat separated light spots SI OkST, S20kS2', S30k3', and S40kS4' with their respective associated centroid wavenumbers kS1 , kS2, kS3 and kS4. Since this range dtc is small compared to the coverage range (overlap range) UE, the range dtc may be regarded as an approximation of a confocal point cPS.

[0407] Figure 25 shows an exemplary end reflector 718 in a reference arm R in a rosette form. The end reflector 718 may be used in one of the above described optical systems.

[0408] As shown in detail 25.1 of Figure 25, the end reflector 718 in a rosette shape comprises a plurality of mirror segments having different curvatures, the mirror segments being in form mirror circle sectors. For example, the end reflector 718 may comprise three, four, six or even eight mirror segments, with adjacent mirror segments each having a different curvature with a radius of curvature rS. The end reflector 718 (for example when used in a chromatic-confocal Michelson interferometer 6) can be movable by a suitable movable device (e.g., actuator such as a piezo-actuator, motor, etc.), in order to be able to adjust its position in the three spatial coordinates. The end reflector 718 may be miniaturized when used in the UV to NIR range.

[0409] In the example shown in Figure 25, the end reflector 718 comprises three convex mirror segments 761 , 762 and 763 each having the radius of curvature rS 1 and three convex mirror segments 771 , 772 and 773 each having the radius of curvature rS2. The different curvatures are shown in the section cut along the line C-C passing through the center of the end reflector 718. shown in details 25.2 and 25.3 of Figure 25. The radius of curvature rS2 is smaller than the radius of curvature rS 1 . The two osculating circles SK1 and SK2

[0410] The mirror segment 771 covers the point of contact BP of the two osculating circles SK1 and SK2 with the radiuses rS1 and rS2, respectively. The point of contact BP is also the point of incidence AR of the main rays S1 R', S2R' and S3R' of reference light beams B1 R, B2R and B3R, respectively. The point of incidence AR lies on the first convex mirror segment 771 with the radius of curvature rS2, the point of incidence AR coinciding with the point of contact BP. The point AR (and BP) lies on the optical axis OAR of the reference arm.

[0411] The mirror area of mirror segment 771 is set such as to be still large enough to reliably reflect a microscopically fine light spot S3R' formed by a reflection of the third reference light beam B3R focused in point AR (and thus BP).

[0412] Since the osculating circles SK1 and SK2 of the osculating spheres as well as the main rays SB1 R, SB2R and SB3R meet at the point of contact BP, it is ensured that the optical path for the main rays SB1 R, SB2R and SB3R in the reference arm R is always the same and thus also for the spatially coherent bundle parts BD1 R and BD2R and the light bundle B1 R.

[0413] It should be noted that only the parts BD1 R and BD2R of the light beams B1 R and B2R, respectively, come to detection after confocal discrimination in a detection unit (for example the detection unit 162). In detail 25.3 of Figure 25 the parts that pass through the respective confocal discriminator are indicated by a rough hatching Practically, a division of the wavefront of the bundles B1 R and B2R occurs due to the effect of confocal discrimination, because the parts of the light beams reflected at a mirror of the end reflector not fitting in the radius of curvature of the mirror cannot form a sharp focus during confocal discrimination and are thus excluded from detection. The loss of light intensity due to the splitting of the wavefront by the effect of confocal discrimination is, however, not critical, since usually the intensity of the reference light exceeds that of the light from the object arm anyway.

[0414] With the above arrangement of the reference arm with the end reflector 718, the following light spots S1 R’, S2R’, S3R’ are formed:

[0415] - A virtual light spot S1 R’ at the center M1 of the mirror segment 761 , formed by the first reference light beam B1 R;

[0416] - A virtual light spot S2R’ at the center M2 of the mirror segment 771 , formed by the second reference beam B2R;

[0417] - A light spot S3R’ at the incident point AR (coinciding with the point BP) formed by the third reference beam.

[0418] The three spots are spatially separated from each other in the depth direction, wherein the following conditions apply: t12R’ = rS1 = r - rS2r1 ; t23R’ = rS2; t13R‘ = rS1 , wherein: r1 , r2 and r3 denote the radius of curvature of the wavefront of a first, second and third multispectral reference light beams B1 R, B2R and B3R, respectively, in the reference arm R at a point WR immediately in front of an end reflector, without any other optical component between the point WR and the end reflector.

[0419] The object arm 0 of a the dual-beam interferometer may be also configured such that three light spots S1 OkST, S20kS2’ and S30kS3’ with the centroid wavenumbers kS1 , kS2 and kS3, respectively. Detail 25.3 of Figure 25 shows the conditions in the object arm 0 with the optical axis OAO in the object arm. As described above, ideally, all four light spots SI OkST, S20kS2’ and S30kS3’ formed within a range dtc that is small compared to the coverage region UE.

[0420] Figure 26 shows an exemplary multispectral light source unit 106 configured to generate six multispectral spatially coherent light beams B1 to B6 of different wavefront curvatures. Light beams B1 to B3 are from a multispectral, broadband point light source with light of centroid wavenumber kS1 , and light beams B4 to B6 are formed from a multispectral, broadband point light source with light of centroid wavenumber kS2. The multispectral light source unit 106 may be used in the above described optical systems, for example in combination with the end reflector 718.

[0421] The multispectral light source unit 106 comprises a multispectral, broadband light source 150, a 50:50 beam splitter 181 , a first fixed (i.e., not movable) rosette reflector 744 in a split beam path in a transmission arm in the light source unit 106, a second fixed rosette reflector 745 arranged in a split beam path in the reflection arm in the light source unit 106, a beam splitter with a color splitter layer 182 for beam splitting and beam combining in the light source unit 106 and a focusing lens 5. A collor spliter layer 182 may also be used in a light source unit 107 or 107a.

[0422] The broadband light source 150 may comprise an SLD system, for example be an SLD system emitting light in the spectral range of 750 nm to 959 nm with an integrated collimator lens. Thus, the broadband light source 150 emits light with substantially plane wavefronts. The multispectral, broadband light source 150 has two overlapping spectral regions, shown in details Det. 26.4 and 26.5 of Figure 26.

[0423] The light emitted from the multispectral, broadband light source 150 passes the 50:50 beam splitter 181 for coupling in transmission. Only the used light beams (i.e., the transmitted light beams) are shown in Figure 26, the light reflected away from the beam splitter 181 is not shown.

[0424] Downstream of the transmitted light, a beam splitter 182 with a color splitter layer is arranged for beam splitting and beam unification. The light which passes beam splitter 182 reaches the fixed rosette reflector 744 with spherically curved mirrors in the outer form of circle segments which have radii of curvature rS1 to rS3, that may be comparatively large. To be noted is, however, that the radii of curvature rS1 to rS3 are depicted quite exaggerated for the sake of better viewability. The first rosette reflector 744 is arranged in the transmission arm in the light source unit 106. The light reflected from the beam splitter 182 is incident on the fixed rosette reflector 745, which is also formed with spherically curved mirrors in the form of circular segments, which, however, are curved somewhat more than those of the rosette reflector 744, but still have comparatively large radii of curvature rS4 to rS6. This is also shown quite exaggerated with respect to the curvature of the mirrors. Det. 26.2 and 26.3 show schematically a front view of the rosette reflectors 744 and 745, respectively.

[0425] The multispectral light source unit 106 of Figure 26 generates at its output at a point lying on the optical axis of the light source unit W six multispectral, spatially coherent light beams B1 to B6 of different wavefront curvatures, which are not shown in Figure 26.

[0426] In the reference arm R of a dual-beam interferometer, such as the previously described chromatic-confocal Michelson interferometer 6, the respective wavefronts WF1 R to WF6R of the resulting reference light beams B1 R to B6R corresponding to the light beams B1 to B6 are shown symbolically in detail 26.1 of Figure 26 together with the associated light spots S1 R' to S6R'. The light spots S1 R' to S6R' are separated in the depth of the reference arm R. Here, the light spots S1 R' to S3R' are formed with light from the spectral range HWB 1 , while the light spots S4R' to S6R' are formed with light from the spectral range HWB 2.

[0427] The dual-beam interferometer may be in particular, a chromatic-confocal Michelson interferometer (not shown), wherein the reference arm is split into two sub-arms R1 and R2. In each of the sub-arms, an end-reflector in a rosette form may be employed.

[0428] The two end-reflectors in the respective sub-arms R1 and R2 may, for example, be two fixed rosette reflectors optically coupled via a beam splitter with a color splitter layer of the same optical specification as beam splitter 182. The angular orientation and their segmentation of the rosette end reflectors in the sub-arms R1 and R2 may be generally the same as rosette reflectors 744 and 745 of light source unit 106, but the radii of curvature of the mirrors may be generally different.

[0429] Figure 27 shows schematically a region in the object arm 0 of a dual-beam interferometer, which may be used in an optical system comprising the multispectral light source unit 106 generating six spatially coherent light beams B1 to B6 of different wavefront curvatures and having spectra with different centroid wavenumbers kS1 to kS6. Details 27.1 and 27.2 show schematically the spectra of two exemplary light beams.

[0430] Figure 27 shows an ideal case, in which the light spots corresponding to the centroid wave number of the light beams B1 to B6 in the object arm coincide in a single point cPO123456. In other words, a sixfold optical probing of each measurement point of the measured object 8 is realized. In many cases, two-, three- or four-fold optical probing of a measured object 8 is sufficient, i.e. , probing with only two, three or even four light beams, although four-fold optical probing is also technically demanding. The main rays SB1 O to SB6O of the respective light beams in the object arm 0 as well as their wavefronts WF1 to WF6 before the beam splitting, which have a different curvature in a point W, are shown in Fig. 27.

[0431] The point cPO123456 is generated in the object arm 0 by the six light beams B1 O1 , B2O2, B3O3, B4O4, B5O5, B6O6 with their main rays SB1 O, SB2O, SB3O, SB4O, SB5O, SB6O, whereby it is assumed that the main rays are always at least approximately collinear even of light of different wavenumbers. The six light beams B1 Ok11 , B20k12, B30k13, B40k21 , B50k22, B60k23 are represented graphically only symbolically and only by three light beams. The six light beams B1 Ok11 , B2Ok12, B30k13, B40k21 , B50k22, B60k23 are created by diffraction at a diffractive-optical element 70, such as a diffractive-optical element described above. Each of the six light beams B1 Ok11 , B20k12, B30k13, B40k21 , B50k22, B60k23 currently generates a sharply imaged light spot S1 Ok11 ', S20k12', S30k13', S40k21 ', S50k22', S60k23' on the measured object 8 at the point cP0123456. This is because their always different wavenumbers k12, k12, k13, k21 , k22, k23 in the respective sharp light spots S1 Ok11 ', S20k12', S30k13', S40k21 ', S50k22', S60k23' on the measured object 8 match the current depth position of the measured object 8 at the moment of measurement. Each temporal change of the depth position of the measured object 8 also simultaneously changes the wavenumbers k11 , k12, k13, k21 , k22 and k23, which currently forms a sharp light spot S1 Ok11 ', S20k12', S30k13', S40k21 ', S50k22', S60k23' on the measured object 8. The wavenumbers k11 , k12, k13, k21 , k22 and k23 are each at least approximately in the center of their associated wavelet on the wavenumber axis, which is a consequence of confocal discrimination.

[0432] Figure 28 shows schematically a detection unit 164 for six light beams B1 to B6, in which there is confocal discrimination at only one location, given by the fiber end 474 of the single-mode fiber 475 comprised in the detection unit 164. In other words, the fiber end 474 of the single-mode fiber 475 acts as a confocal discriminator. The detection unit 164 may be used in combination with the multispectral light source unit 106 described in connection with Figure 26, for example.

[0433] In particular, the detection unit 164 may be employed for detection of the resulting wavelets for the arrangements shown in Figures 26 and 27. In the detection unit 164, two fixed rosette reflectors 744a and 745a may be arranged for this purpose. The two fixed rosette reflectors may be identical in construction to the rosette reflectors 744 and 745 from the arrangement shown in Figure 26. This ensures that mirror segments of the same curvature from the light source unit 107 (and 107a) and the detection unit 164 are always in optical contact.

[0434] The light coming from the chromatic-confocal Michelson interferometer 6 with chromatic depth splitting of light spots in the object arm 0 enters the detection unit 164 via the collimator objective 791 arranged in the detection beam path before the detection unit 164. The light is coupled via the broadband beam splitter 185 in the detection unit 164 and the color beam splitter 186 in the detection unit 164 splits the light into two spectral ranges which meet the rosette reflectors 744a and 745a. The rosette reflectors have different radii of curvature rS1 to rS3 in the repeating mirror segments of the rosette reflector 744a and different radii of curvature rS4 to rS6 in the repeating mirror segments of the rosette reflector 745a, which exactly balance the depth position of the light spots S1 R' to S6R' as well as that of the light from the object arm 0.

[0435] The light from the object arm 0 comprises the at least approximately sharply focused light spots on the measured object 8 around the respective matching wavenumber k1 to k6 and is in two spectral regions. Accordingly, there is a comparatively small spectral range from which the respective narrowband spectral wavelet W1 to W6 emerges. The spectral wavelets W1 to W6 are shown in Figure 29.

[0436] After passing the color beam splitter 186 and the broadband beam splitter 185, the interfering light comes via the focusing objective 792 for confocal discrimination at the confocal discriminator 474, which is formed as the fiber end of the single-mode fiber 475. The focusing objective 792 is arranged in the detection beam path. One light spot S1 R" to S6R" from each of the reference arms R1 and R2 and one light spot S10k1" to S60k6" from the object arm 0 form six pairs of interfering light spots, as schematically shown in Figure 28. Each pair is formed from light in a narrow spectral range around the centroid wavenumber k11 , k12, k13, k21 , k22 and k23 that exactly matches the current depth position zp of the currently measured point of the measured object 8. The detection unit 164 comprises further a fiber-coupled single-channel spectrometer 281 configured to detect light in the corresponding spectral range. For example, the single-channel spectrometer 281 may be configured to detect light in the wavelength range from 700 nm to 1000 nm with a fast line sensor with 2000 pixels then enables detection of the interfering light in the form of six spectral wavelets W1 to W6. The frequency and the position of the spectral wavelets W1 on the wavenumber axis depends only on the current depth position zp of the measured object 8 and may be determined by known methods.

[0437] Figure 29 shows schematically the six resulting wavelets W1 to W6 which can be generated by a light source unit 106 and a dual-beam interferometer 6 and detected by a suitable detection unit, such as the dual-beam interferometer and the detection units described above.

[0438] In the example shown in Figure 29, six reference light beams B1 R to B6R exist in the dual-beam interferometer s. A measurement point cP0123456 on the measured object 8 is optically scanned by six object light beams S1 Ok11 ', S20k12', S30k13', S40k21 ', S50k22' and S60k23'. Wavelets W1 to W3 are formed of light in the spectral range of half-width HWB1 and the exact matching wavenumbers are k11 , k12, k13. The wavelets W4 to W6 are formed of light in the spectral position range of the half-width HWB2 and the exactly matching wavenumbers are k21 , k22, k23. The phase phi of the wavelets over the wavenumber k generally lies on a straight line. There may be deviations from the straight line in the edge regions of each wavelet. Accordingly, the edge regions of each wavelet may be excluded from the subsequent calculations.

[0439] Figure 30 shows schematically the optical setup of another exemplary multispectral light source unit 107 that may be used in the optical systems described above. The optical setup of the multispectral light source unit 107 is similar to that of the multispectral light source unit 106 described above, however rotating rosette reflectors are used instead of fixed reflectors.

[0440] In particular, the light source unit 107 comprises two rotating rosette reflectors 746 and 747 each having different radii of curvature r1 to r6, the radius of curvature rS6 being smaller than rS4 and rS4 being smaller than rS3. The structure of the rosette reflectors 746 and 747 is similar to that of the rosette reflectors 744 and 745, however the reflectors 746 and 747 are not fixed but are rotating (see details 30.1 and 30.2 of Figure 30 for a schematic drawing of the reflectors 746 and 747). In Figure 30 curvatures of rosette reflectors 746 and 747 are drawn strongly exaggerated, for the sake of better visualization.

[0441] The first rotating rosette reflector 746 may be arranged on a high speed precision motorized bearing with motor drive, not shown in the figures. The first rotating rosette reflector 746 is positioned in a transmission arm of a split beam path in the light source unit 107. The second, rotating rosette reflector 747 may also be arranged on a high speed precision bearing with a motor drive, not shown in the figures. The second, rotating rosette reflector 747 is arranged in a reflection arm of the split beam path in the light source unit 107. Similar arrangement is applicable to a modification of the light source unit 107a described further below.

[0442] Furthermore, the multispectral light source unit 107 comprises a broadband free beam laser 151 emitting light in the spectral range of 700 nm to 1000 nm, a 50:50 beam splitter 181 and a beam splitter 182 with a color splitter layer for beam splitting and beam unification. For the sake of an ease of understanding, only the propagation of the used light beams is shown in Figure 30.

[0443] The rosette reflectors 746 and 747 are configured to rotate, for example to rotate rapidly with up to one hundred thousand revolutions per minute. By the rotation of the rosette reflectors 746 and 747, a time-serial optical probing of the measured object 8 with a quasi-full aperture cone is realized. The integration time during detection may be adapted to the period of revolution of the rosette reflectors 746 and 747. Thus, six spectral wavelets W1 to W6 shown in Figure 34 are formed with light from the spectral range of the half-width HWB of the broadband free-beam laser 151 , as shown in Detail 30.3 of Figure 30.

[0444] Figure 31 shows schematically the optical setup of another exemplary multispectral light source unit 107a, which is a modification of the multispectral light source unit 107 described above and which may be used in the optical systems described above. The multispectral light source unit 107a comprises two rosette reflectors 746 and 747 (see details 31.1 and 31.2 of Figure 31 ) that have a structure and are arranged as described above in connection with the multispectral light source unit 107.

[0445] The multispectral light source unit 107a comprises further a multispectral, broadband, fiber-coupled light source 152. The multispectral, broadband, fiber-coupled light source unit 152 has an optical structure largely corresponding to the optical structure of the multispectral, broadband, fiber-coupled light source 151 described in connection with Figure 30. However, unlike the arrangement shown in Figure 30, the detection of light from the chromatic-confocal Michelson interferometer 6, which is not shown here, is also performed in the multispectral, broadband, fiber-coupled light source 152. In other words, the multispectral, broadband, fiber-coupled light source 152 has also the function of detection unit.

[0446] The multispectral light source unit 107a may be used in combination with a dual-beam interferometer, such as a chromatic-confocal Michelson interferometer having a split reference beam path with two reference arms R1 and R2 as described above and as shown, for example in Figure 32. This arrangement enables the reflection of six reference light beams BR1 to BR6, whose depth-separated light spots SR1 ' to SR6' are shown in Figure 31 . Confocal discrimination of the light coming from the chromatic- confocal Michelson interferometer occurs at the fiber end 474 of the single-mode fiber 475 comprised in the multispectral light source unit 107. From there the light passes via an y-coupler 133 and a single-mode fiber 129 into a fiber-coupled single-channel spectrometer 281 . Here, six wavelets W1 through W6 are detected, as shown in Figure 34. To prevent the light reflected in the inbound path from entering the fiber end 474 of the single-mode fiber 475, a slight misalignment of the rosette reflectors 746 and 747 is required.

[0447] Figure 32 describes an arrangement with a split reference beam path in a chromatic- confocal Michelson interferometer, such as the above described chromatic-confocal Michelson interferometer, 6. The split reference beam path has two reference subarms R1 and R2. In this arrangement, the light beams coming from the multispectral light source unit 107 or 107a according to Figure 30 and Figure 31 is coupled into the sub-arms R1 and R2 by a color splitter layer 785 with an edge characteristic, i.e. , light of one spectral range is transmitted, and light of other spectral range is reflected. The transmission area and the reflecting ranges are separated by a steep flank. The steep flank may extend in a spectral range of equal or less of 10% the wavelength at 50% transmission of the light. The color splitter layer 785 is configured for beam splitting of the incoming beams in the first reference arm R1 and the second reference arm R2, respectively, depending on their wavelength. In the first reference sub-arm R1 , an end reflector 7171 is arranged. The end reflector 7171 has three concave, spherically curved mirrors 751 , 752 and 753 arranged concentrically and centered on the optical axis 0AR1 of the first reference sub-arm R1 (see detail 32.1 of Figure 32). The concave spherically curved mirrors have respective radii of curvature rS1 to rS3 (rS 1 < rs2 < rS3), whereby the inner mirror 751 has the radius of curvature rS 1 and the outermost mirror 753 has the radius of curvature rS3. The three spherically curved concave mirrors 751 , 752 and 753 are rotationally symmetric.

[0448] In the reference arm R2 there is also an end reflector 7172 with three concave, spherically curved mirrors 764, 765 and 766, arranged concentrically and centered on the optical axis 0AR2 of the second reference sub-arm R2 (see detail 32.2 of Figure 32). The radii of curvature of the three concave mirrors are rS4 to rS6, respectively, whereby the inner mirror 764 has the radius of curvature rS1 and the outermost mirror 766 has the radius of curvature rS3. The three spherically curved concave mirrors 764, 765 and 766 are rotationally symmetric.

[0449] Generally, the 7171 and 7172 end reflectors need not be identical, but may exhibit different mirror curvatures, thereby realizing an arrangement in which all six mirrors in the resulting arrangement with a split reference beam path have different radii of curvature rS1 to rS6.

[0450] The centers of the osculating circles SK1 , SK2 and SK3 of the concave mirrors 751 , 752 and 753, respectively are located at M1 , M2 and M3 in reference sub-arm R1 . The centers of the osculating circles SK4, SK5 and SK6 of the concave mirrors 764, 765 and 766, respectively are located at M4, M5 and M6 in reference sub-arm R2. The centers M1 , M2 and M3 lie on the optical axis 0AR1 of the reference sub-arm R1 and the centers M4, M5 and M6 lie on the optical axis 0AR2 of the reference sub-arm R2.

[0451] The three multispectral, spatially coherent light beams B1 R, B2R and B3R with the main rays SB1 , SB2 and SB3, each with different wavefront curvature, formed of light with shorter wavelengths, enter the reference arm R1 via the color splitter layer 785 by transmission and are reflected by the end reflector 7171 . Thereby, a light spot S1 R' is formed at the center M1 of the osculating circle SK1 of the first spherically curved concave mirror 751 , a light spot S2R' is formed at the center M2 of the osculating circle SK2 of the second spherically curved concave mirror 752 and a third light spot S3R' is formed at the center M3 of the osculating circle SK3 of the third spherically curved concave mirror 753. The main rays SB1 , SB2 and SB3 of the spatially coherent light beams B1 R, B2R and B3R propagating the in reference sub-arm R1 hit the point of incidence AR1 , which is also the point of contact BP1 of the three osculating circles SK1 , SK2 and SK3. Thus, the light of the three light beams is reflected back into itself and always has the same optical path in the reference sub-arm R1 .

[0452] The three multispectral, spatially coherent light beams B4R, B5R and B6R with the main rays SB4, SB5 and SB6, each with different wavefront curvature, and formed of light with longer wavelengths, pass through the color splitter layer 785 by reflection into the reference arm R2 and are reflected there by the end reflector 7172. Thereby, a light spot S4R' is formed at the center M4 of the osculating circle SK4 of the fourth spherically curved mirror 764, a light spot S5R' is formed at the center M5 of the osculating circle SK5 of the fifth spherically curved mirror 756 and a sixth light spot S6R' is formed at the center M6 of the osculating circle SK6 of the sixth spherically curved mirror 766. The main rays SB4, SB5 and SB6 of the light beams B4R, B5R and B6R strike the point of incidence AR2, which is also the point of contact BP of the three osculating circles SK4, SK5 and SK6. Thus, the light beams B4R, B5R and B6R are reflected back into themselves. Accordingly, the light of the light beams B4R, B5R and B6R always has the same optical path in the reference sub-arm R2.

[0453] Further, the optical paths in the two reference arms R1 and R2 are the same, since the points of contact BP1 and BP2 lie on a circle K12 with a center TR on the optical axis OAR in the color splitting layer 785, which is also the point of intersection of the optical axes 0AR1 and 0AR2. Thus, the equality of the distances TRBP1 and TRBP2 is also valid. This can be achieved by means of a position adjustment of the end reflector 7171 and / or the end reflector 7172. In the example shown in Figure 32, the position of the end reflector is adjustable with a piezo actuator 743 connected to the end reflector 7172. The optical set up shown in Figure 32 allows the use of six spatially coherent light beams with wavefronts of different curvature and the generation of six spectral wavelets W1 to W6 which are at least approximately in phase.

[0454] Figure 33 shows schematically a detection unit 165 for six light beams B1 to B6, in which there is confocal discrimination at only one location by the fiber end 474 of the single-mode fiber 475 comprised in the detection unit 164. In other words, the fiber end 474 of the single-mode fiber 475 acts as a confocal discriminator. The detection unit 165 may be used in combination with the multispectral light source units 107 and 107a and / or the dual-beam interferometers with split reference paths described in connection with Figures 30 and 31 , for example.

[0455] The optical setup of the detection unit 165 is similar to that of the detection unit 164 described above, with the difference that two rotating rosette reflectors 748 and 749 are used instead of fixed reflectors. Each of the two rotating rosette reflectors 748 and 749 may be arranged on a high speed precision bearing with a motor drive (not shown in Figure 33).

[0456] The two rotating rosette reflectors 748 and 749 may be the same in structure to the rotating rosette reflectors 746 and 747 from the arrangements according to Figures 30 and 31 and may run in a strict synchronization with them, which is achieved by a control system. This ensures that mirror segments of the same curvature from the light source units 107 and 107a and the detection unit 165 are always in optical contact.

[0457] The light coming from the chromatic-confocal Michelson interferometer with chromatic depth splitting of light spots in the object arm O (such as the chromatic-confocal Michelson interferometer 6) enters the detection unit 165 via the collimator objective 791 of the detection unit. The light is coupled into the detection path via the broadband beam splitter 185 and the color beam splitter 186 splits the coupled light into two spectral ranges which meet the respective rosette reflectors 748 and 749. The rosette reflector 748 comprises repeating mirror segments having different radii of curvature rS 1 to rS3 (see detail 33.1 of Figure 33). The rosette reflector 749 comprises repeating mirror segments having different radii of curvature rS4 to rS6 (see detail 33.2 of Figure 33). The segments of the rosette reflectors 748 and 749 exactly balance the depth position of the light spots S1 R' to S6R' and also that of the light from the object arm 0.

[0458] The light from the object arm 0 is in two spectral ranges and comprises the at least approximately sharply focused light spots on the measured object 8 around the respective matching wavenumber k1 to k6. Thus, there is a comparatively small spectral range from which the respective rather narrowband spectral wavelet W1 to W6 emerges. The spectral wavelets W1 to V\ / 6 are shown in Figure 34.

[0459] After passing the color beam splitter 186 and the broadband beam splitter 185, the interfering light arrives via the focusing objective 792 of the detection unit 165 for confocal discrimination at the confocal discriminator 474, which is formed as the fiber end of the single-mode fiber 475. One light spot S1 R" to S6R" each from the reference arms R1 and R2 and one light spot S10k1" to S60k6" each from the object arm 0 form six pairs of interfering light, as shown schematically in the lower left portion of Figure 32. The pairs of interfering light of formed of light from a small spectral range, namely the spectral range around the respective centroid wavenumber k1 to k6 that exactly matches the current depth position zp of the measurement point of the measured object 8.

[0460] The detection unit 165 comprises further a fiber-coupled single-channel spectrometer 281. The fiber-coupled single-channel spectrometer 281 may, for example, be configured for the wavelength range from 700 nm to 1000 nm with a fast line sensor (e.g., of a fast line sensor with about 2000 pixels) then enables detection of the interfering light in the form of six spectral wavelets W1 to W6. Their frequency and position on the wavenumber axis depends only on the current depth position zp of the measurement point of the measured object 8 and is an output variable for the calculation of the same. Figure 34 shows schematically the six emerging wavelets W1 to W6 which the spectrometer 281 of the detection unit 165 of Figure 33 detects in combination with a light source unit 107 according to Figure 30 and a dual-beam interferometer with a split reference arm with sub-arms R1 and R2 in an arrangement according to Figure 32. The evaluation of the resulting spectral wavelets W1 to W6 may be made by using known evaluation methods, such as the methods described in the publications WO 2019 / 120470 A1 and US 11 248 900 B2.

[0461] Figure 35 shows schematically the optical setup of an exemplary multispectral light source 108 that may be used in the optical systems described above. The multispectral light source 108 is particular used for measurements on object in the mechanical engineering field (preferably in a real-world, working environment). Such objects can generally be subjected to a comparatively high light intensity without the risk of destruction. The multispectral light source 108 emits four light beams having different wavefront curvatures in a paraxial vicinity of a point W at the output of the multispectral light source unit 108.

[0462] The multispectral light source comprises a free beam broadband laser 151 (optionally with a beam expander and / or collimator) with an output power of at least 100 mW, for example a broadband laser emitting light the spectral range 700 nm to 1000 nm. The multispectral light source 108 comprises further an optical isolator 142 arranged downstream of the broadband laser, a double beam splitting unit 187 and a focusing lens 5 arranged downstream of the double beam splitting unit 187.

[0463] The double splitting unit 187 has two highly reflective, low loss beam splitter surfaces 188 and 189. The reflectance of each of the beam splitter surfaces 188 and 198 may be, for example, between 60% and 90%. Each of the weakly spherically curved broadband beam splitter surfaces 188 and 189 may have a splitter ratio of 85:15 in favor of reflection. The radiuses of curvature rT1 and rT2 of the beam splitter surfaces 188 and 189 are different, which the radius curvature rT1 of the beam splitter surface 188 being less than the radius of curvature rT2 of the beam splitter surface 189, as shown in Detail 35.1 of Figure 35. The multispectral light source unit 108 is configured to generating a chain of effective light spots separated in a depth-direction, as indicated schematically in Figure 35. The generated light spots lie on a straight line.

[0464] It is important to note that generally there is no dispersive material between the two beam splitting surfaces 188 and 189, i.e., air in the normal case. Only indicated symbolically in detail 35.1 of Figure 35 is the fact that the two beam splitting surfaces 188 and 189 are each mounted on a weak meniscus lens made of glass. However, the curvature of the two beam splitting surfaces 188 and 189 is shown extremely exaggerated in detail 35.1 for the sake of better viewability.

[0465] Thus, multiple light beams with different curvature of the wavefronts can be generated at the output of the multispectral light source unit 108 after passing the focusing lens 5 by multiple reflections, wherein and the angles of the beams are drawn magnified here.

[0466] In the reference arm of a chromatic-confocal dual-beam interferometer 6, a chain of depth separated light spots S1 R1 to S1 R4 are formed from the respective four light beams (reference light beams) emitted from the effective light spots and propagating in the reference arm R of a chromatic-confocal Michelson interferometer s, Detail 35.2 of Figure 35 shows symbolically the different wavefront curvatures WF1 to WF4 of the four focused light beams in the reference arm R of the chromatic-confocal Michelson interferometer 6.

[0467] The multispectral light source 108 may have, for example two low-loss beam splitter surfaces 188 and 189 with a reflectance of 85% each. In this arrangement, the first light beam exiting the light source unit 108 still has a light intensity of around 2%, the second of about 1 .6%, the third of about 1 .2% and the fourth of about 0.8%, relative to the light intensity of the input beam entering the double beam splitting unit 187. Other light beams are typically also produced at the output of the light source unit 108, but their light intensity is much lower as compared to the light intensity of the first to fourth light beams and may be neglected. Such light beams may, however, produce an undesirable stray light. The chromatic-confocal Michelson interferometer 6 may be designed with an end reflector 117 according to a design shown in Figure 24 in the reference arm R for reflecting the four reference light beams B1 R to B4R. The light returning from the reference arm R after having been reflected on the end reflection and the light returning from the object arm 0 after having been reflected by the measured object 8 passes the focusing objective 5, the two beam splitting surfaces 188 and 189, the 50:50 broadband beam splitter 181 and a focusing objective 792 in the detection unit 166. The focusing objective focuses 792 the light at an end 474 of a single-mode fiber 475. Confocal discrimination is performed at the single-mode fiber end 474 of the singlemode fiber 475. The transmitted light passes through the single-mode fiber 475 into the fiber-coupled single-channel spectrometer 281 , where four spectral wavelets W1 to W4 are detected. As described above, depth information may be obtained by analyzing the spectral wavelets using methods known in the art.

[0468] Figure 36 shows schematically the optical setup of an yet another exemplary multispectral light source 109 that may be used in the optical systems described above. As in the multispectral light source 108, multiple spatially coherent light beams are generated using the principle of multiple reflections.

[0469] The multispectral light source 109 comprises a free beam broadband laser 151 that may be the same as the broadband laser described in connection with Figure 35. The multispectral light source 109 comprises further a beam splitter surface 181 arranged downstream of the broadband laser 151 , a pellicle beam splitter 214 arranged downstream of the beam splitter surface 181 , a spherically curved beam splitter surface 189 arranged downstream of the pellicle beam splitter 214 and an focusing lens arranged downstream of the beam splitter surface. Further, the multispectral light source 109 comprises a plane mirror 183.

[0470] The beam splitter surface 181 may be a beam splitter surface with a splitter ratio of 85: 15 in favor of reflection. The beam splitter surface 189 is a weakly spherically curved, low-loss beam splitter surface 189, having for example a splitter ratio of 85: 15 in favor of reflection. As schematically indicated in Detail 36.1 of Figure 36, the beam splitter surface 189 may be applied to a weak meniscus lens made of glass, with only air between the beam splitter surface 189 and the pellicle beam splitter 214 and the plane mirror 183. Detail 36.1 shows the broadband beam splitter surface 189 with a curved substrate.

[0471] With the above arrangement, a chain of effective light spots lying on a straight line and separated from each other in a depth direction is generated by the multispectral light source unit 109, as schematically indicated in Figure 36.

[0472] In a reference arm of a chromatic-confocal Michelson interferometer 6 (which may be designed with an end reflector 117 according to a design shown in Figure 24) four depth-separated light spots S1 R’ to S4R’ are formed, as shown in Detail 36.2 of Figure 36.

[0473] The light returning from the reference arm R and the light returning from the object arm 0 having been reflected by the measured object 8 passes the focusing objective 5, the beam splitting surface 189, the 50:50 broadband beam splitter surface 181 and a focusing objective 792 in the detection unit 166. The focusing objective 792 focuses the light at an end 474 of a single-mode fiber 475. Confocal discrimination is performed at the single-mode fiber end 474 of the single-mode fiber 475. The transmitted light passes through the single-mode fiber 475 into the fiber-coupled single-channel spectrometer 281 , where four spectral wavelets W1 to W4 are detected. As described above, depth information may be obtained by analyzing the spectral wavelets using methods known in the art.

[0474] Further, the present disclosure relates to the following examples and modifications thereof, which may be implemented in the optical systems described above:

[0475] Further example 1

[0476] The further example 1 relates to an exemplary optical system for chromatic-confocal spectral-domain OCT with multiple wavelets or for chromatic-confocal dual-beam interferometry that may be implemented in one of the optical systems described in connection with Figures 1 to 35 and the above mentioned aspects and examples.

[0477] The optical system for chromatic-confocal spectral-domain OCT with multiple wavelets or for chromatic-confocal dual-beam interferometry may comprise a chromatic- confocal dual-beam interferometer, which is formed with a non-zero optical path difference, with an achromatic reference arm with an end reflector and with a chromatic object arm, in which both chromatic optical means for depth splitting of light spots and at least one measured object are arranged. The chromatic-confocal dual-beam interferometer may be, for example, the above described chromatic-confocal dualbeam interferometer 6 or 61 .

[0478] Further, upstream, in an illumination beam path, there is arranged a multispectral light source unit, such as the above described light source unit 100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109. The multispectral light source unit 100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109, which may be a multispectral light source unit in real time or a quasi-multispectral light source unit formed time-serially by means of light sources which can be tuned rapidly in time in spectral domain as a quasi-multispectral light source unit.

[0479] In the illumination beam path, there is arranged at least approximately achromatic focusing optics (an exemplary achromatic focusing unit, such as the achromatic focusing unit 5, 51 , 511 , 52 described above). For the sake of brevity, the at least approximately achromatic focusing optics is also referred to as focusing achromatic optics or achromatic optics. The achromatic focusing optics may consist of or may comprise an achromatic focusing lens 5, 52, 53. The focusing achromatic optics 5, 51 , 511 , 52 may be arranged completely upstream of the chromatic-confocal dual-beam interferometer with a reference arm R and an object arm O or may be at least partially integrated into the chromatic-confocal dual-beam interferometer prior to the beam splitting. The focusing achromatic optics may be a part of the multispectral light source unit, such as for example in the examples described in connection with the figures. The focusing achromatic optics 5, 51 , 511 , 52 is formed with an optical axis. The focusing achromatic optics, in interaction with all used optical components from the light source unit up to the end reflector in the achromatic reference beam path, generates an at least approximately optically diffraction-limited image for a light with a given light spectrum emitted from multispectral light source unit in the form of diffraction-limited light spots (reference light spots).

[0480] Detection may be performed by a respective detection unit. For example, for a multispectral light source unit in real time the detection may be performed by a spectrometer or by a dual-beam evaluation interferometer. In case of light sources that can be rapidly tuned in time in spectral domain as a quasi-multispectral light source unit, detection may be performed by using fast photosensors.

[0481] A real-time multispectral light source unit may comprise an array of two or more superluminescent diodes or of a broadband laser light source.

[0482] Further, at the output of the multispectral light source unit 100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109, several, but at least two and preferably not more than twelve, at least approximately spatially coherent light beams B1 , B2, ... Bi, with i=3, 4, ..., of multispectral light are formed.

[0483] The wavefronts WF1 , WF2, ... WFi, with i=3, 4, ...12, of the spatially coherent light beams B1 , B2, ... Bi, ..., are at least approximately spherical at least in the paraxial region around the optical axis OAS of the focusing achromatic optics. The focusing achromatic optics may be arranged at the output of the light source unit 100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109, in which case the optical axis OAS of the focusing achromatic optics and the optical axis of the light source unit 100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109 coincide.

[0484] The wavefronts WF1 , WF2, ... WFi, ..., of the spatially coherent light beams B1 , B2, ... Bi, ..., are formed with different, predetermined radii of curvature r1 pW, r2pW, ripW, ..., formed at a common point W located downstream of the light source unit 100, 101 , 102, 103, 104, 104a, 104a 105, 106, 107, 107a, 108, 109, in particular downstream of the focusing achromatic optics and upstream of the entrance El of the chromatic- confocal dual-beam interferometer, such as the chromatic-confocal dual-beam interferometer 6, 61 described above. The predetermined values of the radii of curvature r1 pW, r2pW, ripW, ..., of the wavefronts WF1 , WF2, WFi, ... may differ significantly from each other. The light emitted by means of the first light beam B1 has a centroid wavelength lambda_S1 and a centroid wavenumber kS1 , respectively, wherein kS1 =2Pi I lambda_S1.

[0485] The chromatic-confocal dual-beam interferometer is formed with an optical path difference xp that is greater than ten times or equal to ten times and less than ten thousand times or equal to ten thousand times the centroid wavelength lambda_S1 .

[0486] With the help of the focusing achromatic optics 5, 51 , 511 , 52, a first multispectral light beam B1 R (first reference light beam) with an aperture angle alpha_1 , which is produced from the light beam B1 after beam splitting in the chromatic-confocal dualbeam interferometer 6, 61 , is formed in the reference arm R, microscopically small and at least approximately diffraction-limited light spot S1 R' is formed in the reference arm R by the first reference light beam. Further, a second multispectral, microscopically small and at least approximately diffraction-limited light spot S2R' is formed in the reference arm R by a second multispectral light beam B2R (second reference light beam), wherein the second reference light beam which is produced from a second light beam B2 after the beam splitting in the chromatic-confocal dual-beam interferometer 6, 61 . Accordingly at the end reflector in the reference arm, multispectral the light spots S1 R', S2R' are formed by diffraction limited imaging.

[0487] In case that more than two reference beams BiR are formed from respective light beams Bi after beam splitting in the chromatic-confocal dual-beam interferometer 6, 61 , further multispectral, microscopically small and at least approximately diffraction limited light spots SiR' are formed in the reference arm R. A first reference light spot S1 R' of the plurality of reference light spots may be positioned as follows:

[0488] - in the center M1 of a convex, spherically curved mirror 72, 732 on the end reflector 711 , 713, 716 in the reference arm R (see e.g., Figures 1 to 3); or

[0489] - in the center M1 of a concave, spherically curved mirror 74 on the end reflector 712 in the reference arm R. (see e.g., Figure 4); or

[0490] - in the center M1 of an inner convex, spherically curved mirror 731 on the end reflector (715) in the reference arm R (see e.g., Figure 14); or

[0491] - in the center M1 of an inner concave spherically curved mirror 751 on the end reflector (714, 715, 717) in the reference arm R (see e.g., Figs. 19, 22, 24);

[0492] - in the center M1 of a concave, spherically curved mirror 781 on the end reflector 782 in the reference arm R1 (see e.g., Figure 23); or

[0493] - in the center M1 of the osculating sphere SK2 of a convex, spherically curved mirror (771 ), formed as a segment of a circle on the end reflector 718 in rosette form in the reference arm R (see e.g., Figure 25); or

[0494] - in the center M1 of a central, concave, spherically curved mirror 751 , on the end reflector (7171 ) in the reference arm R1 (see e.g., Figure 32).

[0495] A second reference light spot and optionally a third (or further reference spots) may be located for example at the following positions:

[0496] - in the case of a single convex spherically curved mirror 72 on the end reflector 711 , a second reference light spot S2R' may be positioned at the point of incidence AR on the optical axis OAR on the one spherically curved mirror (72) in the reference arm R (see e.g., Figure 1 );

[0497] - in the case of an inner convex spherically curved mirror 731 and an outer convex spherically curved mirror 732 on an end reflector 713 in the reference arm R, a second reference light spot S2R' may be positioned at the center M2 of the convex spherically curved mirror 731 (see e.g., Figure 3);

[0498] - in the case of a single concave, spherically curved mirror 74 on the end reflector 712 in the reference arm R, a second reference light spot S2R' may be positioned in the point of incidence AR on a miniaturized plane mirror 741 in the reference arm R, wherein the miniaturized plane mirror 741 with the point of incidence AR is arranged in the intersection of the osculating circle SK1 of the one spherically curved mirror 74 with the optical axis OAR (see e.g., Figure 4);

[0499] - in the case of an inner convex spherically curved mirror 731 and an outer concave spherically curved mirror 752 on an end reflector 715 in the reference arm R, a second reference light spot S2R' may be positioned at the point of incidence AR of the convex, spherically curved mirror 731 , which is also the point of contact BP of the osculating circles SK1 and SK2. Further, a third light spot S3R' may be positioned in the center M2 of the second concave spherically curved mirror 752 (see e.g., Figure 14);

[0500] - in the case of an inner convex spherically curved mirror 731 and an outer convex spherically curved mirror 732 on an end reflector 713 in the reference arm R, the second reference light spot S2R' may be positioned at the center M2 of the convex, spherically curved mirror 731 and a third reference light spot S3R' may be positioned in the point of incidence AR of the convex, spherically curved mirror 731 , which also represents the point of contact BP of the osculating circles SK1 and SK2 (see e.g., Figure 18);

[0501] - in the case of an inner concave spherically curved mirror 751 and an outer convex spherically curved mirror 732 on an end reflector 716 in the reference arm R, a second reference light spot S2R' may be positioned at the point of incidence AR, which is also the point of contact BP of the two osculating circles SK1 and SK2 on the concave spherically curved mirror 751. Further, a third reference light spot S3R' may be positioned at the center M2 of the inner concave spherically curved mirror 751 (see e.g., Figure 19);

[0502] - in the case of an inner concave spherically curved mirror 751 and an outer concave spherically curved mirror 752 on an end reflector 714 in the reference arm R, a second reference light spot S2R' may be positioned at the center M2 of the outer concave, spherically curved mirror 752 and a third reference light spot S3R' may be positioned at the point of incidence AR on the inner concave spherically curved mirror (751 ), which is also the point of contact BP of the two osculating circles SK1 and SK2 (see e.g., Figure 22); - in the case of a concave spherically curved mirror 781 on an end reflector 782 in a reference sub-arm R1 of a split reference arm, a second reference light spot S2R' may be positioned at the point of incidence AR1 on the concave spherically curved mirror 781 on the optical axis 0AR1 of the first reference sub-arm. Further, a third reference light spot S3R' may be positioned at the center M2 of a concave spherically curved mirror 783 in the second sub-arm R2 of the split reference arm (see e.g., Figure 23);

[0503] - in the case of three concave spherically curved mirrors 751 , 752, 753 on an end reflector 717 in the reference arm R, a second reference light spot S2R' may be positioned at the center M2 of the central convex spherically curved mirror 752 and a third reference light spot S3R' is positioned at the center M2 of the concave spherically curved mirror 751 in the reference arm R (see e.g., Figure 24);

[0504] - in the case of six convex, spherically curved mirrors 761 , 762, 763, 771 , 772, 773 on an end reflector 718 in rosette form in the reference arm R, a second reference light spot S2R' may be positioned in the center M2 of the convex, spherically curved mirror 771 and a third reference light spot S3R' may be positioned at the point of incidence AR on the convex, spherically curved mirror 771 , which is also the point of contact BP of the two osculating circles SK1 and SK2 of the two concave mirrors 762, 771 (see e.g., Figure 25);

[0505] - in the case of a split reference arm having a first reference sub-arm R1 with three concave spherically curved mirrors 751 , 752, 753 on an end reflector 7171 , a second reference light spot S2R' may be positioned at the center M2 of the concave spherically curved mirror 752 and a third reference light spot S3R' is positioned at center M2 of the concave spherically curved mirror 753 in reference arm R (see e.g. Figure 32).

[0506] The effective, multispectral, microscopic and at least approximately diffraction limited reference light spots S1 R', S2R', SiR', ..., are positioned in a depth direction of the reference arm R, R1 , R2 and are separated from each other by certain depth distance(s) due to the predetermined curvature of the wavefronts WF1 , WF2, WFi, ..., at the output of the light source unit 100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109 and the focusing effect of the focusing achromatic optics 5, 51 , 511 , 52.

[0507] For the depth distance t12R' separating a first and a second reference light spots S1 R' and S2R', the following relationship may be derived from the wave-optical depth of focus: lambda_S1 / [sin(alpha_1 )]2< t12R' < 100*lambda_S1 / [sin(alpha_1 )]2wherein: lambda_S1 denotes the centroid wavelength of the first reference light beam B1 R in the reference arm R; and angle alpha_1 denotes the aperture angle of the first reference light beam B1 R at the light spot S1 R' in the reference arm R.

[0508] The miniaturized spherical mirror 72, 721 , 732, 74, 751 , 762, 781 may have a radius (rS 1 ) of its curvature smaller than or equal to

[0509] 100 * lambda_S1 / [sin(alpha_1 )]2.

[0510] The depth distance t12R' separating the two reference light spots S1 R' and S2R' may be made equal to the radius rS1 (i.e., t12R' = rS1 ) in case:

[0511] (i) the second reference light spot S2R' is positioned in the reference arm R at a point of incidence AR on the spherically curved mirror 72, 721 , 732, 74, 751 , 762, 781 , or

[0512] (ii) the second light spot S2R' is positioned at a point of intersection GO of the osculating circle SK1 of the spherically curved mirror 72, 721 , 732, 74, 751 , 762, 781 with the optical axis OAR.

[0513] The depth distance t12R' separating the two reference light spots S1 R' and S2R' may be made equal to the absolute value of the difference rS1 - rS2 (i.e., t12R' = | rS1 - rS2 | ) in case a second miniaturized spherically curved mirror 731 , 752 with a radius rS2 of its curvature is arranged in the reference arm and a second reference light spot S2R' is positioned at the center M2 of the second spherically curved mirror 731 , 752.

[0514] In the chromatic object arm 0 of the chromatic-confocal dual-beam interferometer 6, 61 , the light, split at a beam splitting layer 60 (of a beam splitting unit) and entering the object arm is further chromatically split by a chromatic depth splitting unit and reaches the measured object 9. The chromatic depth splitting unit may consist of or may comprise at least one diffractive-optical element 70, 701 , 702. With the help of the achromatic focusing optics 5, 51 , 52, 511 (in connection with the other optical elements in the object arm upstream of the measured object 8), the light is sharply focused onto the measured object, thereby forming two or more approximately diffraction-limited light spots S10k1 ', S20k2', ... SiOki', ..., (object light spots) on the measured object 8 (i.e. , in the object space). The two or more approximately diffraction-limited light object light spots S10k1 ', S20k2', ... SiOki', ..., are formed from light having current wavenumbers k1 , k2, ki, ... resulting from (or corresponding to) the current depth position of the measured object 8.

[0515] Further, in the chromatic object arm 0, extended stretches (areas or ranges) of light spots dzc1_H, dzc2_H, ... dzci_H, i=3, 4, ... 12, are formed in the depth of the object arm 0 with the help of the chromatic depth splitting unit 70, 701 , 702 from light emitted from the respective effective multispectral microscopic light emitting spots S1 , S2, S3, ...Si, .... The extended stretches of light spots dzc1_H, dzc2_H, ... dzci_H, ..., are at least approximately collinear and at least partially overlap in the depth direction, thereby forming an overlapping region (coverage region) UE, in which the measured object 8 is arranged.

[0516] The light is reflected by the measured object 8 and is recombined with the light from the reference arm R at the beam splitting layer 60 comprised in the beam splitting unit of the interferometer.

[0517] The two or more multispectral object light spots S10k1", S20k2", SiOki", ..., which are at least approximately diffraction limited, are associated with confocal discriminators 116, 117, 118, 261 , 262, 263, 271 , 272, 471 , 472, 473, 474, which are separated in a predetermined depth. The confocal discriminators 116, 117, 118, 261 , 262, 263, 271 , 272, 471 , 472, 473, 474 confocally discriminate both the light from the reference arm R and the light from the object arm 0 to thereby form combined light spots S1 R", S2R",...SiR", S1 Ok1", S20k2", ... SiOki",... from the light from the reference arm R and the light from the object arm 0 transmitted at by confocal discriminators 116, 117, 118, 261 , 262, 263, 271 , 272, 471 , 472, 473, 474.

[0518] The light of two or more combined light spots S1 R", S2R",...SiR", S10k1", S20k2", ... SiOki", ..., transmitted at the confocal discriminators 116, 117, 118, 261 , 262, 263, 271 , 272, 471 , 472, 473, 474 may be detected, for example by a fiber-coupled singlechannel spectrometer 281 or by evaluation spectrometers 28, 29 arranged in the detection unit (16) as corresponding two or more wavelets W1 , W2, ... Wi, ... and thus a chromatic-confocal, spectral dual-beam interferometer is formed.

[0519] The light of two or more combined light spots S1 R", S2R",...SiR", S10k1", S20k2", ... SiOki" , ..., transmitted at the confocal discriminators 116, 117, 118, 261 , 262, 263, 271 , 272, 471 , 472, 473, 474 may be alternatively detected as a spatial interferogram rl by an evaluation Michelson interferometer 282 arranged in the detection unit 163 and thus a combination of two coupled dual-beam interferometers is formed.

[0520] Based on the detected light (e.g., by the fiber-coupled single-channel spectrometer 281 or at least one spectrometer 28, 29), the depth distance zp of the current measurement point cPO12 of the measured object is calculated by using digital computing unit 31 . More specifically, the depth distance zp is calculated either from the at least two detected wavelets W1 , W2, ...Wi, ... with the current wavenumbers k1 , k2, ...ki, ... or from the spatial interferogram rl detected by employing an evaluation Michelson interferometer 282 and a line light detector (e.g,, a fast CMOS line camera 283). Further example 2

[0521] In a further example 2 based on the further example 1 , the spatially coherent, multispectral light beam B1 has the centroid wavenumber kS1 , the spatially coherent multispectral light beam B2 has the centroid wavenumber kS2. Optionally, there are further spatially coherent multispectral light beams Bi, wherein a spatially coherent multispectral light beam Bi (wherein i = 3, 4, ..., wherein preferably i < 12), has the centroid wavenumber kSi and all centroid wavenumbers (kS1 , kS2, ... kSi, ...) differ from each other by at least 1 %, further optionally by at least 3%.

[0522] Further example 3

[0523] In a further example 3 based on the further example 1 or 2, in the system for chromatic- confocal spectral-domain OCT with multiple wavelets or for chromatic-confocal dualbeam interferometry, the at least approximately diffraction-limited object light spots SI OkST, S20kS2', .... SiOkSi', ... in the object arm 0 at their assigned centroid wavenumbers kS1 , kS2, ... kSi, ... may be positioned in a depth range dtc, which is at most 50% of the overlap range (coverage range or coverage region) UE resulting from the ranges dzc1_H and dzc2_H, dzci_H with i=3, 4, ... 12 at the chromatic depth splitting of light in the object arm 0 of the multispectral light beams B1 O, B2O, ... BiO with i=3, 4, ...12.

[0524] Further example 4

[0525] In a further example 4 based on any of the further example 1 to 3, the at least approximately diffraction-limited light spots SI OkST, S20kS2', .... SiOkSi' with i=3, 4, ...12 in the object arm 0 at their associated centroid wavenumbers kS1 , kS2, ... kSi with i=3, 4, ...12 are positioned in a depth range dtc which is at most 10% of the overlap range (coverage area or coverage range) UE resulting from the overlapping multispectral stretches dzc1_H. dzc2_H, dzci_H with i=3, 4, ... 12 in the chromatic depth splitting of light of the multispectral light beams B1 O, B2O, ... BiO, with i=3, 4, ...12, in the object arm 0 - Further example 5

[0526] In a further example 5, based on any one of the further examples 1 to 4, at least two light spots S1 OkS1 ', S20kS2' in the object arm 0 at their assigned centroid wavenumbers kS1 and kS2 are positioned at least approximately in a confocal point cPS during the chromatic depth splitting of light of the multispectral light beams B1 O, B2O. This represents an ideal case, which, however, can still be approximated quite well with two light spots SI OkST, S20kS2' in the object arm 0.

[0527] Further example 6

[0528] In a further example 6, based on any of the further examples 1 to 5, more than two and no more than 12 multispectral, spatially coherent light beams B1 , B2, ... Bi, with i=3, 4, ... 12, may be formed, whose wavefronts WF 1 , WF2, ... WFi, with i=3, 4, ... 12 have different, predetermined radii of curvature r1 pW, r2pW, .... ripW, with i= 3, 4, ... 12. Optionally, from each light beam B1 , B2, ... Bi with i=3, 4, ... 12 after beam splitting by means of a beam splitting layer 60 in the chromatic-confocal dual-beam interferometer 6, 61 a reference light beam B1 R, B2R, ... BiR with i=3, 4, ... 12 with a corresponding light spot S1 R', S1 R', ... SiR' with i=3, 4, ... 12 (reference light spot) is formed in the reference arm R or in a reference arm R1 and R2, wherein the reference light spots are formed at different depths.

[0529] Further example 7

[0530] In a further example 7, based on any of the further examples 1 to 6, there may be exactly one point of contact BP, BP1 , BP2 of all osculating circles SK1 , SK2, ... Ski with i=3, 4, ...12 in a reference arm (R, R1 , R2). The osculating circles SK1 , SK2, ... Ski with i=3, 4, ...12, represent cross-sections of the osculating spheres of the spherically curved mirrors 731 , 732, 751 , 752, 753, 761 , 762, 763, 764, 765, 766, 771 , 772, 773 arranged in a reference arm R, R1 , R2 on the optical axis OAR. Further example 8

[0531] In a further example 8, based on any of the further examples 1 to 7, optionally a light spot S3R' (reference light spot) in the reference arm R or R1 impinges on a spherically curved mirror 751 at the point of incidence AR and the point of incidence AR coincides with the point of contact BP of both osculating circles SK1 , SK2.

[0532] Further example 9

[0533] In a further example 9, based on any of the further examples 1 to 8, optionally a light spot S4R' in the reference arm R or R1 or R2 impinges on a spherically curved mirror (i with i=3, 4, 5 ...12) in the point of incidence AR and the point of incidence AR coincides with the point of contact BP of all three osculating circles SK1 , SK2, SK3.

[0534] Further example 10

[0535] In a further example 10, based on any of the further examples 1 to 9, optionally a light spot SiR' with i=3, 4, 5 ...12 is positioned in the reference arm R or R1 or R2 in the center Mi with i=3, 4, 5 ...12 of the osculating sphere of a spherically curved mirror i with i=3, 4, 5 ...12. The center of the osculating sphere of a spherically curved mirror coincided with the center (center of curvature) of the respective mirror.

[0536] Further example 11

[0537] In a further example 11 , based on any of the further examples 1 to 10, the light source unit 102 may have h two multispectral point light sources 110, 120 and a singlecoupling beam splitter 191 with a color splitter layer adapted to the centroid wavenumbers kS1 , kS2 of the two multispectral point light sources 110, 120. The centroid wavenumber kS2 of the multispectral point light source 120 differs by at least 5% from the centroid wavenumber (kS1 ) of the multispectral point light source 110. Further example 12

[0538] In a further example 12, based on any of the further examples 1 to 10, the light source unit 103, 104, 104a may have three effective multispectral point light sources 110, 120, 130. The centroid wavenumbers kS2, kS3 of the multispectral point light sources 120, 130 differ from the centroid wavenumber kS1 of the multispectral point light source 110 by at least 5% and from each other by at least 5%. In other words, the difference between a centroid wavenumber to the next closest centroid number may be at least 5%.

[0539] Further example 13

[0540] In a further example 13, based on further example 12, the light source unit 103 may have three effective multispectral point light sources 110, 120, 13, a first beam splitting layer 221 and a second beam splitting layer 222 in a beam splitter group 22. The first beam splitting layer 221 and the second beam splitting layer 222 are adapted to the centroid wavenumbers kS1 , kS2, kS3 of the three multispectral point light sources 110, 120, 130. For example, the light from the first effective multispectral point light source 110 may be transmitted at the first beam splitting layer 221 somewhat more strongly than the light from the second effective multispectral point light source 120. At the second beam splitting layer 222, the light from the third effective multispectral point light source 130 may be reflected somewhat more strongly than the light from the second effective multispectral point light source 120.

[0541] Further example 14

[0542] In a further example 14, based on any of the further examples 1 to 10, the light source unit 104a may comprise a beam splitter group 23 with a three-part cemented prism 24 with two color splitter layers 195, 196. The beam splitter group 23 may be constructed in prism form like a prism for known 3-chip camera technique. Further example 15

[0543] In a further example 15, based on any of the further examples 1 to 14, a light source unit with “i” multispectral point light sources, wherein i is an integer number equal to or greater than 4 and equal to or smaller than 12 is optionally formed, whose respective centroid wavenumbers kSi differ from one another by at least 3%.

[0544] Further example 16

[0545] In a further example 16, based on any of the further examples 1 to 15, at least one end reflector 717, 7171 , 7172 in the reference arm may be formed with multiple centered spherically curved mirrors 752, 753, 765, 766 in ring form with different radii of curvature rS2, rS3, rS5, rS6, whose osculating spheres meet in a point of contact BP, BP1 , BP2 on the optical axis OAR, 0AR1 , 0AR2 in the reference arm R R1 , R2.

[0546] Further example 17

[0547] In a further example 17, based on any of the further examples 1 to 16, the chromatic depth splitting unit optionally comprises or consists of a diffractive-optical element having a focusing power, for example a diffractive-optical element having a positive or negative focusing power. The focusing power of the diffractive-optical element 70, 701 , 702 may undercompensated or overcompensated by suitable optical element(s) arranged in the object arm 0 or in the reference arm R. This ensures that the optical path difference xp equal to zero is reliably avoided when positioning the measured object 8 in the overlap region UE.

[0548] Further example 18

[0549] In a further example 18, based on further example 17, the focusing power of the diffractive-optical element 70, 701 is optionally positive. Further example 19

[0550] In a further example 19, based on further example 18, the positive focusing power of the diffractive-optical element 70 may be at least undercompensated or overcompensated by a curved substrate back surface 703 of the diffractive-optical element 70 with at least approximately negative achromatic refractive power.

[0551] Further example 20

[0552] In a further example 20, based on any of further examples 17 to 19, the focusing power of the diffractive-optical element 701 may be undercompensated or overcompensated by a thin converging lens 705 with at least approximately achromatic negative refractive power arranged in the reference arm R.

[0553] Further example 21

[0554] In a further example 21 , based on further example 17, wherein the focusing power of the diffractive-optical element is optionally negative. Further, the negative focusing power of the diffractive-optical element 702 may be undercompensated or overcompensated by a curved substrate back surface 706 with at least approximately positive achromatic refractive power.

[0555] Further example 22

[0556] In a further example 22, based on further example 20, the negative focusing power of the diffractive-optical element 702 is undercompensated or overcompensated by a thin diverging lens with at least approximately achromatic refractive power in the object arm O.

[0557] Further example 23

[0558] In a further example 23, based on any one of the further examples 1 to 22, the end reflector 718 may be formed in rosette shape with multiple spherically curved mirror segments 761 , 762, 763, 771 , 772, 773 with alternating different curvatures, whose osculating spheres meet at a single point of contact BP on the optical axis OAR in the reference arm R and the mirror segments 761 , 762, 763 of a first group each have the radius of curvature rS 1 and the mirror segments 771 , 772, 773 of a second group each have the radius of curvature rS2.

[0559] The end reflector in rosette form is not limited to this example and may be formed, for example, from three or more groups of mirror segments of different radii of curvature arranged in triples, quadruples, etc. In another example, the rosette form end reflector may be formed with spherically curved mirrors in the form of circular segments with radii of curvature rS1 , rS2, rS3, rS4, rS5, rS6 of the spherically curved mirrors that differ pairwise.

[0560] Further example 24

[0561] In a further example 24, based on further example 23, the rosette form end reflector may be a fixed rosette reflector 744, 745. For example, the rosette form end reflector may be a fixed rosette reflector with spherically curved mirrors in the form of circular segments with radii of curvature rS1 , rS2, rS3, rS4, rS5, rS6 of the spherically curved mirrors that differ pairwise.

[0562] Further example 25

[0563] In a further example 25, based on further example 23, the rosette form end reflector may be a rotating end reflector, for example a rapidly rotating rosette form end reflector. In an example, the end reflector in rosette form may be a (rapidly) rotating end reflector 746, 747, 748, 749 m with spherically curved mirrors in the form of circular segments with radii of curvature rS1 , rS2, rS3, rS4, rS5, rS6 of the spherically curved mirrors which differ in pairs. Further example 26

[0564] In a further example 26, based on any of the further examples 1 to 25, the reference arm R is a split reference arm, i.e. a reference arm that is split into multiple partial or sub-reference arms R1 , R2. The number of sub-reference arms is not limited to 2 but may be 3, 4 or more.

[0565] Further example 27

[0566] In a further example 27, based on any of the further examples 1 to 25, an array (719) of end reflectors is preferably arranged in reference arm R.

[0567] Further example 28

[0568] In a multispectral light source as described above in connection with one of the examples and further examples, comprising point light source(s), the main rays of the point light sources, after being reflected by means of beam splitters, may lie at least approximately on a straight line g defined by the connecting line of two light spots S1 and S2. In the simplest case of centered downstream optics, the straight line g may coincide with the optical axis of the centered downstream optics. Usually, in centered systems, the pupil center is also on the optical axis of the subsequent optic. In case of an off-axis position of the straight line g, preferably, the straight line aims at the pupil center of the subsequent optics.

[0569] The subsequent optics with imaging optical elements, up to the reference arm R can have a single-stage or two-stage telecentric design, in which case the straight line g may aim at the pupil center of the subsequent optics. Thus, in a two-stage telecentric imaging stage, the straight line g can also be parallel to the optical axis. In the case of a single-stage telecentric imaging stage with a telecentric aperture in the focal plane, the straight line g may be inclined to the optical axis and may aim at the center of the pupil, which is then located in the front focal plane. In the above described optical systems, it is possible to generate several pairs, triples, quadruples (i.e., several n-tuples, wherein n= 1 , 2, 3, 4, ...) of reflected light points or spots laterally shifted in front of the following optics. Thus, it is possible to generate or form an array of reflected light points or spots.

[0570] Further, in the case of multiple point light sources, the light spots S1 , S2 , S3... Si (i = 4, 5, 6... 12) preferably lie on a straight line g after the mirroring-in and the main rays of the light beams may coincide with the straight line g.

[0571] In a further exemplary optical system (no figure), the multispectral light source unit may be configured to emit two multispectral and spatially coherent light beams B1 and B2 with a particularly broad light spectrum, wherein all light beams have the same centroid wavenumber kS1. The two light beams B1 and B2 each have a different curvature of the wavefront at a point W at the output of a light source unit with the thus two different radii of curvature rp1 and rp2 of the wavefronts. At the point WR, the light beams B1 R and B2R have two different radii of curvature r1 and r2 of the two wavefronts. In this optical system, a single reflector may be used for the light source unit and for the detection unit. This reflector may have two different curvatures rSL1 and rSL2 laterally distributed or arranged in its reflection field. The reflector may, for example, be designed as a concentric mirror arrangement or as a mirror arrangement in a rosette form or as a mirror arrangement with statistically laterally distributed mirror areas. Thus, there is no full pupil, since laterally distributed / arranged mirror areas exist. A single end reflector is also arranged in the dual-beam interferometer (not to be confused with the reflector of the light source unit and detection unit). The end reflector in the reference arm has two mirrors, which have radiuses of curvatures rS 1 and rS2. The end reflector can, for example, be formed as a concentric mirror arrangement or as a mirror arrangement in rosette arrangements or as a mirror arrangement with statistically distributed mirror areas.

[0572] In yet a further exemplary optical system (no figure), the multispectral light source unit is configured to emit three multispectral and spatially coherent light beams B1 , B2 and B3 with differently curved wavefronts at a point W at the output of the multispectral light source unit with the thus three mutually different radii of curvature rp1 , rp2 and rp3 of the wavefronts. At a point WR, the light beams B1 R to B3R have three different radii of curvature r1 , r2 and r3 of the three wavefronts. All multispectral light beams exhibit a broad light spectrum with the same centroid wavenumber kS1 . In this optical system, a single reflector may be used for the light source unit and for the detection unit. The reflector has three different curvatures rSL1 , rSL2 and rSL3 laterally distributed / arranged in its reflection field. The reflector can, for example, be designed as a concentric mirror arrangement or as a mirror arrangement in a rosette form or as a mirror arrangement with statistically distributed mirror areas. In the interferometer, there is a single end reflector (not to be confused with the reflector of the light source unit and detection unit) with three mirrors, which have the curvatures rS 1 rS2 and rS3, respectively. The end reflector in the reference arm may, for example, be designed as a concentric mirror arrangement or as a mirror arrangement in a rosette form or as a mirror arrangement with statistically distributed mirror areas.

[0573] In yet a further exemplary optical system (no figure), there are four multispectral and spatially coherent light beams B1 , B2, B3 and B4 each with different curvature of the wavefront at a point W at the output of a light source unit with the thus four mutually different radii of curvature rp1 , rp2 , rp3 and rp4 of the wavefronts. At a point WR, the light beams B1 R to B4R have four different radii of curvature r1 , r2, r3 and r4 of the four wavefronts. The first and the second light beams have the centroid wavenumber kS1 . The third and fourth light beams have the centroid wavenumber kS2. In this optical system, two preferably identical reflectors are used for the light source unit and for the detection unit. The two reflectors have two different curvatures rSL1 and rSL2 in their reflection field and are preferably arranged at different depths to produce different wavefront curvatures. The two reflectors can, for example, be designed as concentri...

Claims

Claims1 . An optical system for chromatic confocal spectral-domain optical coherence tomography or for chromatic confocal two-beam interferometry, comprising: a multispectral light source unit (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109) configured to generate a plurality of spatially coherent multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...), wherein each of the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) is formed by multispectral light having a spectrum with a centroid wavelength lambda (S_1 , S_2, S_i, with i = 3, 4, ...) and a corresponding centroid wavenumber (kS1 , kS2, kSi, with i = 3, 4, ...), an achromatic focusing unit (5, 51 , 511 , 52) having an optical axis (OAS) at its output, wherein the achromatic focusing unit (5, 51 , 511 , 52) is configured such that the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) generated from the multispectral light source unit (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109) and passed through the achromatic focusing unit (5, 51 , 511 , 52) have at least approximately spherical wavefronts (W1 , W2, Wi, with i = 3, 4, ...) in at least a paraxial region around a common point (W) located on the optical axis (OAS) at the output of the achromatic focusing unit (5, 51 , 511 , 52), wherein the wavefronts (W1 , W2, Wi, with i = 3, 4, ...) have different radii of curvatures (r1 pW, r2pW, ripW, with i = 3, 4, ...); a chromatic confocal two-beam interferometer (6, 61 ) having a non-zero optical path difference (xp) arranged downstream of the multispectral light source unit (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109), wherein the chromatic confocal two-beam interferometer comprises: an achromatic reference arm (R, R1 , R2) having an optical axis (OAR), wherein at least one end reflector (711 , 712, 713, 714, 715, 716, 717, 718, 782, 784) is arranged in the reference arm (R, R1 , R2); an object arm (O), wherein a chromatic depth splitting unit (70, 701 , 702)is arranged in the object arm, a beam splitting unit (22, 60) configured to split each of the multispectral light beams (B1 , B2, Bi) which has passed through the achromatic focusing unit (5, 51 , 511 , 52) into a pair of a multispectral reference light beam (B1 R, B2R, BiR, with i = 3, 4, ...) in the reference arm and a multispectral object beam (B1 O, B2O, BiO, with i = 3, 4, ...) in the object arm (0); a detection unit (16, 161 , 162, 163, 164, 165, 166) configured to detect interfering light reflected by the at least one end reflector (711 , 712, 713, 714, 715, 716, 717, 718, 782, 784) in the achromatic reference arm (R, R1 , R2) and multispectral light reflected from a measured object (8) arranged in the object arm (0), said detection unit (16, 161 , 162, 163, 164, 165, 166) comprising a confocal discriminator unit and at least one spectrometer (281 , 28, 29), evaluation dual-beam interferometer (282) or fast detector (283), wherein the optical system is configured such that: in the reference arm (R; R1 , R2), each of the multispectral reference light beams (B1 R, B2R, BiR, with i = 3, 4, ...) forms a respective substantially diffraction limited reference light spot (S1 R’, S2R’, SiR’, with i = 3, 4, ...), and wherein the reference light spots (S1 R’, S2R’, SiR’, with i = 3, 4, ...) formed by the different multispectral reference light beams (B1 R, B2R, BiR, with i = 3, 4, ...) are spatially separated and lie on a straight line, which is substantially coincident with the optical axis (OAR) of the reference arm (R; R1 , R2); and in the object arm (0), each of the multispectral object beams (B10, B20, BiO, with i = 3, 4, ...) is chromatically split by the chromatic depth splitting unit (10, 701 , 702) and forms a respective multispectral stretch (dzc1_H, dzc2_H, dzci_H, with i = 3, 4, ...) of diffraction limited light spots extended in a depth direction of the object arm (0), wherein the multispectral stretches (dzc1_H, dzc2_H, dzci_H, with i = 3, 4, ...) of diffraction limited light spots are at least approximately collinear and at least partially overlap in the depth direction, thereby forming an overlapping region (UE), and wherein a current diffraction limited object light spot (S10kT, S20k2’, SiOki', with i = 3, 4, ...) of each of the multispectral stretches (dzc1_H, dzc2_H, dzci_H, with i = 3, 4, ...) of diffraction limited light spots, the current diffraction limited object light spot (S10kT, S20k2’, SiOki', with i = 3, 4, ...) having a current wavenumber (k1 , k2, ki, with i = 3,4, ...), is focused at a current measurement point of the measured object (8) in the overlapping region (UE); the confocal discriminator unit confocally discriminates light from the reference arm (R, R1 , R2) after reflection on the at least one end reflector (711 , 712, 713, 714, 715, 716, 717, 718, 782, 784) and light from the object arm (0) after reflection on the measured object (8), and the at least one spectrometer (281 , 28, 29), evaluation dual-beam interferometer (282) or fast detector (283) detects the confocally discriminated light.

2. The optical system according to claim 1 , wherein the centroid wavelengths (S_1 , S_2, S_i, with i = 3, 4, ...) or the centroid wavenumbers (kS1 , kS2, kSi, with i = 3, 4, ...) of the multispectral spatially coherent light beams (B1 , B2, Bi, with i = 3, 4, ...) emitted by the multispectral light source unit (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109) are different from each other.

3. The optical system according to any one of the preceding claims, wherein the optical path difference (xp) of the chromatic-confocal dual-beam interferometer (6, 61 ) is equal to or greater than ten times and equal to or less than ten thousand times one of the centroid wavelengths of the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) generated by the multispectral light source (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109); and / or wherein the optical path difference (xp) is substantially the same for all wavelengths of the light spectra of the multispectral light beams (B1 , B2, Bi, with i = 3,4. ...) entering the chromatic-confocal dual-beam interferometer (6, 61 ).

4. The optical system according to any one of the preceding claims, wherein the optical system is configured such that diffraction limited object light spots (S1 OST, S20kS2’, SiOkSi', with i = 3, 4, ...) of each of the multispectral stretches (dzc1_H, dzc2_H, dzci_H, with i = 3, 4, ...) of diffraction limited light spots having the centroid wavenumbers (kS1 , kS2, kSi) coincide in a confocal point (cPS) in the object arm (O); ordiffraction limited object light spots (S10ST, S20kS2’, SiOSki', with i = 3, 4, ...) of each of the multispectral stretches (dzc1_H, dzc2_H, dzci_H, with i = 3, 4, ...) of diffraction limited light spots having the centroid wavenumbers (kS1 , kS2, kSi, with i = 3, 4, ...) are positioned in a depth range (dtc), which is at most 50% or at most 30% or at most 10% of the overlapping region (UE).

5. The optical system according to any one of the preceding claims, wherein the at least one end reflector (711 , 712, 713, 714, 715, 716, 717, 718, 782, 784) comprises at least a first spherically curved mirror (72, 74, 721 , 731 , 732, 74, 751 , 752, 753, 781 , 783) or mirror segment (761 , 762, 763, 771 , 772, 773), wherein a center point (Mi, with i = 3, 4, ...) of the first spherically curved mirror or mirror segment lies on the optical axis (OAR) of the reference arm (R; R1 , R2) or, in case of a split reference arm with two or more sub-arms (R1 , R2), on an optical axis of a sub-arm (R1 , R2) of the split reference arm, and wherein a first reference light spot (S1 R') of the plurality of reference light spots (S1 R’, S2R’, SiR’, with i = 3, 4, ...) is formed at the center point (Mi, with i = 3, 4, ...) by a first reference light beam (B1 R) of the plurality of reference light beams (B1 , B2, Bi, with i = 3, 4, ...).

6. The optical system according to claim 5, wherein the end reflector (711 , 782, 784) comprises a single first spherically curved mirror (72, 74, 781 , 783), and wherein a second reference light spot (S2R’) of the plurality of reference light spots (S1 R’, S2R’, SiR’, with i = 3, 4, ...) is formed by a second reference light beam (B2R) from the plurality of reference light beams (B1 , B2, Bi, with i = 3, 4, ...) at a point of incidence (AR) of a main ray of the second reference light beam (B2R) on the first spherically curved mirror (72, 74, 711 , 781 , 783); or at a point of intersection (GO) of an osculating sphere (SK1 ) of the first spherically curved mirror (72, 74, 711 , 781 , 783) with the optical axis (OAR) of the reference arm (R) or, in case of a split reference arm, an optical axis of a sub-arm (R1 , R2) of the split reference arm.

7. The optical system according to claim 5,(i) wherein the at least one end reflector (713, 714, 715, 716, 717, 718) further comprises at least one second spherically curved mirror (731 , 732, 74, 751 , 752, 753) or mirror segment (761 , 762, 763, 771 , 772, 773), and wherein a second reference light spot (S2R’) of the plurality of reference light spots is formed by a second reference light beam (B2R) from the plurality of reference light beams (B1 , B2, Bi, with i = 3, 4, ...) at a point of incidence (AR) of the main ray of the second reference light beam (B2R) on one of the plurality of spherically curved mirrors (731 , 732, 74, 751 , 752, 753) or mirror segments (761 , 762, 763, 771 , 772, 773), or at a point of intersection (GO) of an osculating sphere (SK1 , SK2) of one of the spherically curved mirrors (721 , 731 , 732, 74, 751 , 752, 753) or mirror segments (761 , 762, 763, 771 , 772, 773), with the optical axis (OAR) of the reference arm or, in case of a split reference arm, an optical axis of a sub-arm (R1 , R2) of the split reference arm, or at a center point (M2) of the at least one second spherically curved mirror or mirror segment (761 , 762, 763, 771 , 772, 773); or(ii) wherein the end reflector (712) further comprises a plane mirror (741 ) and the first spherically curved mirror (74) is arranged concentrically around the plane mirror (741 ), and wherein a second reference light spot (S2R’) of the plurality of reference light spots (S1 R, S2R, SiR’, with i = 3, 4, ...) is formed by a second reference light beam (B2R) of the plurality of reference light beams (B1 , B2, Bi, with i = 3, 4, ...) at a point of incidence (AR) of a main ray of the second reference light beam (B2R) on the plane mirror (741 ).

8. The optical system according to claim 7, wherein the end reflector (713, 714, 715, 716, 717, 718) comprises a plurality of concentric spherically curved mirrors (731 , 732, 74, 751 , 752, 753) or a plurality of spherically curved mirror segments (761 , 762, 763, 771 , 772, 773) arranged in a rosette form, wherein the osculating spheres of the plurality of spherically curved mirrors (731 , 732, 74, 751 , 752, 753) or mirror segments (761 , 762, 763, 771 , 772, 773) contact each other in a single contact point (BP), and wherein optionally the point of incidence (AR) of a main ray of the second reference light beam (B2R) coincides with the contact point (BP).

9. The optical system according to any one of the preceding claims, wherein the end reflector (711 , 712, 713, 714, 715, 716, 717, 718, 782, 784) is a fixed end reflector or a movable end reflector; and / or an array (719) of end reflectors is arranged in the reference arm (R; R1 , R2).

10. The optical system according to any one of the preceding claims, wherein the confocal discriminator unit comprises a plurality of confocal discriminators (271 , 272, 471 , 472, 473, 474), each confocal discriminator (471 , 472, 473, 474), being configured to confocally discriminate a combined light spot formed from light coming from a reference light spot (S1 R’, S2R’, SiR’, with i = 3, 4, ...) and a corresponding current object light spot (S1 OST, S20kS2’, SiOki', with i = 3, 4, ...), wherein the reference light spot and the corresponding object light spot are formed from light of a single one of the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...); or wherein the confocal discriminator unit comprises a beam combining unit configured to combine light coming from the plurality of reference light spots (S1 R’, S2R’, SiR’, with i = 3, 4, ...) and the corresponding current object light spots (S1 OST, S20kS2’, SiOki', with i = 3, 4, ...), and a confocal discriminator configured to confocally discriminate the combined light.11 . The optical system according to any one of the preceding claims, wherein the chromatic depth splitting unit comprises a diffractive-optical element (70, 701 , 702) with a negative or positive focusing power.

12. The optical system according to claim 12, wherein the object arm (0) and / or the reference arm (R) comprises further at least one optical compensating element configured to overcompensate or undercompensate the focusing power of the diffractive-optical element (70, 701 , 702).

13. A method for chromatic confocal spectral-domain optical coherence tomography or for chromatic confocal two-beam interferometry, comprising: providing an optical system for chromatic confocal spectral-domain opticalcoherence tomography or for chromatic confocal two-beam interferometry according to any one of the above claims; generating, by the multispectral light source unit (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109) a plurality of spatially coherent multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...), wherein each of the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) is formed by multispectral light having a spectrum with a centroid wavelength lambda (S_1 , S_2, S_i, with i = 3, 4, ...) and a corresponding centroid wavenumber (kS1 , kS2, kSi, with i = 3, 4, ...), forming, by the achromatic focusing unit (5, 51 , 511 , 52), the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) emitted from the multispectral light source unit (100, 101 , 102, 103, 104, 104a, 105, 106, 107, 107a, 108, 109), such that after passing through the achromatic focusing unit (5, 51 , 511 , 52), the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) have at least approximately spherical wavefronts (W1 , W2, Wi, with i = 3, 4, ...) in at least a paraxial region around a common point (W) located on the optical axis (OAS) at the output of the achromatic focusing unit (5, 51 , 511 , 52), wherein the wavefronts (W1 , W2, Wi, with i = 3, 4, ...) have different radii of curvatures (r1 pW, r2pW, ripW, with i = 3, 4, ...); splitting, by the beam splitting unit (22, 60) of the chromatic confocal two-beam interferometer (6, 61 ), each of the multispectral light beams (B1 , B2, Bi, with i = 3, 4, ...) which has passed through the achromatic focusing unit (5, 51 , 511 , 52) into a pair of a multispectral reference light beam (B1 R, B2R, BiR, with i = 3, 4, ...) in the reference arm and a multispectral object beam (B1 O, B2O, BiO, with i = 3, 4, ...) in the object arm (O), wherein in the reference arm (R; R1 , R2), each of the multispectral reference light beams (B1 R, B2R, BiR, with i = 3, 4, ...) forms a respective substantially diffraction limited reference light spot (S1 R’, S2R’, SiR, with i = 3, 4, ...), and wherein the reference light spots (S1 R’, S2R’, SiR’, with i = 3, 4, ...) formed by the different multispectral reference light beams (B1 R, B2R, BiR, with i = 3, 4, ...) are spatially separated and lie on a straight line, which is substantially coincident with the optical axis (OAR) of the reference arm (R; R1 , R2); and wherein in the object arm (O), each of the multispectral object beams (B10, B2O, BiO, with i = 3, 4, ...) is chromatically split by the chromatic depth splitting unit (10, 701 ,702) and forms a respective multispectral stretch (dzc1_H, dzc2_H, dzci_H, , with i =3, 4, ...) of diffraction limited light spots extended in a depth direction of the object arm (0), wherein the multispectral stretches of diffraction limited light spots (dzc1_H, dzc2_H, dzci_H, , with i = 3, 4, ...) are at least approximately collinear and at least partially overlap in the depth direction, thereby forming an overlapping region (UE), and wherein a current diffraction limited object light spot (S10kT, S20k2’, SiOki', with i = 3,4, ...) of each of the multispectral stretches (dzc1_H, dzc2_H, dzci_H, with i = 3, 4, ...) of diffraction limited light spots, the current diffraction limited object light spot (S10kT, S20k2’, SiOki', with i = 3, 4, ...) having a current wavenumber (k1 , k2, ki), is focused at a current measurement point of the measured object (8) in the overlapping region (UE); confocally discriminating, by the confocal discriminator unit, combined light from the reference arm (R, R1 , R2) after reflection on the at least one end reflector and light from the object arm (0) after reflection on the measured object (8), and detecting, by the at least one spectrometer (281 , 28, 29), evaluation dual-beam interferometer (282) or fast detector (283), the confocally discriminated light.

14. The method according to claim 13, further comprising processing the detected light to obtain depth information of the current measurement point.

15. The method according to claim 13 or 14, further comprising scanning a plurality of measurement points of the measured object (8) and carrying out the method according to claim 13 or 14 for each of the plurality of measurement points.

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