Single-frame ramp interferometer

Abstract

An interferometer (10) is provided for measuring the surface (108) or optical thickness of an optically smoothed test object (122), wherein the interferometer (10) is configured to simultaneously illuminate the optically smoothed test object with multiple object waves (14.1, 14.2, 14.3) having different wavelengths from each other, and to superimpose the object waves (14.1, 14.2, 14.3) deformed by the illuminated test object onto a coherent reference wave on an image capturing device (K), and to decompose the spectrum of the superimposed interferogram into wavelength-specific partial interferograms.

Description

Technical Field

[0001] This invention relates to an interferometer and method for measuring the surface or optical thickness of an optically smooth test object. The interferometer can be used to measure optical surfaces and is a precise and rapid measuring device. Background Technology

[0002] Interferometers utilize the wave properties of light. When a known reference wave is superimposed on a wave of an object that is coherent with it and is typically deformed by the surface of the test object, regions with extinction and regions with optical amplification appear. The resulting intensity images and interferograms contain information about the deviation of the test object from the desired shape and can be evaluated.

[0003] An interferometer having the features of the preamble of claim 1 and a method having the preamble of claim 10 are considered to be known.

[0004] In one embodiment, a known interferometer is used, for example, in [Garbusi, Eugenio; Pruss, Christof; Osten, Wolfgang: “Interferometer for precise and flexible asphere testing”, Optics Letters, 33, 2973-5 (2009), 10.1364 / OL.33.002973], for surface measurement of the surface or optical thickness of an optically smoothed component. The interferometer is configured to illuminate the optically smoothed test object using multiple illumination configurations, and to superimpose an object wave onto a reference wave coherent with the object wave on at least one image capturing device (K) to produce an interferogram, wherein the object wave is altered by reflection from the test object at the surface being measured or by transmission through the test object.

[0005] As a result, each illumination configuration is the totality of object waves simultaneously illuminating the test object. Each object wave is preferably emitted from a point light source. Therefore, an illumination configuration can also be understood as the totality of point light sources emitting the object waves. An optically smooth surface should be understood as a reflective surface.

[0006] The increasing use of aspherical or freeform surfaces in optical design allows for higher image quality and more compact sizes in optical systems, but this necessitates measurement techniques for the manufacturing process, which can be integrated with production. The production of optics with the aforementioned accuracy requirements is only possible if the production machine receives feedback from the measurement techniques, allowing subsequent calibration steps to occur in a targeted manner. In final quality control, such as in the case of replicating optics, surface measurements must confirm the quality of the optics and detect deviations in the manufacturing process as quickly as possible. For this feedback, fast, flexible, and still highly accurate measurement techniques are essential.

[0007] Previous solutions can be categorized into scanning systems and surface measurement systems. Scanning systems measure the test object point-by-point (e.g., tactile or optical coordinate measuring machines) or subsequently by calculating and combining individual small-area regions (e.g., Zygo Verifire aspherical scanning interferometer, QED SSiA stitched interferometer).

[0008] Due to the inherent sequential approach, a stable measurement environment is required, and the measurement process can last from minutes to hours. Therefore, integration with production becomes more difficult; achieving a high rate of 100% testing (e.g., injection molding of precision optics) is impossible.

[0009] So-called zero-lenses hold great potential for applications in rapid measurement techniques. These are lenses that are recalculated and manufactured for each new type of aspherical or freeform surface, adapting the interferometer's wavefront to the specific test object. However, the potential speed advantage is outweighed by the high cost and long lead times of zero-lenses.

[0010] A flexible interferometric technique with low measurement uncertainty that can be used for rapid measurement of aspherical and freeform lenses is the tilting wave interferometer (TWI) invented by the Institut für Technische Optik.

[0011] TWI achieves measurement of the test object in less than a minute through comprehensive detection and calculation of systematic errors of special lighting and instruments, without the need for complex zero-lens.

[0012] The measurement of dimensional accuracy for optical components requires resolution within a fraction of the wavelength A used (e.g., A / 100, where A is, for example, 400 nm to 800 nm in the visible light range). This translates to resolution in the single-nanometer range. This can be considered a solvable problem for applications involving spherical or planar surfaces, thanks to the use of interferometers that measure surfaces in a planar manner.

[0013] However, unlike measurements of spherical and planar surfaces, interferometry of aspherical surfaces presents a problem in the field of optical measurement technology, and the solution remains unsatisfactory in many areas. The root of these problems lies in the fact that zero-point testing of aspherical surfaces always requires the production of special refractive or diffractive lenses as suitable zero-point lenses for aspherical surfaces.

[0014] Standardized refractive and diffractive zero lenses for aspherical surfaces can now be produced; diffractive structures are used to shape the spherical object wave of the interferometer lens in a manner suitable for generating the wave of the test object. Because aspherical surfaces are produced in a wide variety of forms, producing adaptive zero lenses requires significant time and cost.

[0015] Existing solutions cannot measure aspherical or freeform surfaces in a single camera exposure. Many consecutive individual measurements must be captured, making known methods susceptible to environmental influences. Only zero-lens measurements are possible within a single camera exposure cycle; however, due to the single zero-lens requirement, this method is too complex and expensive for many applications.

[0016] It is known that the interferogram resulting from the superposition of two wavefronts can be evaluated using different methods; that is, the phase difference between the two wavefronts (hereinafter referred to as phase) can be determined based on the intensity distribution that can be recorded by a camera. In interferometry, this phase carries information about the object being tested, which is why its determination is essential for any interferometric technique. Known methods for determining the phase include the very common phase-shifting method, in which multiple camera images must be captured. However, methods are also known that can determine the phase from only one camera image.

[0017] According to the abstract of WO2005052502A2, a phase difference sensor is known for measuring the spatially resolved phase difference between an orthogonally polarized reference beam and a test wavefront. The sensor is constructed as a pixelated phase mask aligned with and imaged on a pixelated detector array. Each adjacent pixel of the phase mask measures a predetermined relative phase shift between a circularly orthogonally polarized reference beam and a test beam. Thus, multiple phase-shift interferograms can be synthesized simultaneously by combining pixels with the same phase shift. It should be possible to combine multiple phase-shift interferograms to calculate the phase difference between the reference wavefront and the test wavefront. Any configuration of the interferometer that generates the orthogonally polarized reference beam and object beam can be combined with the phase difference sensor to provide a single-trigger measurement for simultaneous phase shifts.

[0018] Another method that can determine the phase from just one camera image is the so-called carrier frequency method, in which the phase can be obtained from the sideband modulation principle, see, for example, Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, “Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry”, J. Opt. Soc. Am. 72, 156-160 (1982).

[0019] The latest, fastest, and most flexible method, TWI, typically requires four phase measurements in different lighting distributions, with each phase measurement requiring five camera captures.

[0020] The publication (X. Tian et al., "Snapshot multi-wavelength interference microscope", Opt. Express 26, 18279–18291 (2018), DOI: 10.1364 / OE.26.018279) discloses a single-image multi-wavelength interferometer for surface measurements, in which the surface is illuminated with light of different colors incident from the same direction, and in which the light emitted by the surface is decomposed into wavelength-indicated partial interferograms. Tian et al. thus achieved a method to simultaneously obtain phase information about all three wavelengths required for multi-wavelength interferometry in each pixel. Summary of the Invention

[0021] In view of this background, the object of the present invention is to provide a method and measuring apparatus of the type described at the beginning, which can be used to accurately measure optical surfaces in a way that reduces time consumption without the need for a zero lens.

[0022] This objective is achieved by an interferometer having the features of claim 1 and a method having the features of claim 10.

[0023] The interferometer according to the invention is characterized in that each illumination configuration is implemented by a separate color channel and / or by an object wave having a wavelength range with a center wavelength specific to that wavelength range, wherein a first illumination configuration in the illumination configuration has only an object wave with a wavelength range having a first center wavelength, and wherein a second illumination configuration in the illumination configuration has only an object wave with a wavelength range having a second center wavelength, and wherein a third illumination configuration in the illumination configuration has only an object wave with a wavelength range having a third center wavelength, the first center wavelength being different from the second center wavelength, the third center wavelength being different from the first center wavelength and the second center wavelength, and wherein the interferometer is configured to simultaneously illuminate a test object from discretely different (i.e., non-parallel) directions using the object waves of the first illumination configuration, the object waves of the second illumination configuration, and the object waves of the third illumination configuration, and after interacting with the test object, superimpose the object waves returning from the test object on an image capturing device, and wherein the image capturing device is configured to decompose the spectrum of the superimposed interferogram into wavelength-specific partial interferograms.

[0024] This invention enables the sequential switching of a lighting configuration from one lighting configuration to the next previously used lighting configuration in parallel over time by using spectral information.

[0025] Each illumination configuration is achieved using a separate color channel and / or a separate center wavelength. This means that the configuration is not only illuminated by a light source, but also by, for example, three or four laser light sources of different wavelengths. This invention allows for the measurement of precision optical devices such as aspherical and freeform surfaces with low measurement uncertainty in a very short time. Therefore, the measurement processes required for optical device production can be integrated into production, which can significantly improve productivity by saving test logistics and measurement time. By detecting the entire morphology of the test object in a single exposure, the effects of unstable environmental conditions are minimized. New measurement methods are made possible for measuring moving test objects. Single-frame TWI measurement technology enables interferometric measurements of variable test objects, such as moving test objects in continuous production. The dynamic behavior of variable-shaped test objects, such as corneas, vibrating surfaces, or astronomical mirrors, can also be measured with high measurement accuracy using the single-frame TWI measurement technology according to the invention.

[0026] The ability to flexibly measure highly aspherical and freeform surfaces in a single exposure without using a special zero-lens is novel. This invention makes it possible to replace the sequential changes in the illumination configuration required for TWI with parallel, wavelength-separated illumination schemes, which simultaneously enables phase evaluation methods based on individual images.

[0027] Preferably, the fourth illumination configuration in the illumination configuration has only an object wave with a fourth center wavelength in the wavelength range, wherein the fourth center wavelength is different from the first center wavelength, the second center wavelength, and the third center wavelength, and wherein the interferometer is configured to simultaneously illuminate the test object from discrete different directions using the object waves of the first illumination configuration, the second illumination configuration, the third illumination configuration, and the fourth illumination configuration, and after interacting with the test object, superimpose the object waves returning from the test object on the image capturing device, and wherein the image capturing device is configured to decompose the interferogram spectrum generated by the superposition into wavelength-specific partial interferograms.

[0028] Another preferred embodiment is characterized in that the interferometer is configured to simultaneously illuminate the optical smoothing component using a first illumination configuration and a second illumination configuration, or simultaneously illuminate the optical smoothing component using a first illumination configuration, a second illumination configuration, and a third illumination configuration, or simultaneously illuminate the optical smoothing component using a fourth illumination configuration. Each illumination configuration may have an object wave or a group of object waves, wherein the group of object waves may also have up to several hundred object waves.

[0029] Preferably, the object wave propagates in the closest adjacent direction, that is, the object wave is emitted, for example, from two point light sources arranged adjacent to each other, and does not contain wavelengths from the same wavelength range. In principle, it is also conceivable that the point light source is constructed as a virtual point light source.

[0030] More preferably, the interferometer is configured to separate the interferogram using a suitable color filter, such that each pixel of the camera detector detects interferogram information for essentially only one wavelength range.

[0031] Preferably, the object wave source is a point light source.

[0032] A further preferred embodiment is characterized in that the point light sources are arranged such that each light source is arranged at the center of an imaginary regular hexagon, wherein the hexagons are adjacent to each other in a plane without gaps, and the point light sources are arranged such that a light source having light with a central wavelength or a light source emitting light with a wavelength of one wavelength range within the central wavelength range at its center is adjacent only to a hexagon having light with a different wavelength or a wavelength of another wavelength range at its center.

[0033] Preferably, the point light sources are arranged such that each light source is arranged at the center of an imaginary regular square, wherein the squares are adjacent to each other in a plane without gaps, and wherein a square having a light source at its center that emits light with a central wavelength or light with a wavelength of one wavelength range is adjacent only to a square having a light with another wavelength or another wavelength range at its center.

[0034] Furthermore, preferably, in addition to the color filter, the image capturing device (K) also has polarization filters with different orientations on each pixel.

[0035] Another preferred embodiment is characterized in that the plurality of point light sources for each center wavelength and / or for each wavelength range and / or for each color channel are coherent laser sources, Keplerian telescopes, and point light source arrays composed of a transparent substrate (e.g., a glass substrate), wherein microlenses are provided, the light incident side of the microlenses facing the telescope, and the focal point of the microlenses is located in a plane on the light emitting side of the substrate, an aperture array is located in said plane, and object waves of one wavelength range are emitted from the microlenses. The microlenses may be designed, for example, to be refractive, i.e., having a continuous surface, or to be designed to be diffractive, i.e., as Fresnel zone plates, or as a hybrid design of these embodiments.

[0036] Preferably, the multiple point light sources for each center wavelength and / or for each wavelength range have a structure that couples to the fiber end of a laser source via one or more optical couplers, the laser source emitting light with a center wavelength.

[0037] Other advantages can be found in the instruction manual and accompanying drawings.

[0038] It should be understood that, without going beyond the scope of this invention, the above features and those features which will still be explained below can be used not only in the separately specified combinations, but also in other combinations or individually. Attached Figure Description

[0039] Embodiments of the present invention are shown in the accompanying drawings and explained in more detail in the following description. The drawings are shown in schematic form as follows:

[0040] Figure 1 Examples of embodiments of known interferometers are shown;

[0041] Figure 2 The light-emitting side of a point light source array is shown, wherein the point light sources are arranged in rows and columns;

[0042] Figure 3 The wavefronts of three objects, tilted relative to each other, are shown reaching an optically smooth surface;

[0043] Figure 4a The outline of the interferogram captured by four point light sources in each row and column is shown, with all point light sources turned on simultaneously;

[0044] Figure 4b The pattern of four interferograms captured using the same interferometer is shown, resulting in captures in chronological order;

[0045] Figure 5 It is a plan view of a colored mosaic point light source array with the first arrangement of point light sources;

[0046] Figure 6 A side view of a colored mosaic point light source array is shown;

[0047] Figure 7 This is a side view of an alternative point light source array based on optical fibers;

[0048] Figure 8 It is a plan view of a second-row colored mosaic point light source array with point light sources;

[0049] Figure 9 The Thyman Green tilt wave interferometer is shown;

[0050] Figure 10 shows the Fizeau tilt wave interferometer;

[0051] Figure 11 The Mach-Zehnder interferometer is shown; and

[0052] Figure 12 A flowchart is shown as an example of an embodiment of the method according to the present invention. Detailed Implementation

[0053] Figure 1 An interferometer operating using a tilted wavefront is illustrated in detail. Light from a coherent laser source L with only one center wavelength (defined as the wavelength of a narrow wavelength range with maximum intensity) is split into a test beam path and a reference beam path by a first polarization beam splitter ST1. The light in the test beam path is expanded by a Keplerian telescope consisting of a first microscope objective M1 and a collimating lens L1, and illuminates a point source array PLQA. PLQA consists of a quartz glass substrate in which diffractive microlenses are positioned above the collimating lens on their light-incident side. The focal points of the microlenses are located in a plane on the rear side of the substrate forming the light-outcryogenic side, where chromium is vapor-deposited on the rear side of the substrate. The chromium layer has a spatial filter aperture corresponding to each microlens, which removes unwanted diffraction orders from the microlenses and serves as a spatial filter for the wavefront. Additionally, after PLQA, there is a pinhole array AA, which is displaceable relative to PLQA and transmits light only from each fourth microlens.

[0054] Figure 2 A schematic plan view of the light-emitting side of the point light source array is shown, where the point light sources are arranged in rows and columns, each point light source having one of the microlenses and one aperture of the pinhole array. The number of pinhole apertures is one-quarter of the number of microlenses.

[0055] The rows and columns are arranged at right angles to each other. The pinhole array covers all other point light sources. By moving the pinhole array parallel to the rows and columns, a four-point light source structure can be formed. In this way, four lighting configurations that are tilted relative to each other can be produced.

[0056] As an example, Figure 3 Three wavefronts 14.1, 14.2, and 14.3, which are tilted relative to each other, are shown for an object wave reaching an optically smooth surface.

[0057] Figure 1 It also shows that light emitted from a point source passes through a second beam splitter ST2 and is collimated by a collimating lens L2, thereby producing an array of wavefronts of object waves with different inclinations.

[0058] The wavefront of the object wave is transformed into a spherical wavefront by the interferometer objective lens O to compensate for the fundamental curvature of the test object SUT. After reflection on the test object SUT, the wavefront propagates back to the second beam splitter ST2, where it is reflected into the image capture beam path. An aperture A with a square opening lies in the Fourier plane of the interferometer 10 and shields all areas of the wavefront with a fringe density exceeding the Nyquist frequency of the image capture device K (e.g., a camera). After aperture A, light passes through the imaging optics AO and reaches the image capture device K.

[0059] In the reference beam path, the polarization of the light emitted from the first polarization beam splitter ST1 is rotated by π using a λ / 2 plate W, so that the polarization of the reference wave matches the polarization of the object wave (test wave) reflected from the test object SUT. The beam is deflected by two mirrors S1 and S2, one of which is mounted on a piezoelectric actuator PS to enable evaluation of the wavefront using a phase-shifting method. The light is expanded by a Keplerian telescope consisting of a second microscope objective M2 and a lens L3, and then transformed into a spherical wavefront by a lens L4. The focal point of the reference wavefront is located at the center of aperture A. Subsequently, the light passes through the imaging optics AO and reaches the image acquisition device K, where it interferes with the light from the test wavefront in the plane of the image acquisition device K.

[0060] Each point light source typically produces a measurement range in the form of a small interferogram (“block”) on the image capture device K. The pinhole array AA is shifted four times to complete the measurement. At each of the four positions, a partial measurement is performed by determining the phase using phase-shifting interferometry (PSI), i.e., the phase difference between the object wave and the reference wave caused by the different optical path lengths of the object wave and the reference wave in the block region.

[0061] The distance between the light sources 12 in the PLQA is chosen such that light reaches the image capturing device at all points on the surface of the test object located within the numerical aperture NA of the interferometer objective lens O, and the areas of the image capturing device covered by adjacent point light sources slightly overlap. This ensures that information about the entire surface of the test object is included in the measurement.

[0062] The four partial measurements are then calculated to produce the overall measurement result. The reason for dividing the measurements into four parts is to avoid overlap between regions from different point light sources—so-called partial measurement patches. Only in this way can the interferogram of the partial measurements be evaluated with low measurement uncertainty.

[0063] Depending on the form of the test object, each illumination configuration (point light source configuration) generates one or more tiles. The precise distribution of the tiles depends on the shape of the test object, its location, and the design of the interferometer used.

[0064] However, it is impossible to use standard interferometric methods to assess these overlapping regions because the resulting interferogram is no longer generated by two wavefronts, but rather produces multiple beam interference.

[0065] Figure 4a An interferogram on an image capture device K is schematically illustrated. The interferogram has been captured using four point light sources per row and column, with all point light sources simultaneously activated. Interferogram patches generated by the light from each point light source are then superimposed. The interferogram cannot be evaluated in the shaded, overlapping areas.

[0066] Figure 4b Four patterns of interferogram 18, captured using the same interferometer, are shown, generated only when every other point light source is switched on for four different illumination configurations. These patterns are captured sequentially, one at a time for each case. For example, if only the even-numbered point light sources are switched on in each row and column of the Cartesian arrangement of the point light sources, one example of an illumination configuration is generated (other configurations: even-numbered point light sources in a row switched on with odd-numbered point light sources in a column; odd-numbered point light sources in a row switched on with even-numbered point light sources in a column; odd-numbered point light sources in a row switched on with odd-numbered point light sources in a row). Pattern 18 is then not superimposed, thus allowing for evaluation. Figure 4a The pattern is in Figure 1 When all 18 tiles of b are placed on top of each other, or in Figure 4bThe patch 18 and all the point light sources 18 generated simultaneously contribute to the exposure of the image capturing device.

[0067] Based on four phase measurements, the shape deviation of the test object (SUT) from its nominal shape can be determined. Since at least three image captures (typically five or more) must be obtained for phase measurements using phase-shift interferometry, compared to the carrier frequency method described above which allows phase measurements to be performed using only one image, and therefore at least 12 or more reasonably 20 camera images must be captured sequentially for a complete measurement, a measurement time period in the half-minute range is required to date. During this time period, unstable environmental conditions such as vibration, drift, and air turbulence can distort the measurement results.

[0068] In the method according to the invention, instead of sequential captures of interferograms with a single center wavelength generated in each case, individual captures of camera images with object waves having different wavelengths and thus generating different interferograms occur, which can be separated from each other by wavelength-dependent filtering.

[0069] Specifically, the interferometer according to the invention is characterized in that a first object wave propagating along a first discrete different direction has a first center wavelength, a second object wave propagating along a second discrete different direction has a second center wavelength, and a third object wave propagating along a third discrete different direction has a third center wavelength; and the interferometer is configured to simultaneously illuminate an optical smoothing component using the first object wave, the second object wave, and the third object wave, or to simultaneously illuminate the optical smoothing component using four object waves. This illumination configuration is, for example, in… Figure 3 Three object waves 14.1, 14.2, and 14.3 are generated as the first object wave, the second object wave, and the third object wave, respectively, wherein these object waves have different wavelengths from each other.

[0070] Therefore, the basic idea of ​​this invention is to parallelize the sequential selection of illumination constructs by using spectral information, and thus utilize a single capture instead of non-parallel capture of multiple illumination constructs.

[0071] In the interferometer according to the invention, each illumination configuration is achieved by a separate color channel and / or by a wavelength range having a center wavelength. The color or wavelength is separated from other illumination configurations / wavelengths at the image capture device K by a wavelength selective filter.

[0072] In this case, the illumination configuration is not limited to a single object wave; instead, the illumination configuration can comprise multiple object wavefronts that are discretely tilted relative to each other. Wavefronts tilted in discrete, different directions are associated with wave normals that are not parallel to each other as directions of propagation. These object waves illuminate the test object at discrete, different angles, or from discrete, different, and therefore non-parallel directions. This means that one or more wavefronts reach each point on the test object being measured, and that the propagation directions of these wavefronts are different.

[0073] This means that an interferometer exposed at a single point in time is not only illuminated by an illumination structure and / or an object wave with a central wavelength, but also simultaneously illuminated by, for example, three or four object waves of different wavelengths.

[0074] Each laser source is integrated into the structure in such a way that its light illuminates the test object at an angle different from that of other laser sources. This results in... Figure 3 The wavefronts shown belong to object waves with different wavelengths / colors. The desired result is to produce colored overlapping interferometric patches on the image capture device.

[0075] The image capture device K is preferably designed such that it can distribute the interferogram obtained from its wavelength through spatially resolved wavelength selection. This resolves interference superposition. An exemplary image capture device is manufactured by Sony and widely advertised under the name "Polysens".

[0076] Therefore, it is necessary to select the illumination angle of each illumination structure so that adjacent interference patches 18 do not have the same color / wavelength, because otherwise interference superposition areas with the same wavelength will appear.

[0077] Figure 5 A plan view of a point light source array 20 of an interferometer according to the present invention is shown. The point light source array 20 is characterized in that the point light sources 12 are arranged such that each point light source 12 is positioned at the center of an imaginary regular hexagon, wherein the hexagons are adjacent to each other in a plane without gaps, and such that a hexagon at the center emitting light of a central wavelength is adjacent only to a hexagon at the center emitting light of a different wavelength. Point light sources emitting light of the same wavelength are represented by the same shade of their associated hexagon. This allows the required number of illumination configurations to be reduced to three.

[0078] Different wavelengths can be found in different ranges within the visible light spectrum or adjacent spectral ranges (near-infrared or ultraviolet), such that the center wavelength represents light of different colors. This applies to all examples of the embodiments and is therefore generally applicable to interferometers according to the invention.

[0079] In this arrangement of point light sources on a two-dimensional grid with hexagonal unit cells, three light sources of different wavelengths are sufficient to meet the condition that adjacent elements must have different colors, while simultaneously fully illuminating the test object. If red light (e.g., 630nm-690nm), green light (e.g., 532nm), and blue light (e.g., 440nm-460nm) are used, all three illumination configurations can be registered in a single image capture, assuming an RGB color camera is used as the image capture device K, for example, a camera with a Bayer filter array before the camera pixels. The point light source array 20 can also be referred to as a color mosaic point light source array.

[0080] Figure 6 A side view of a color mosaic point light source array 20 is shown. The color mosaic point light source array produces point light sources with only one center wavelength. These are transformed by collimating lenses 5 into wavefronts that are approximately planes tilted relative to each other. The color mosaic point light source array consists of a microlens array 2 and an optional filter array 4. The microlens array 2 focuses the arriving light onto an aperture array 3, and the filter array 4 allows light with only one of n wavelengths to pass through each microlens. For each microlens, only one of the n wavelengths is ideally focused in the aperture plane. In the context of the preferred embodiment, n = 3 or 4. Figure 5 In the case of the color mosaic point light source array shown, n=3. Therefore, the color mosaic point light source array generates multiple point light sources from the arriving light wavefront with n different wavelengths.

[0081] As Figure 5 Alternatively, a fiber optic array fed by n=3 or 4 or even more light sources of different wavelengths can be used as an alternative to the microlens-based color mosaic point light source array 20. Figure 7 An array of point light sources based on optical fibers is shown. Light from a laser 6 with different wavelengths (n=3) is coupled into optical fiber 7, which is split into m fibers at a 1 / m coupler 8. A point light source is formed at the light exit end of optical fiber 6, and... Figure 5 and 8 The arrangement is shown. Here, point sources emitting light of the same wavelength are also represented by the same symbol. Therefore, Figure 7 An embodiment is shown in which multiple point light sources 12 for each center wavelength have optical fiber ends that are coupled to a laser source 6 that emits light with a center wavelength via an optical coupler 8.

[0082] Each set of point light sources 12, coupled to one of the lasers 6, forms an illumination structure. However, according to Figure 5The arrangement of the three illumination configurations ensures that the interferogram patches do not overlap. Parallel capture of these three illumination configurations is successful in a single capture because they are activated simultaneously, and the test object is thus simultaneously illuminated by wavelengths (or color channels) specific to the respective illumination configurations, and the image capture is spectrally filtered.

[0083] Therefore, for example, by using a local color filter before each pixel, the superimposed but color-distinguishable interference patches 18 can be separated again on the image capturing device K.

[0084] As a favorable result, all three / four lighting configurations can be displayed in a single image capture.

[0085] Phase assessment can traditionally be performed using a phase shifter; however, this requires capturing, for example, five separate images that can be captured in a fast sequence, making a typical measurement time of about one second possible.

[0086] Another implementation uses the previous lighting structure, where point light sources are arranged on a two-dimensional grid with square unit cells. However, this requires four light sources of different wavelengths (or four color channels). In this case, the color / wavelength channels on the image capture device can also be separated by a filter array in front of the pixels of the image capture device. In this case, the filter array is designed such that the filter of each pixel primarily transmits only one wavelength, while other wavelengths are blocked.

[0087] Figure 8 A colored mosaic point light source array is shown, wherein the point light sources are arranged such that each light source is positioned at the center of an imaginary regular square, wherein the squares are adjacent to each other in a plane without gaps, and wherein a square with a point light source at its center emitting light of a central wavelength (i.e., light of one color channel) is adjacent only to a square with a point light source at its center emitting light of another wavelength (i.e., light of another color channel). Point light sources 12 emitting light of the same wavelength are represented by the same shadow of their associated squares. Thus, this embodiment works with four illumination configurations and / or color channels.

[0088] In addition to the regular arrangements mentioned above, lighting configurations can also be used with irregular structures of point light sources, as long as it is ensured that no two adjacent point light sources use the same wavelength. For some test configurations, it is advantageous to position the point light sources such that they are not located on a flat surface, but rather, for example, on a curved surface, or on a conical, cylindrical, or pyramidal surface.

[0089] The present invention enables the capture of the form of a test object in a single camera image as follows: the light reflected or transmitted by the test object SUT (test wave, or object wave reflected or transmitted by the test object SUT) is modified by a corresponding polarizer and phase delay plate in such a way that circularly polarized light falls onto the image capture device K.

[0090] The reference wavefront required for interference is modified in such a way that circularly polarized light falls onto the image capturing device K. The relative phase between the object wave and the reference wave is changed by polarizers with different orientations before the pixels of the image capturing device K, making it possible to determine the phase being measured using conventional phase-shifting algorithms.

[0091] Technically, the present invention can be implemented very compactly using Sony's color sensor, which has been on the market for several months. In addition to color filters, the color sensor also has polarization filters with different orientations on each pixel of the image capture device.

[0092] Alternatively, instead of polarization-based phase assessment, a carrier frequency-based phase assessment method can also be used (see Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, “Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry”, J. Opt. Soc. Am. 72, 156-160 (1982)).

[0093] The present invention is not limited to the interferometer described herein as an application example, but can also be used, for example, in conjunction with Fizeau, Michelson or Mach-zehnder type tilt wave interferometers.

[0094] Figure 9 A Thyman Green interferometer is schematically shown, comprising an illumination unit 100, a beam splitter 102, a collimating lens 104, an interferometer objective lens 106, a test object surface 108, an interferometer aperture 110, an imaging lens 112, and an image capturing device K.

[0095] Figure 10 schematically illustrates a Fizeau tilt wave interferometer having an illumination unit 100, a beam splitter 102, a collimating lens 104, a Fizeau interferometer objective lens 114, a test object surface 108, an interferometer aperture 110, an imaging lens 112, and an image capturing device K.

[0096] The interferometer described here operates using light reflected from the test object. Figure 11A Mach-Zehnder interferometer is shown, which operates using light transmitted from a test object. The interferometer has an illumination unit 100, a collimating lens 104, a first beam splitter 118, a first reflector 120, a test object 122 transmitting the light, a second beam splitter 124, an imaging lens of the interferometer aperture, an image capturing device, and a second reflector 126 in the reference beam path.

[0097] Depending on the type of interferometer, the light from the illumination unit 100, which generates the desired light waves, reaches the transmission test object 122 and / or the reflection test object surface 108 along different paths. The illumination unit 100 is, for example, a colored mosaic point light source array 20 described further above. There, the light interacts with the test object 122 / test object surface 108, thus carrying the desired information. For example, if the surface topography of the test object surface is being measured, a Thyman Green type or Fizeau type structure can be selected, or if the variable being measured is its optical thickness, a Mach-Zehnder type structure is selected and the test object is used for transmission. After interacting with the test object, test wavefronts of different wavelengths generated by this interaction reach the image capture device K, where the wavefronts are superimposed on a corresponding coherent reference wavefront. By using light of different wavelengths, all point light sources can emit light simultaneously during at least a portion of the exposure time in the image capture device. Since all point light sources are simultaneously activated, measurement information is generated simultaneously during image capture across the entire test object.

[0098] Spectral separation of the interferogram generated by superposition on the image capturing device K can be achieved through various techniques. In addition to the filter mask mentioned above before the pixels of the image capturing device, spectral channels can be implemented using separate cameras / image capturing devices for each color or center wavelength. In this case, light from the test object is distributed to different image capturing devices by a beam splitter, and spectral selection is performed either by a dichroic beam splitter layer at the beam splitter or by an absorption filter directly in front of the image capturing device.

[0099] The interferogram separated by the color filter first shows a sinusoidal intensity band. During phase evaluation, the phase difference is calculated based on the sinusoidal intensity band, and the path difference between the reference wave and the object wave is calculated from this. The measured characteristics of the test object are determined based on the path difference.

[0100] According to existing technology, phase assessment can be performed using a time phase shifter, i.e., by capturing a sequence of interferograms, where a phase change between the object and the reference wave is selectively inserted between captures, typically via the displacement of a mirror in the beam path. This requires capturing, for example, five separate images that can be captured in a fast sequence, allowing for a typical measurement time of approximately one second.

[0101] The capture of a test object in a single image capture is achieved by the present invention as follows.

[0102] Interferograms of spectral superpositions of three or more illumination configurations are preferably split into separate interferograms by spectral selection, where each separate interferogram corresponds to one illumination configuration. These separate interferograms are each evaluated using a method for evaluating individual interferograms. A preferred embodiment of single interferogram evaluation is evaluation via polarization methods (e.g., U.S. Patent Nos. 7,777,895, 6,304,330, 6,552,808, and 7,230,717). In this case, light from the test object is modified by a polarizer and a phase delay plate in such a way that circularly polarized light falls onto the image capture device. The reference wavefront required for interference is modified in such a way that opposing circularly polarized light falls onto the image capture device. The relative phase between the object wave and the reference wave is altered by polarizers of different orientations before the pixels of the image capture device, allowing the phase measured from a set of four pixels to be determined using conventional phase-shifting algorithms.

[0103] Technically, the invention can be implemented very compactly using a polarization image sensor that is currently available from Sony and has been on the market for a few months. This polarization image sensor has four polarization filters with different orientations on each pixel, in addition to color filters, and the polarization filters are arranged between the photodiode layer and the lens layer of the image capture device chip.

[0104] In another preferred embodiment, the phase of each interferogram is determined by a carrier frequency method, see, for example, Mitsuo Takeda, Hideki Ina, and Seiji Kobayashi, “Fourier Transform Method for Fringe Pattern Analysis of Computer-Based Topographic and Interferometric Surveys,” J. Opt. Soc. Am. 72, 156-160 (1982). In this case, the object wave is superimposed on a reference wave that is strongly tilted relative to the object wave. The phase is then evaluated using a demodulation method from the high-frequency interferogram band.

[0105] Figure 12 A flowchart is shown as an example of a method for measuring the surface or optical thickness of an optically smooth test object according to the present invention.

[0106] In the first step 130, the optically smoothed test object is illuminated with multiple object waves from discrete different directions. In this case, the first object wave arriving from the first direction has a first center wavelength. The second object wave propagating along the second direction among the discrete different directions has a second center wavelength, and the third object wave propagating along the third direction among the discrete different directions has a third center wavelength. The first center wavelength is different from the second center wavelength, and the third center wavelength is different from the first center wavelength and the second center wavelength. The test object is simultaneously illuminated with the first object wave, the second object wave, and the third object wave. After interaction with the test object, in step 132, the object waves originating from the test object are superimposed on the image capturing device. In the third step 134, the interferogram generated by the superposition is spectrally decomposed into wavelength-specific partial interferograms.

[0107] In a preferred embodiment, the interferometer 10 is configured to illuminate an optically smoothed test object using a plurality of illumination configurations, each illumination configuration being generated by a plurality of object waves illuminating the test object. The object waves, affected by the test object through reflection on the surface 108 being measured or through transmission through the test object 122, are superimposed on a reference wave coherent with the object waves on at least one image capture device K to produce an interferogram. In this case, each illumination configuration is implemented by a separate color channel and / or a separate center wavelength, wherein the first illumination configuration has only object waves 14.1, 14.2, 14.3, the object waves 14.1, 14.2, 14.3 of the first illumination configuration having a first center wavelength, and wherein the second illumination configuration has only object waves 14.1, 14.2, 14.3, the object waves 14.1, 14.2, 14.3 of the second illumination configuration having a second center wavelength, and wherein the third illumination configuration has only object waves 14.1, 14.2, 14.3, the object waves 14.1, 14.2, 14.3 of the third illumination configuration having a second center wavelength, and wherein the third illumination configuration has only object waves 14.1, 14.2, 14.3, the object waves ... Waves 14.1, 14.2, and 14.3 have a third center wavelength, wherein the first center wavelength is different from the second center wavelength, wherein the third center wavelength is different from the first center wavelength and the second center wavelength, and wherein the interferometer 10 is configured to simultaneously illuminate the test object 122 from discrete different directions using object waves constructed by the first illumination, object waves constructed by the second illumination, and object waves constructed by the third illumination, and after interacting with the test object 122, the object waves returning from the test object 122 are superimposed on the image capturing device K, and wherein the image capturing device K is configured to decompose the spectrum of the superimposed interferogram into wavelength-specific partial interferograms.

[0108] Preferably, the fourth illumination configuration has only an object wave having a fourth center wavelength, and the fourth center wavelength is different from the first center wavelength, the second center wavelength, and the third center wavelength.

[0109] Further preferably, the object waves propagating in the closest adjacent directions do not have the same center wavelength.

[0110] Another preferred embodiment is characterized in that the image capturing device K is configured to separate the interferogram by a color filter, such that each pixel of the image capturing device K detects only one wavelength of interferogram information.

[0111] Preferably, the interferometer has multiple point light sources 12, from which object waves illuminating the illumination structure are emitted simultaneously.

[0112] More preferably, the point light sources 12 are arranged such that each point light source 12 is arranged at the center of an imaginary regular hexagon, wherein the hexagons are adjacent to each other in a plane without gaps, and such that the hexagon with a point light source 12 emitting light with a central wavelength at its center is adjacent only to the hexagon with a point light source 12 emitting light with another wavelength at its center.

[0113] Another preferred embodiment is characterized in that the point light sources 12 are arranged such that each point light source 12 is arranged at the center of an imaginary regular square, wherein the squares are adjacent to each other in a plane without gaps, and such that a square with a point light source 12 emitting light of a central wavelength at its center is adjacent only to a square with a point light source 12 emitting light of another wavelength at its center.

[0114] Preferably, in addition to the color filter, the image capturing device K also has polarization filters with different orientations on each pixel.

[0115] Furthermore, preferably, the multiple point light sources 12 for each center wavelength are arranged as a coherent laser source L, a Kepler telescope, and a point light source array PLQA composed of a transparent substrate, wherein a microlens is provided, the light incident side of the microlens faces the telescope, and the focal point of the microlens is located in the plane of the light emitting side of the substrate, and the aperture array is located in the plane, from which object waves with a center wavelength are emitted.

[0116] Another preferred embodiment is characterized in that the interferometer according to any one of claims 1 to 8 is characterized in that a plurality of point light sources 12 for each center wavelength have a structure that is coupled to the optical fiber end of a laser source 6 via one or more optical couplers 8, the laser source 6 emitting light having the center wavelength.

[0117] In a preferred embodiment of the method, an optically smoothed test object 122 is illuminated using multiple illumination configurations 14.1, 14.2, and 14.3, each illumination configuration being generated by multiple object waves that illuminate the test object. The object waves, affected by the test object 122 through reflection on the measured surface 108 or through transmission through the test object 122, are superimposed on a reference wave coherent with the object waves on at least one image capturing device K, thereby generating an interferogram. Each illumination configuration is implemented through a separate color channel and / or a separate center wavelength, wherein the first illumination configuration has only object waves 14.1, 14.2, 14.3 with a first center wavelength, and the second illumination configuration has only object waves 14.1, 14.2, 14.3 with a second center wavelength, and the third illumination configuration has only object waves 14.1, 14.2, 14.3 with a third center wavelength, wherein the first center wavelength is different from the second center wavelength, and the third center wavelength is different from the first center wavelength and the second center wavelength, and wherein the test object 122 is simultaneously illuminated from discrete different directions by the object waves of the first illumination configuration, the object waves of the second illumination configuration, and the object waves of the third illumination configuration, and after interacting with the test object 122, the object waves returning from the test object 122 are superimposed on the image capturing device K, and wherein the image capturing device K is configured to decompose the interferogram spectrum generated by the superposition into wavelength-specific partial interferograms.

Claims

1. An interferometer (10) for optically smoothing a surface (108) or a surface measurement of an optical thickness of a test object (122), wherein, The interferometer (10) is configured to illuminate the optically smoothed test object using a plurality of illumination configurations, wherein each illumination configuration is generated by a plurality of object waves (14.1, 14.2, 14.3) illuminating the test object, and the interferometer (10) is configured to superimpose the object waves affected by the test object through reflection on the surface (108) being measured or through transmission through the test object (122) onto a reference wave coherent with the object waves (14.1, 14.2, 14.3) on at least one image capturing device (K) to generate an interferogram, characterized in that each illumination configuration is realized by object waves having wavelengths of a continuous wavelength range having a center wavelength specific to the wavelength range, wherein a first illumination configuration of the illumination configuration has only object waves (14.1, 14.2, 14.3) having a wavelength range having a first center wavelength, and wherein a second illumination configuration of the illumination configuration has only object waves (14.1, 14.2, 14.3) having a wavelength range having a second center wavelength. The object waves (14.1, 14.2, 14.3) in the illumination configuration, wherein the third illumination configuration in the illumination configuration has only object waves (14.1, 14.2, 14.3) with a third center wavelength in the wavelength range, wherein the first center wavelength is different from the second center wavelength, wherein the third center wavelength is different from the first center wavelength and the second center wavelength, wherein the object waves propagating in the closest adjacent directions do not contain wavelengths in the same wavelength range, and wherein the interferometer (10) is configured to simultaneously illuminate the test object (122) from discrete different directions using the object waves of the first illumination configuration, the object waves of the second illumination configuration and the object waves of the third illumination configuration, and after interacting with the test object (122), superimpose the object waves returning from the test object (122) on the image capturing device (K), and wherein the image capturing device (K) is configured to decompose the interferogram spectrum generated by the superposition into wavelength-specific partial interferograms.

2. The interferometer (10) according to claim 1, characterized in that The fourth illumination configuration in the illumination configuration has only an object wave with a fourth center wavelength in the wavelength range, wherein the fourth center wavelength is different from the first center wavelength, the second center wavelength, and the third center wavelength, and wherein the interferometer is configured to simultaneously illuminate the test object from discrete different directions using the object waves of the first illumination configuration, the second illumination configuration, the third illumination configuration, and the fourth illumination configuration, and after interacting with the test object, superimpose the object waves returning from the test object onto the image capturing device, and wherein the image capturing device is configured to decompose the interferogram spectrum generated by the superposition into wavelength-specific partial interferograms.

3. The interferometer according to any of the preceding claims, characterized in that The interferometer has multiple point light sources from which object waves illuminating the illumination structure are simultaneously emitted.

4. The interferometer (10) according to claim 1 or 2, characterized in that The image capture device (K) is configured to use a color filter to separate the interferogram, such that each pixel of the image capture device (K) detects interferogram information for only one wavelength range.

5. The interferometer (10) according to claim 3, characterized in that The point light sources (12) are arranged such that each point light source (12) is arranged at the center of an imaginary regular hexagon, wherein the hexagons are adjacent to each other in a plane without gaps, and the point light sources (12) are arranged such that the hexagon with a point light source (12) at its center emitting light of a wavelength range having a central wavelength is adjacent only to the hexagon with a point light source (12) at its center emitting light of a wavelength range having a central wavelength.

6. The interferometer (10) according to claim 3, characterized in that The point light sources (12) are arranged such that each point light source (12) is arranged at the center of an imaginary regular square, wherein the squares are adjacent to each other in a plane without gaps, and wherein a square with a point light source (12) at its center emitting light of a wavelength range having a central wavelength is adjacent only to a square with a point light source (12) at its center emitting light of a wavelength range having a central wavelength.

7. The interferometer (10) according to claim 4, characterized in that In addition to the color filter, the image capturing device (K) also has polarization filters with different orientations on each pixel.

8. The interferometer (10) according to claim 1 or 2, characterized in that The plurality of point light sources (12) for each of the wavelength ranges are an arrangement of a coherent laser source (L), a Kepler telescope, and a point light source array (PLQA) consisting of a transparent substrate, wherein a microlens is provided, the light incident side of the microlens facing the telescope, and the focal point of the microlens is located in a plane on the light emitting side of the substrate, and an aperture array (AA) is located in the plane, emitting an object wave of one wavelength range from the microlens.

9. The interferometer of claim 1 or 2, wherein, The plurality of point light sources (12) for each of the wavelength range have a structure that is coupled to the fiber end of a laser source (6) via one or more optical couplers (8), the laser source (6) emitting light having the center wavelength.

10. A method for surface measurement of the surface (108) or optical thickness of an optically smooth test object (122), wherein, The optically smoothed test object (122) is illuminated using multiple illumination configurations (14.1, 14.2, 14.3), each illumination configuration consisting of multiple object waves used to illuminate the test object, and the object waves are emitted by the multiple point light sources (12), wherein the object waves affected by the test object through reflection on the surface (108) being measured or through transmission through the test object (122) are superimposed on a reference wave coherent with the object waves on at least one image capturing device (K) to produce an interferogram, characterized in that each illumination configuration is implemented by object waves having a wavelength range having a center wavelength specific to the wavelength range, wherein the first illumination configuration of the illumination configuration has only object waves (14.1, 14.2, 14.3) with a wavelength range having a first center wavelength, and wherein the second illumination configuration of the illumination configuration has only wave waves The illumination configuration includes a long range of object waves with a second center wavelength (14.1, 14.2, 14.3), wherein the third illumination configuration in the illumination configuration has only object waves with a third center wavelength (14.1, 14.2, 14.3) in the wavelength range, wherein the first center wavelength is different from the second center wavelength, wherein the third center wavelength is different from both the first and second center wavelengths, wherein the object waves emitted by the two closest adjacent point light sources do not contain wavelengths in the same wavelength range, and wherein the test object (122) is simultaneously illuminated from discrete different directions by the object waves of the first illumination configuration, the object waves of the second illumination configuration, and the object waves of the third illumination configuration, and after interacting with the test object (122), the object waves returning from the test object (122) are superimposed on the image capturing device (K), and wherein the interferogram generated by the superposition is spectrally decomposed into wavelength-specific partial interferograms.

Images