Optical measurement device and multiple mirror

Abstract

To provide a device capable of reducing a structure size and making quality of a result independent of a measuring direction.SOLUTION: There is provided a multiple mirror (10) for multiplying a single incoming wavefront (12) of electromagnetic radiation (11) into a plurality of outgoing wavefronts (14). The multiple mirror (10) includes: at least one first mirror (16) on which an incident wave front (12) first impinges; and a second mirror (18) on which the wave front is finally reflected, where these mirror surfaces are overlapped in a moving direction of the first wave front (12). The first mirror (16) is partially transparent to the electromagnetic radiation (11) and the second mirror (18) is fully reflective.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The invention relates to a multimirror for multiplying an incident wavefront according to the preamble of claim 1. The invention further relates to an apparatus for optically measuring a surface according to the preamble of claim 6. [Background technology]

[0002] A method for measuring the phase of radiation, in particular optical radiation, and a device for measuring the phase of radiation, in particular optical radiation, are known from DE-A 198 56 400 (B4) and DE-A 010 30 059 (B1). In the known device, a body is illuminated with coherent radiation of a given frequency. The body may have a diffusely reflecting surface. The radiation reflected by the body or transmitted through the body or the transparent medium is projected by imaging optics onto an image plane, in which a sensor is located. The sensor is preferably a surface sensor. The surface sensor may be part of a framing camera with corresponding lens optics.

[0003] The known structure is multi-part and bulky. It is large and contains many components and lenses. Therefore, the known structure is expensive and complex. Furthermore, the known structure allows only measuring small areas, which are shaded in the diagram. Furthermore, the image quality depends on the measurement direction. Summary of the Invention [Problem to be solved by the invention]

[0004] One objective underlying the present invention is to create a device which makes it possible to reduce the structure size and at the same time make the quality of the results independent of the measurement direction. [Means for solving the problem]

[0005] This object is achieved according to the invention by means of the features of the independent claims. The dependent claims set out elaborations and alternatives of the invention.

[0006] A multi-mirror is proposed, by which an incident wavefront of electromagnetic radiation is multiplied so that several wavefronts emerge from the multi-mirror after reflection of the radiation. The electromagnetic radiation propagates in the form of waves, the wavefront of each wave being oriented in the direction of propagation. The wavefronts are phase shifted with respect to each other. The multi-mirror thus comprises at least one first mirror, on which the incident wavefront strikes first, and a second mirror, on which the wavefront is reflected last. These mirrors are overlapped in the direction in which the first wavefront moves. To ensure that the wavefront reaches the second mirror regardless of the overlap in the propagation direction of the incident radiation, the first mirror is partially transparent to the electromagnetic radiation, whereas the second mirror is fully reflective. Thus, a part of the incident electromagnetic radiation is reflected by the first mirror, and the remaining part of the incident electromagnetic radiation propagates to the second mirror, where the remaining radiation is also reflected, but with a time delay with respect to the first reflection. This time delay causes a phase shift between the two parts of the reflected radiation. For example, when multiple multimirrors are combined with each other, their phase shifts can be added together and used in interferometry applications. The sandwiched arrangement of multimirrors allows for a very compact design, making the multimirror ideally suited for use in portable measurement devices (especially outside a laboratory). The multimirror may preferably be used in devices for shearography.

[0007] To allow a particularly efficient detection of the contour of the object to be measured, at least one of the mirrors may be capable of polarizing the electromagnetic radiation. For this purpose, the first and second mirrors may preferably polarize the radiation in different directions, which may be aligned preferably orthogonal to each other. For example, if a beam with mixed polarization hits the multi-mirror, a part of the radiation with one uniform polarization direction may be polarized at the first mirror, while a second part with another polarization may pass through and reach the second mirror. The second part may be reflected at the second mirror. The second mirror may be a simple non-polarizing mirror. Alternatively, the second mirror may have a polarizing effect, in particular in a direction orthogonal to the first mirror. Finally, in both cases, two beams of different polarization emerge from the multi-mirror. If one or several multi-mirrors are used, the exiting beams may overlap each other, and beams with the same polarization interfere with each other. When a single multi-mirror is used, one of the two polarized wavefronts (reflected from the multi-mirror) interferes with a similarly polarized portion of the simple wavefront reflected from the simple mirror, and the resulting image may be evaluated with respect to the composition of the measured object.

[0008] Advantageously, at least one of the mirrors may be tilted with respect to at least another mirror. The distance between the mirror surfaces of these mirrors may be changed. The distance between the mirror surfaces may be kept constant in terms of position and / or time or may be changed. The radiation is reflected directly at the mirror surfaces. This distance causes a phase shift, so that using a device with such a multi-mirror, highly accurate measurements can be performed. When using a multi-mirror or multiple multi-mirrors, the image generated by interference can be influenced by tilting. If a multi-mirror is used, one of the two polarized wavefronts (reflected by the multi-mirror) interferes with a similarly polarized part of the simple wavefront reflected by the simple mirror. These can also be shifted with respect to each other in the image plane.

[0009] The mirrors are preferably designed to be flat. Such surface mirrors may be tilted relative to one another so that they subtend an angle. The angles of incidence and reflection of the electromagnetic radiation may thus be offset from one another at the first and second mirrors. This makes it possible to adjust the phase shift of the reflected wavefronts.

[0010] The mirrors are advantageously adjustable relative to one another in a simple manner in that the first mirror is rotatable about a rotation axis, which is located, for example, in the frame of the mirror holder of these mirrors. The rotation axis may, for example, extend through the mirror surface or may be located in front of or behind the mirror surface. The mirror is preferably rotated out of a position parallel to the second mirror until it is at a desired angle with respect to the second mirror. Another rotation axis of such type may be implemented in the second mirror. Both mirrors may have such a rotation axis. Or, only one of the two mirrors may have such a rotation axis. For example, the first or the second mirror may be provided with a rotation axis.

[0011] An apparatus for optically measuring a surface, having a preferably coherent light source (e.g. laser) emitting an electromagnetic beam, may include at least one beam splitter. The beam splitter may be a two-part compound prism made of a material transparent to the radiation. In particular, the apparatus is an interferometer, which measures the surface by interferometry. In order to achieve a compact design of the apparatus and at the same time to achieve maximum measurement accuracy even under harsh environmental conditions, at least one multi-mirror, with at least two mirror surfaces arranged at a distance from each other, is arranged in front of and / or behind the beam splitter in the propagation direction of the electromagnetic beam. Such an apparatus is ideal for use in environments exposed to vibrations. Furthermore, such an apparatus is hardly affected by temperature and hygroscopic variations.

[0012] To strengthen the device, at least one diaphragm may be provided between the multi-mirror and the beam splitter, which further improves the measurement accuracy. Furthermore, the diaphragm can advantageously be used to enable a spatial phase shift.

[0013] The device includes two beam splitters, two shutters and two multi-mirrors, which can improve the measurement accuracy. Thus, by passing the light reflected from the surface of the object to be measured through such a device, the light can be analyzed by interferometry.

[0014] When using such multi-mirrors, the mirror surface may be partially transparent to the electromagnetic radiation. In particular, at least one of the multi-mirrors comprises a mirror that can be tilted relative to another mirror. The wavefront impinging on the multi-mirror is then split into two exiting wavefronts that are phase shifted relative to each other. It is particularly preferred that the two multi-mirrors comprise tiltable mirrors.

[0015] An advantageous development comprises at least one diaphragm having an aperture with a polarizing filter, which may be used to filter the reflected beam and to remove destructive incorrect polarization.

[0016] Such modified beams are particularly suitable for interferometric measurements. Here, optimal results are obtained if at least one diaphragm has two apertures, each of which has a polarizing filter, which are preferably aligned orthogonally to each other. In this way, two beams multiplied by the multimirror can pass through one aperture each and be filtered accordingly. After passing through the filters aligned orthogonally to each other, each beam has a polarization corresponding to the respective filter.

[0017] In addition to or as an alternative to a polarizing filter, the aperture of at least one diaphragm may include a frequency filter, allowing only specific wavelengths to pass through the diaphragm.

[0018] In order that the device ensures optimal measurements in all directions of the spatial extent of the sample, the diaphragm may be designed as a grating diaphragm. The grating diaphragm is preferably specified by the slit width of the aperture in two dimensions. By limiting the slit width in the plane of the diaphragm, it is possible to influence the two dimensions of the measurement radiation by the beam geometry.

[0019] A colour camera is particularly advantageous for the analysis of different wavelengths. The colour camera records the electromagnetic radiation that has passed through at least one multi-mirror. The colour camera is able to distinguish between different wavelengths.

[0020] In one embodiment, the device includes a beam splitter, which is arranged in the direction of propagation so as to be first struck by the electromagnetic radiation. The beam enters the beam splitter as soon as it enters the device. The beam is then split in the beam splitter and directed to a movable mirror in the first direction of propagation. The remaining part of the split beam is directed to a fixed mirror in the second direction of propagation. A diaphragm is respectively arranged between the beam splitter and each of the two mirrors. Each split beam passes through each diaphragm at least twice and merges again in the beam splitter. A virtual double slit is seen from the direction of the camera. This simple configuration makes it possible to create a robust, space-saving device that ensures a high degree of portability.

[0021] According to another development, the device comprises a further second beam splitter, a mirror and a movable mirror, the two beam splitters being arranged diagonally next to each other. An imaginary diagonal may be drawn through some corners of each beam splitter, and one mirror is arranged on each of the two sides of the diagonal arrangement of the beam splitters. The mirrors are arranged approximately parallel to the imaginary diagonal through the beam splitters. One diaphragm is respectively arranged between the beam splitter into which the beam enters the second, i.e. the beam splitter into which the partial beam from the first beam splitter enters, and the mirror. The diaphragm is approximately parallel to the face of the beam splitter into which the beam reflected by the mirror enters. Of course, the diaphragm may be arranged anywhere in the beam path of the two beam splitters, in front of or behind the mirror.

[0022] It will be understood that the features set forth above and to be further described below can be used not only in the combinations specifically stated, but also in other combinations, and the scope of the invention is defined solely by the claims.

[0023] The invention will now be explained in more detail based on an exemplary embodiment and with reference to the associated drawings. [Brief description of the drawings]

[0024] [Figure 1] A multi-mirror with different output beams is shown. [Diagram 2] 1 shows an apparatus for performing optical measurements of a surface using a multi-mirror. [Diagram 3] 1 shows an embodiment having a beam splitter, a fixed mirror, and a movable mirror. [Figure 4] An embodiment is shown having two beam splitters, a fixed mirror, and a movable mirror. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] FIG. 1 shows a multi-mirror 10, which reflects electromagnetic radiation 11. The radiation 11 may consist of coherent waves, as for example a laser. The multi-mirror 10 multiplies an incident wavefront 12 of the radiation 11, so that after reflection several wavefronts 14 emerge from the multi-mirror 10. The wavefronts are multiplied by two mirrors 16, 18, which are parallel to each other and aligned to overlap each other in the propagation direction of the incident and reflected radiation 11. Reflection takes place on the mirror surfaces of the mirrors 16, 18. The mirrors 16, 18 are also spaced apart from each other in the propagation direction. Furthermore, the first mirror 16, on which the radiation 11 strikes in the propagation direction, is partially transparent to the electromagnetic radiation 11, whereas the second mirror 18 is fully reflective.

[0026] Now, when the incoming wavefront 12 hits the first mirror 16, a part of it is reflected back, where the angle of incidence, relative to the mirror surface, corresponds to the angle of reflection. The remaining part is reflected off the mirror surface of the second mirror 18. Because the mirrors 16, 18 are spaced apart from each other, a certain amount of time elapses between the first and second reflections, and therefore the reflection off the first mirror 16 occurs before the reflection off the second mirror 18. This results in a phase shift between the two outgoing wavefronts 14 from the first and second reflections.

[0027] The incident beam 11 is polarized in various directions. The mirrors 16, 18 may be designed such that the reflected radiation is polarized in only one direction. The two mirrors 16, 18 are capable of polarizing the radiation 11 in two mutually orthogonal directions. Each radiation 11 reflected at each of the mirrors 16, 18 is polarized in only one direction. Alternatively, for example, only the first mirror 16 may have a polarizing effect, and the reflected part of the radiation 11 has a uniform polarization. The remaining component of the radiation 11 travels up to the second mirror 18 and has an orthogonal polarization. This second component may be reflected at the second mirror 18, which has no polarizing properties. Nevertheless, the second component emerges from the multi-mirror 10 in a polarized state.

[0028] Fig. 2 shows an apparatus 100 for optical measurement of a surface, which comprises two multi-mirrors 10, two beam splitters 102 and two diaphragms 104. An incident beam 11 is generated by illuminating the surface to be measured with coherent light. The coherent light may be, for example, a laser. If one or two multi-mirrors 10 are used, the phase-shifted wavefronts 14 emerging from the individual multi-mirrors 10 are added together and the radiation 11 reflected by the mirrors 16, 18 may interfere with each other. If one multi-mirror is used, one of the two polarized wavefronts (reflected by the multi-mirrors) interferes with a similarly polarized part of the simple wavefront reflected by the simple mirror.

[0029] The radiation 11 reflected by the surface to be measured first enters the beam splitter 102. The beam splitter 102 may be rectangular in cross section, the incident beam 11 hitting a flat face of the beam splitter 102, which in particular does not face any of the multi-mirrors 10. After the beam 11 enters the first beam splitter 102 and is split into two further beams 11, their wavefronts 12 each hit a multi-mirror 10, by which the beam 11 is again doubled. The wavefronts 14 emerging from each multi-mirror 10 are directed towards the second beam splitter 102. Each doubled and phase-shifted wavefront 14 passes through a diaphragm 104, which is each located on a flat face of the second beam splitter 102. Alternatively, the diaphragm 104 may be located anywhere on the respective beam paths between the first beam splitter 102 and the second beam splitter 102. In the second beam splitter 102, the phase-shifted output beam 11 coming from the multi-mirror 10 is doubled again, and two wavefronts 15, each with the same phase, are directed to the camera. The beams from the two multi-mirrors 10 are merged and split into two bundles, each of which exits from one side of the beam splitter 102. During the merger, the beams 11 interfere with each other to form an interference image, which can be spectrally resolved, for example, by Fourier analysis.

[0030] The two beam splitters 102 are positioned in a row at an angle to the beam paths entering and exiting the multi-mirror 10. The multi-mirror 10 is positioned approximately between the beam splitters 102 and to the left and right of the beam splitter 102. The two diaphragms 104 are positioned on the flat surface of the second beam splitter 102 and face the first beam splitter 102. Alternatively, the diaphragms 104 may be located anywhere on the respective beam paths between the first beam splitter 102 and the second beam splitter 102. The two diaphragms 104 are approximately perpendicular to each other. Also, the diaphragms 104 may be positioned at a non-perpendicular angle to each other.

[0031] Each of the multi-mirrors 10 includes two mirrors 16, 18, one of which has a rotation axis. For example, the first mirror 16 and / or the second mirror 18 may have a rotation axis. One multi-mirror 10 may have a rotatable first mirror 16 and the other multi-mirror 10 may have a rotatable second mirror. The rotation axes may be aligned perpendicularly to each other. The rotation of the mirrors 16, 18 may be used to generate a phase shift of the multiplied beam 11. The phase shift depends on the angle 19 set between the two mirrors 16, 18 by the rotation. The displacement of the mirrors may also cause a shearing of the radiation 11. The respective interference images of the polarized beams from the first and second mirrors 16, 18 of the two multi-mirrors 10 may be shifted relative to each other by the rotation. The rotation axis may be implemented, for example, in the frame of the multi-mirror 10.

[0032] Upon passing through the second beam splitter 102, the single slit diaphragm 104 appears as a virtual double slit diaphragm on the exit flat surface of the second beam splitter 102. This is because the two phase shifted beams from each multi-mirror 10 merge again after passing through their respective diaphragms 104. This double slit spacing requires a spatial carrier frequency that corresponds to the phase shift in space. The carrier frequency is therefore decoupled from the rotational position of the mirrors 16, 18.

[0033] The mirrors 16, 18 polarize the radiation 11 in different directions depending on the reflection. For example, the partially transparent first mirror 16 can only reflect radiation 11 with a certain polarization, so that the radiation 11 that can pass through is reflected by the second mirror 18. In this situation, the radiation 11 can be polarized in the same direction by the two first mirrors 16. The reflected beams 11 with the same polarization direction can interfere with each other. Similarly, the beam 11 from the second mirror can be polarized in the same direction. Therefore, they can also interfere with each other. In contrast, the beam 11 from the first mirror cannot interfere with the beam 14 from the second mirror, because their polarization directions are different. Now, if the first polarizing mirror 16 of one multi-mirror 10 is tilted with respect to the first polarizing mirror 16 of the other multi-mirror 10, an interference image of the reflected beams 11 is generated. This image may be understood as a sheared image. For example, it may be sheared in the horizontal direction, which allows shearography measurements. Alternatively, the second mirror 18 of one multi-mirror 10 may be tilted towards the second mirror 18 of the other multi-mirror 10, which creates another sheared image due to interference between the reflected beams 11. The shearing may be, for example, vertical.

[0034] In this way, a horizontally sheared wavefront 14 and an associated reference wavefront, and a vertically sheared wavefront and an associated reference wavefront are generated by the two multi-mirrors 10. Thus, carrier frequencies of the respective wavefronts polarized to be capable of interference in the horizontal and vertical directions, respectively, are generated by the use of the diaphragm 104. This results in a spectral separation in Fourier space.

[0035] Both images, generated by interference between the pair of beams 11 reflected from the first and second mirrors 16, 18, are captured by a camera. To evaluate the two sheared images separately, their polarization can be used as a criterion for separation. This can be achieved, for example, by using different spatial carrier frequencies depending on the polarization. To that end, the two diaphragms 104 may have polarization-specific apertures 106. The width of the apertures 106 of the diaphragms 104 determines the frequency width of the images that are proportionally generated by interferometry in the Fourier analysis. The apertures 106 may comprise polarizing filters.

[0036] Additionally or alternatively, the aperture 106 may comprise a frequency filter that allows only radiation 11 of a specific wavelength to pass through. Filtering out a specific wavelength increases the measurement accuracy, since for example the aperture size can be adjusted to only one wavelength, thereby avoiding image errors. Using a color camera makes it possible to analyze the filtered interference images from different wavelengths. For example, a frequency filter can be produced by a color filter.

[0037] FIG. 3 shows an apparatus 100 including a beam splitter 102 arranged in the direction of propagation of an electromagnetic beam 11, which first enters the beam splitter 102. As soon as the beam 11 enters the apparatus, it travels into the beam splitter 102 and is split therein. A part of the beam 11 is directed to a movable mirror 6 in a first direction of propagation 1. The remaining part of the split beam 11 is directed to a fixed position mirror 5 in a second direction of propagation 2. A diaphragm 104 is arranged between the beam splitter 102 and each of the two mirrors 6, 5. Each partial beam obtained through the beam splitter 102 passes through each diaphragm 104 at least twice. That is to say, the partial beam passes through the diaphragm 104 once before being reflected by each mirror 6, 5 and once after being reflected. These partial beams are merged again in the beam splitter 102 and directed to a camera. A virtual double slit 4 is visible from the direction of the camera.

[0038] Fig. 4 discloses a further development including a further second beam splitter 102. A mirror 5 and a movable mirror 6 are also part of the device 100, the two beam splitters 102 being arranged diagonally next to each other. An imaginary diagonal 3 may be placed through some corners of each beam splitter, whereby several corners of each beam splitter 102 are aligned with respect to each other. One of the mirrors 6, 5 is arranged on each of the two sides of the diagonal arrangement of the beam splitter 102. The mirrors 6, 5 are arranged approximately parallel to the imaginary diagonal 3 through the beam splitter 102. The movable mirror 6 may deviate from the parallel arrangement by an adjustment angle, but is resettable to a parallel position. A diaphragm 104 is arranged between the beam splitter 102 into which the beam 11 enters second, i.e. the beam splitter 102 into which the partial beam from the first beam splitter 102 enters, and each of the mirrors 5, 6. The diaphragm 104 is approximately parallel to the face of the beam splitter 102 into which the beam 11 reflected by the mirrors 5 and 6 enters.

[0039] Reference List 1 First Propagation Direction 2 Second Propagation Direction 3 Diagonal 4 Virtual double slit 5 Fixed mirror 6 Movable mirror 10 Multi Mirror 11 Electromagnetic Radiation 12 Incident wavefront 14 Exit wavefront 15 Wavefront towards the camera 16 First Mirror 18 Second Mirror 19 angle 20 Rotational Axis 100 devices 102 Beam splitter 104 Diaphragm 106 Aperture

Claims

1. 1. A multi-mirror for multiplying an incident wavefront (12) of electromagnetic radiation (11) to form a plurality of exiting wavefronts (14), comprising at least one first mirror (16) against which the incident wavefront (12) initially strikes and a second mirror (18) to which the wavefront is ultimately reflected, the mirrors (16, 18) being overlapped in the direction of movement of the first wavefront (12), the first mirror (16) being partially transparent to the electromagnetic radiation (11) and the second mirror (18) being fully reflective.

2. 2. A multi-mirror according to claim 1, characterized in that at least one of the mirrors (16, 18) is capable of tilting relative to at least one other mirror (18, 16).

3. 3. A multi-mirror according to claim 1 or 2, characterized in that the mirrors (16, 18) are flat and can be tilted relative to one another so as to subtend an angle (19).

4. A multi-mirror according to any one of claims 1 to 3, characterized in that the first and / or second mirror (16, 18) are rotatable about an axis of rotation (20), the axis of rotation (20) being in particular mounted on a mounting frame of the mirrors (16, 18).

5. A multimirror according to any one of claims 1 to 4, characterized in that at least one of the mirrors (16, 18) polarizes the electromagnetic radiation, the first mirror (16) preferably reflecting radiation of a particular polarization and transmitting radiation of another polarization, preferably orthogonal to the particular polarization.

6. 1. An apparatus for interferometric optical measurements of a surface, comprising a coherent light source emitting an electromagnetic beam (11) and at least one beam splitter (102), characterized in that at least one multi-mirror (10) comprising at least two mutually spaced mirror surfaces according to any one of claims 1 to 5 is arranged in front of and / or behind the beam splitter (102) in the propagation direction of the electromagnetic beam (12).

7. 7. The device according to claim 6, further comprising at least one diaphragm (104), said at least one diaphragm (104) being arranged between said multimirror (10) and said beam splitter (102).

8. 8. Apparatus according to claim 6 or 7, characterized in that it comprises at least two beam splitters (102), at least two diaphragms (104) and at least one multimirror (10).

9. 9. Apparatus according to any one of claims 6 to 8, characterized in that the multimirror (10) comprises at least one mirror (16, 18), the at least one mirror (16, 18) having a mirror surface and being partially transparent to the electromagnetic radiation.

10. 10. The device according to claim 6, wherein in at least one of the multimirrors (10), a mirror (16, 18) can be tilted relative to the other mirrors (18, 16), whereby a wavefront (12) impinging on the multimirror (10) is split into two exiting wavefronts (14) which are phase shifted relative to each other.

11. Device according to any one of claims 6 to 10, characterized in that at least one diaphragm (104) has an aperture (106) containing a polarizing filter.

12. 12. Apparatus according to any one of claims 6 to 11, characterized in that at least one diaphragm (104) has at least two apertures, each of said at least two apertures having a polarizing filter, said polarizing filters being preferably arranged orthogonal to each other.

13. 13. The device according to claim 6, wherein at least one diaphragm (104) has an aperture (106) with a frequency filter, whereby only certain wavelengths are allowed to pass through said diaphragm (104).

14. The device according to any one of claims 6 to 13, characterized in that the diaphragm (104) is designed as a grating diaphragm and preferably has specific slit widths of the aperture (106) in two dimensions, the slit widths being in particular the same dimension.

15. 15. Apparatus according to any one of claims 6 to 14, characterized in that the camera which records the electromagnetic radiation (11) which has passed through the at least one multi-mirror (10) is designed in particular as a colour camera.

16. The device according to any one of claims 6 to 15, characterized in that a movable mirror (6) is arranged in a first propagation direction (1) of the beam (11) split by the beam splitter (102) and a fixed position mirror (5) is arranged in a second propagation direction (2), a diaphragm (104) is arranged between the beam splitter (102) and each of the mirrors (5, 6), the beam (11) passes through each diaphragm (104) at least twice, the beam (11) is merged again in the beam splitter (102), and a virtual double slit (4) is seen from the direction of the camera.

17. The device according to any one of claims 6 to 16, characterized in that it comprises two beam splitters (102) arranged diagonally next to each other, a fixed mirror (5) is arranged on one of the two sides of the diagonal arrangement and a movable mirror (6) is arranged on the other of the two sides of the diagonal arrangement of the beam splitter (102), and a shutter (104) is arranged between at least one beam splitter (102) and each of the mirrors (5, 6).

18. 18. Apparatus according to any one of claims 6 to 17, characterized in that the fixed mirror (5) and the movable mirror (6) are aligned approximately parallel to an imaginary diagonal passing through the at least two beam splitters (102).

Images