DE102022210483A1 - Interferometric measuring device - Google Patents

Abstract

A measuring device (100) for the interferometric determination of a shape of an optical surface (112) of a test object (110) comprises an illumination module (120) for generating an effective light source (130) in a light source plane (132) of the illumination module, a collimator (210) for Collimation of the measuring light emitted by the effective light source (130), a transparent reference element (220) connected downstream of the collimator (210) with a reference surface (222) facing the surface (112) of the test object (110), with the reference surface (222) and a cavity (230) is formed on the surface (112) of the specimen (110) and a beam splitter (240) which is arranged between the illumination module (120) and the collimator (210) in such a way that the effective light source (130) emits Measuring light passes through to the collimator (210) or is reflected and measuring light reflected from the reference surface (222) and the surface (112) of the test object (110) is superimposed and reflected or passed in the direction of a detector (250). The illumination module (120) has an additional module (160) in the form of a front-end interferometer with a front-end cavity (165), which is optically arranged between an extended light source (LQ) and the light source plane (132) and is configured in such a way that the light from the extended Light source (LQ) two coherent light source images (LQ1, LQ2) can be generated which have an optical path length difference to one another, the measuring light emitted by the effective light source (130) in the light source plane (132) being a superimposition of light from the two coherent light source images (LQ1, LQ2) contains.

Description

FIELD OF APPLICATION AND PRIOR ART

The invention relates to a measuring device for the interferometric determination of a shape of an optical surface of a test object. The measuring device includes a Fizeau interferometer.

A general area of application is the high-precision interferometric measurement of optical surfaces and optically transparent objects. A special area of application is ultra-high-precision interferometric measurement, such as that used in the manufacture of lithographic projection optics. This involves measurement uncertainties below 1/100 of the visible wavelength.

Free-form surface mirrors or free-form lenses are increasingly being used in lithographic projection optics. In the interferometric testing of free-form surfaces (FFF), the method of rotation mediation, which can be used with rotationally symmetrical aspheres, is not applicable due to the asymmetry of the test object. Since the classic calibration with rotational transmission of rotationally asymmetrical errors is not possible, an optical interference signal (speckle effects) remains, which is generated, among other things, by the fine surface structure of the interferometer components.

A classic means of minimizing or avoiding the coherent interference and artifacts caused by interferometer components is to use a large light source. It ensures that fine structures on the components are imaged out of focus, so that their coherent disruptive effects are dampened. Large light sources can be used, for example, in "Michelson" or "Twyman-Green" type two-beam interferometers. Due to various boundary conditions, however, such interferometer types are rarely used.

The workhorse in the optics workshop, particularly in the production of elements of lithographic projection optics, is the "Fizeau" type interferometer (Fizeau interferometer). A Fizeau interferometer measuring device includes an illumination module that produces an effective light source. Light emitted from the illumination module is collimated with a collimator. A beam splitter is located between the illumination module and the collimator. The collimated beam passes through a transparent reference element whose exit surface, manufactured with high optical quality, serves as a reference surface through which part of the light is reflected. The transmitted portion propagates further to the test piece surface. The space between the reference surface and the test piece surface is called the cavity of the interferometer. The part reflected from the test piece surface contains information about the aberration caused by the test piece. The wave fronts of both components interfere in the interferometer and are directed via the beam splitter onto the sensor surface of a detector. A sharp image of the sample surface is created on the sensor surface, which is streaked with a striped pattern (the interference pattern). A continuous strip indicates areas of the same air gap thickness. Adjacent fringes, on the other hand, show a change in thickness that corresponds to half the wavelength of the light.

A fundamental law of spatial coherence dictates that the size of the light source must not exceed a certain size if interference images are to be produced with sufficient contrast. If one also takes into account that Fizeau interferometry only works satisfactorily if the Haidinger rings that occur in the case of extended monochromatic light sources remain smaller than half the stripe width, there is a limit to the maximum size of the effective light source.

It has already been proposed to use a circular light source ( DE 10 121 516 A1 accordingly U.S. 2003/030819 A1 ) or an arc-shaped light source ( DE10 2016 213 237 A1 accordingly U.S. 2019/154427 A1 ) to reduce.

TASK AND SOLUTION

Against this background, an object of the invention is to provide an interferometric measuring device of the generic type that offers improved suppression of high- and medium-frequency measuring errors that arise from interference reflections from interferometer components (e.g. from roughness and surface defects, dirt, bubbles and streaks, etc. ).

To solve this problem, the invention provides a measuring device with the features of claim 1. Preferred developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference.

According to one formulation of the invention, a measuring device for interferometrically determining a shape of an optical surface of a test object is provided. The measuring device includes an illumination module for generating an effective light source in a light source plane of the illumination module.

The measuring device also includes a collimator for collimating the measuring light emitted by the effective light source. The collimator can be formed by a single lens or a computer generated hologram (CGH) or can comprise multiple lenses and CGHs, it is also referred to herein as collimating optics. To test spherical, aspheric or free-form surfaces, the collimator can be followed by test or compensation optics (in short: test optics), which can also be formed from individual or multiple lenses or CGHs. A transparent reference element is provided which is connected downstream of the collimator (and possibly the test optics) and has a reference surface facing the surface of the test object, with a cavity having a cavity length d being formed between the reference surface and the surface of the test object. Furthermore, a beam splitter is provided, which is arranged between the illumination module and the collimator in such a way that the measurement light emitted by the illumination module passes through to the collimator or is reflected, and the light reflected from the reference surface and the surface of the test object is superimposed and reflected or passed in the direction of a detector.

The "effective light source" is the light source whose emitted light is collected by the collimator and used as the measuring light for the measurement. The light of the effective light source is usually generated by suitable processing from the light of a primary light source and has properties that are particularly suitable for the measurement. In conventional systems, for example, a laser can be used as the primary light source. A downstream beam expansion optic can generate an expanded laser beam, which hits a stationary or rotating diffuser and forms a secondary light source there in the form of a uniformly illuminated illumination spot or illumination spot of the desired size. This illumination spot, i.e. the secondary light source, acts as the effective light source of the illumination module and emits measurement light towards the collimator.

It is also possible to use an actively radiating primary light source as an effective light source. For example, a super luminescent diode (SLD) can be used as an effective light source. Such light sources have a short coherence length but a larger luminous area.

According to the claimed invention, the illumination module has an additional module in the form of a switching interferometer, which is optically arranged between an extended light source and the light source plane and is configured in such a way that two coherent light source images can be generated or are generated from the light of the light source, which have an optical path length difference to each other, wherein the measuring light emitted by the effective light source in the light source plane contains a superimposition of light from the two coherent light source images. The light source that is imaged to generate the light source images can be a primary light source or an extended secondary light source illuminated with light from a primary light source.

The front-end interferometer has its own cavity, referred to here as the front-end cavity. The upstream interferometer generates two coherent light source images that have a defined optical path difference (OPD) relative to one another. The measuring light emitted by the effective light source then contains a superimposition of light from the two light source images. The light source images can lie in the light source plane, but possibly also outside the light source plane optically in front of it in such a way that light components of the light source images are guided along optical paths to the light source plane and there together form the effective light source.

The optical path length difference in the upstream interferometer should correspond to that of the Haidinger rings of the interferometer and should deviate by less than half a ring. In other words, Haidinger rings should be generated at the output of the front-end interferometer, which essentially correspond to those of the Fizeau interferometer without front-end interferometer. According to another formulation, the upstream interferometer should be designed and adjusted in such a way that a path length difference between beams originating from different light source points is essentially independent of the position of the light source point.

Four coherent light source images are then superimposed in an intermediate light source plane of the interferometer, two of which are “in phase” in such a way that they show a constant phase difference over the surface. These ensure a high-contrast interference image on the sensor surface of the detector.

wobei f die Brennweite des Kollimators und d die Kavitätslänge ist. Die Lichtquelle kann dann praktisch beliebig groß sein, wenn man gleichzeitig Kollimator, ggf. Prüfoptik, Referenzfläche und Okular des Detektors proportional vergrößert.This makes it possible to overcome the restriction mentioned in the introduction regarding the maximum size of the (effective) light source in a Fizeau interferometer without significantly reducing the measuring capacity. The extended effective light source can then exceed the maximum permissible light source diameter ∅ _max for conventional Fizeau interferometers, so that the following condition applies to the light source diameter ∅ for preferred embodiments: $$\varnothing >\frac{\mathrm{8th}ƒ\mathrm{i.e}}{4\mathrm{i.e}-\lambda }\sqrt{\frac{\lambda }{2\mathrm{i.e}}\left(1-\frac{\lambda }{\mathrm{8th}\mathrm{i.e}}\right)}={\varnothing }_{max}$$

where f is the focal length of the collimator and d is the cavity length. The light source can then be of practically any size if the collimator, possibly test optics, reference surface and eyepiece of the detector are proportionally enlarged at the same time.

The optical path length difference or the phase delay caused thereby can be set by specifying the upstream cavity length d _VSK . The difference in optical path length corresponds to twice the length of the upstream cavity.

The phase delay or the optical path length difference can be permanently specified by the optical structure of the additional module or the upstream interferometer, in particular by the optical structure of the upstream cavity. According to a further development, however, the difference in optical path length can be set in a targeted manner by means of a setting device over a certain setting range, with stepless setting preferably being provided. This enables an exact adjustment of the effect of the additional module on the measurement conditions. The adjustment device can, for example, have an adjustment mechanism for (continuously) displacing a mirror or a lens.

According to one development, the additional module or the upstream interferometer has an interferometer arrangement that is configured to receive measurement light in an entry plane and to generate phase-modified measurement light from it in an exit plane. The additional module comprises a first beam splitter in the form of a partially transparent mirror for receiving measurement light from the entrance plane and for splitting the beam path into a first and a second optical path, with measurement light reflected by the first beam splitter along the first optical path and transmitted by the first beam splitter Measuring light propagated along the second optical path in the direction of the exit plane. The arrangement further comprises a second beam splitter in the form of a partially transparent mirror for combining the first and the second optical path in such a way that the first and the second optical path run together between the second beam splitter and the exit plane. Furthermore, a first mirror is provided, which is arranged in the first optical path such that measurement light is reflected to the second beam splitter, and a second mirror is arranged in the second optical path such that measurement light is reflected to the second beam splitter. With this arrangement, it is possible in different ways to preset a desired path length difference between the first optical path and the second optical path with high precision.

The interferometer arrangement is designed to effect optical imaging along both optical paths, so that a light source image is generated from a light source at a light source position in the input plane.

In some embodiments, a first objective is provided, which is arranged between the entrance plane and the first beam splitter. Additionally or alternatively, a second lens is provided, which is arranged between the second beam splitter and the exit plane. The lenses contribute to the creation of the light source images. However, the use of such lenses is not mandatory. For example, for the purpose of imaging, the first mirror and/or the second mirror can be designed as a concave mirror, ie as a mirror with refractive power.

Different types of interferometer arrangements are possible. In some embodiments, the first beam splitter and the second beam splitter are formed by the same beam splitter. There is therefore only one optical element which acts as a beam splitter with a dual function, namely on the one hand to split the part of the beam path coming from the light source into the two optical paths and on the other on the other hand, for merging the optical paths after reflection at the respective mirrors in the paths. The interferometer arrangement can thus be constructed in the manner of a Michelson interferometer. As an alternative, it is also possible for the first beam splitter and the second beam splitter to be two beam splitters arranged separately from one another. An interferometer arrangement can thus be constructed in the manner of a Mach-Zehnder interferometer.

The use of a front-end interferometer will usually lead to unwanted reflections, which can lead to loss of contrast in the interference pattern and impair the measurement accuracy. If you want to make the contrast as high as possible, the unwanted reflections should be hidden or eliminated in some other way. In some embodiments, it is provided for this purpose that at least one optical element of the first optical path and/or at least one optical element of the second optical path is misaligned or can be misaligned in a controlled manner such that light source images from the first and second optical paths are in a suitable plane laterally offset from each other. If necessary, the two light source images can be offset laterally with respect to one another without overlapping. Unneeded parts from unneeded optical paths can then be blocked by an aperture. The misalignment can be achieved, for example, by tilting one of the mirrors from a reference position. Alternatively or additionally, one or more optical elements can be decentered without tilting and/or undesired reflections can be suppressed in a different way. This can help compensate for any loss of interference contrast by introducing a front-end interferometer.

In particular, if tuning the cavity length of the upstream cavity alone is not sufficient to compensate for the path differences between the optical paths contributing to the high-contrast interference image, provision can be made to introduce at least one additional correction element in the additional module, which may be, for example, a lens with a refractive power, a Compensation system with multiple lenses, a computer generated hologram (CGH) or a lens element with a mirror surface (Mangin mirror).

Alternatively or additionally, it can be provided that the first mirror and/or the second mirror has a curved mirror surface and/or an aspherical mirror surface. One of the mirrors or both mirrors can be designed as a concave mirror, for example, so that lenses in the optical paths can be omitted.

It is possible to design one of the mirrors or both mirrors as actively deformable mirrors, so that the deformation (deviation of the mirror surface shape from a flat reference shape, for example) can be specifically adjusted and changed if necessary. This enables precise adjustments to the measurement conditions.

The measurement system can work with a quasi-monochromatic primary light source. It is also possible that the primary light source is a broadband light source. It should then be ensured that the path length difference of the reflexes interfering in the sensor surface or camera plane of the detector is within the temporal coherence length of the primary measuring light.

In order to determine the form deviation of test objects, a series of interferograms is often required, the fringe patterns of which have been changed in a specific sequence. So-called phase shifting methods are used for this purpose in order to change the phase difference in a suitable manner over time. In some embodiments, the measuring device is designed to set phase shifts. Various devices can be provided for the purposeful phase change (for example constant component and possibly additional tilting), including: a device for generating a change in the position of the test object and/or reference; a device for generating a change in position of the first mirror and/or the ballast cavity; a device for changing the refractive indices of the optical media in the reference and/or test arm; means for changing the indices of refraction of the optical media in the two arms of the ballast cavity; a device for changing the wavelength.

A variant of the method includes recording a series of interferograms to determine wavefronts while simultaneously shifting the phase position of the test object, the reference and the two mirrors of the upstream cavity with the respective fringe frequencies ω _P , ω _R , ω ₁ , ω ₂ . Such a series of interferograms is preferably recorded, where ω _R and ω ₁ or ω _P and ω ₂ are embodied as high-frequency signals that cannot be time-resolved by the receiver. The measuring device is then configured accordingly in order to implement this procedure.

character list

• 1 zeigt schematisch eine konventionelle Messvorrichtung mit einem Fizeau-Interferometer;
• 2 zeigt ein Ausführungsbeispiel einer Messvorrichtung, bei der in das Beleuchtungsmodul ein Zusatzmodul in Form eines Vorschaltinterferometers mit Vorschaltkavität derart integriert ist, dass das von der effektiven Lichtquelle emittierte Messlicht eine Überlagerung von phasenversetztem Licht zweier kohärenter Lichtquellenbilder enthält;
• 3A bis 3F zeigen Beispiele für Zusatzmodule mit einem Michelson-Interferometer mit Feldwinkelkorrektur;
• 4A und 4B zeigen Beispiele für Zusatzmodule mit einem Michelson-Interferometer mit Bildebenenkorrektur;
• 4C zeigt ein Beispiel für ein Zusatzmodul, bei dem die Teilbilder der effektiven Lichtquelle auf den Spiegeln des Zusatzmoduls liegen;
• 5A und 5B zeigen Beispiele für Zusatzmodule mit einem Mach-Zehnder-Interferometer mit Feldwinkelkorrektur;
• 6 zeigt ein Beispiel für ein Zusatzmodul mit einem Mach-Zehnder-Interferometer mit Bildebenenkorrektur;
• 7 zeigt ein Beispiel mit Feldwinkelkorrektur und Ausblendung unerwünschter Strahlanteile zur Unterdrückung von Störreflexen;
• 8 zeigt ein Beispiel für ein Zusatzmodul mit Mach-Zehnder-Interferometer mit Bildebenenkorrektur zur Erzeugung eines lateralen Versatzes der Lichtquellenbilder;
• 9A und 9B zeigen Beispiele zum Abgleich von optischen Wegen mittels refraktiver optischer Elemente.

Further advantages and aspects of the invention result from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures.:

• 1 shows schematically a conventional measuring device with a Fizeau interferometer;
• 2 shows an exemplary embodiment of a measuring device in which an additional module in the form of an upstream interferometer with upstream cavity is integrated into the illumination module in such a way that the measuring light emitted by the effective light source contains a superimposition of phase-shifted light from two coherent light source images;
• 3A until 3F show examples of additional modules with a Michelson interferometer with field angle correction;
• 4A and 4B show examples of add-on modules with a Michelson interferometer with image plane correction;
• 4C shows an example of an additional module, in which the partial images of the effective light source lie on the mirrors of the additional module;
• 5A and 5B show examples of additional modules with a Mach-Zehnder interferometer with field angle correction;
• 6 shows an example of an add-on module with a Mach-Zehnder interferometer with image plane correction;
• 7 shows an example with field angle correction and suppression of undesired beam components to suppress interference reflections;
• 8th shows an example of an additional module with a Mach-Zehnder interferometer with image plane correction for generating a lateral offset of the light source images;
• 9A and 9B show examples of the adjustment of optical paths using refractive optical elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Theoretical principles of the invention are explained below and possibilities for practical implementation are illustrated using exemplary embodiments.

The 1 shows schematically a measuring device 100 of the prior art (SdT) with a Fizeau interferometer 200 for interferometric determination of the shape of an optical surface 112 of a specimen 110, which is held in a specimen holder, not shown.

The measuring device 100 has an illumination module 120 with a primary light source 125 which, during operation, generates measuring light of a measuring wavelength in the visible wavelength range, e.g. at a wavelength of approx. 532 nm. Optical elements of the lighting module are used to receive the measurement light and to generate an extended effective light source 130 in a light source plane 132 of the lighting module.

In the example of 1 a laser is used as the primary light source 125 . Downstream beam expansion optics 127 generate an expanded laser beam, which strikes a rotating diffuser disk 128 and forms a secondary light source there in the form of a uniformly illuminated illumination spot or illumination spot of the desired size. This illumination spot, ie the secondary light source, acts as an effective light source 130 of the illumination module and emits measuring light in the direction of a collimator 210 or collimation optics 210.

The collimator is collimation optics 210 with one or more converging lenses, which are used to collimate the measurement light emitted by the effective light source 130 of the illumination module. A transparent reference element 220 is located at a distance behind the collimator 210 in the parallelized beam path. The reference element 220 has a front optical surface facing the collimator and a rear optical surface 222 facing the surface 112 of the specimen 110, which serves as a reference surface 222 for the measurement. A cavity 230 is formed between the reference surface 222 and the surface 112 of the test piece 110, the distance between the test piece surface 112 and the reference surface 222 is the cavity length d.

Between the illumination module 120 and the collimator 210, a beam splitter 240 in the form of a plane-parallel, partially transparent plane plate is arranged at an angle in the beam path. Measuring light arriving from the illumination module 120 or from the effective light source 130 can pass through to the collimator. Measuring light reflected from the reference surface 222 and measuring light reflected from the surface 112 of the specimen 110 are superimposed, impinge on the beam splitter 240 from the side facing the collimator and are superimposed and reflected in the direction of a detector 250 . The detector 250 has a CCD camera 251 with a flat sensor surface 252 and with camera optics 255 connected upstream. The arrangement is such that light source images are formed in an intermediate image plane 254 at a distance in front of the camera and are imaged onto the sensor surface 252 by means of the camera optics. An interferogram IF with interference fringes is created there. A diaphragm 258 is arranged in the intermediate image plane 254 .

The inventor has recognized certain inherent limitations of such prior art measurement systems. These should initially be based on 1 be explained. First and foremost, it is important that the measuring light does not come from a point-like effective light source, but from an effective light source 130 that has a certain extent or size in the light source plane, e.g. a diameter of the order of one or several millimeters, depending on the focal length of the collimator, the cavity length and the wavelength.

According to the rules of spatial coherence, the light source size 2θ must not exceed a certain size if interference images with sufficient contrast are to be produced. Parameter θ is the acceptance angle at which half the effective light source appears from the collimator 210. The following then applies: $$\text{cos}\mathrm{\theta }>1-\frac{\lambda }{4\mathrm{i.e}}$$

If a punctiform light source were to be displaced perpendicularly to the optical axis 202 of the interferometer in the light source plane 132, a quadratic reduction in the optical path length difference (OPD) around the axis point would be registered in the camera plane. When using an extended monochromatic light source, the classic Haidinger rings HR could be observed in the light source intermediate image plane in front of the eyepiece of the detector. It has been recognized that Fizeau interferometry works satisfactorily only when the Haidinger rings are <1/2 fringes. In other words, the contrast of the interference fringes on the camera decreases with the number of Haidinger rings passed by the aperture and decreases to zero when exactly one Haidinger ring passes. ½ Haidinger ring is sufficient for a usable interference contrast.

wobei f die Brennweite des Kollimators, d die Kavitätslänge und λ Wellenlänge des Messlichts (Messwellenlänge) ist.Taking into account that Fizeau interferometry only works satisfactorily if the Haidinger rings that occur with extended monochromatic light sources remain smaller than half the stripe width, the maximum possible light source diameter ∅ _max is then given by: $${\varnothing }_{max}<\frac{\mathrm{8th}ƒ\mathrm{i.e}}{4\mathrm{i.e}-\lambda }\sqrt{\frac{\lambda }{2\mathrm{i.e}}\left(1-\frac{\lambda }{\mathrm{8th}\mathrm{i.e}}\right)}$$

where f is the focal length of the collimator, d is the cavity length and λ is the wavelength of the measuring light (measuring wavelength).

In practice, this limits the possibility of suppressing high- and medium-frequency measurement errors caused by interference reflections from interferometer components (e.g. due to roughness and surface defects, dirt, bubbles and streaks, etc.).

These limitations can be overcome by utilizing the teachings of the invention. 2 shows an embodiment of a measuring device 100. The optical components of the interferometer can be identical to those of the prior art ( 1 ) be. The same reference numbers are used for the same or corresponding components as in 1 .

There are significant differences in the lighting module 120 . Here the lighting module 120 has an additional module 160 in the form of a series interferometer 160 . The additional module 160 or the upstream interferometer has an upstream cavity 165 which is arranged optically between a light source LQ and the light source plane 132 . The light source LQ can be a primary light source or an extended secondary light source illuminated with light from a primary light source (cf. 1 ).

The optical components of the additional module are configured in such a way that during operation two coherent light source images LQ2, LQ2 are generated from the light of the light source LQ, which have an optical path length difference (OPD) to one another. The measuring light emitted by the effective light source 130 in the light source plane 132 contains a superimposition of light from the two coherent light source images LQ1, LQ2.

In other words: seen from the collimator, two light source images LQ1, LQ2 that are phase-shifted with respect to one another are “visible”. These can completely superimpose one another spatially (as in this example) or appear offset from one another (examples explained later). In the example, the light source images LQ1, LQ2 are created in the light source plane 132, in other embodiments also at a different location.

In the example, the additional module or the upstream interferometer 160 has an interferometer arrangement in the manner of a Michelson interferometer. This is configured to receive measurement light from the light source LQ at a light source position (the position of the light source LQ) in an entry plane 162 and to generate phase-modified measurement light from the effective light source 130 at the light source position of the effective light source 130 in an exit plane 164, which in the example corresponds to the light source plane 132 .

The additional module or the interferometer arrangement includes a beam splitter ST in the form of a partially transparent mirror for receiving measuring light from the light source LQ and for splitting the beam path into a first and a second optical path. The measurement light reflected by the beam splitter ST propagates along the first optical path (transmitted light path) and the measurement light transmitted by the beam splitter propagates along the second optical path in the direction of the light source position of the exit plane. Entry level and exit level are perpendicular to each other. A first mirror SP1 in the form of a plane mirror is arranged in the first optical path in such a way that it reflects measurement light back to the beam splitter ST. A second mirror SP2 in the form of a plane mirror is arranged in the second optical path and reflects measurement light back to the beam splitter. A first objective OB1 is arranged between the light source position in the entrance plane and the beam splitter ST. A second objective OB2 is arranged between the beam splitter ST and the light source position in the exit plane. In this configuration, the beam splitter ST is also used to combine the first and second optical path in such a way that the portions of the first and second optical path reflected by the two mirrors SP1, SP2 are between the beam splitter ST and the light source position (effective light source 130) in the light source plane 132 run together.

The first optical path and the second optical path have a predetermined optical path length difference from one another. This corresponds to twice the length d _VSK of the upstream cavity. This can be continuously adjusted by axial displacement of the second plane mirror SP2.

The additional module 160 or the upstream cavity 165 generates two coherent light source images LQ1, LQ2, which in this example are completely overlapping in the light source plane 132 and form the effective light source 130.

The optical path length differences of the light source images should correspond to that of the Haidinger rings of the interferometer. Four coherent light source images are then superimposed in the light source intermediate image plane 254, two of which are “in phase”, ie show a constant phase difference over the surface. They ensure a high-contrast interference image on the camera.

1. 1. OP1: „Lichtquelle LQ- Objektiv OB1 - Planspiegel SP1 - Objektiv OB2 - Kollimator 210 - Prüfling 110- Kollimator 210 - Lichtquellenzwischenbild LQZ1“
2. 2. OP2: „Lichtquelle LQ - Objektiv OB1 - Planspiegel SP1 - Objektiv OB2 - Kollimator 210 - Referenzfläche 222 - Kollimator 210 - Lichtquellenzwischenbild LQZ2“
3. 3. OP3: „Lichtquelle LQ - Objektiv OB1 - Planspiegel SP2 - Objektiv OB2 - Kollimator 210 - Prüfling 110 - Kollimator 210 - Lichtquellenzwischenbild LQZ3“
4. 4. OP4: „Lichtquelle LQ - Objektiv OB1 - Planspiegel SP2 - Objektiv OB2 - Kollimator 210 - Referenzfläche 222 - Kollimator 210- Lichtquellenzwischenbild LQZ4“

For a better understanding, the light paths are considered below. The extended light source LQ is imaged to infinity by the first objective OB1. The beam splitter ST directs the light coming from the first objective OB1 in the direction of the plane mirrors SP1 and SP2. After reflection, the rays are imaged to the second objective OB2 and from there into the light source plane 132 in which the effective light source 130 lies. Two light source images are created here, with that of plane mirror SP2 differs from that coming from plane mirror SP1 by an adjustable phase delay 2*d _VSK . Parameter d _VSK is the upstream cavity length. There are four different light paths (OP = optical path):

1. 1. OP1: "Light source LQ - lens OB1 - plane mirror SP1 - lens OB2 - collimator 210 - test object 110 - collimator 210 - light source intermediate image LQZ1"
2. 2. OP2: "Light source LQ - objective OB1 - plane mirror SP1 - objective OB2 - collimator 210 - reference surface 222 - collimator 210 - light source intermediate image LQZ2"
3. 3. OP3: "Light source LQ - objective OB1 - plane mirror SP2 - objective OB2 - collimator 210 - test object 110 - collimator 210 - light source intermediate image LQZ3"
4. 4. OP4: "Light source LQ - objective OB1 - plane mirror SP2 - objective OB2 - collimator 210 - reference surface 222 - collimator 210 - light source intermediate image LQZ4"

Only the interference of OP1 with OP4 leads to a high-contrast interference image. All other superimpositions form more than ½ Haidinger rings and superimpose the camera image with an even brightness background. A high-contrast interference pattern is created when the following condition is met in a first approximation: $${\mathrm{i.e}}_{VSK}=\mathrm{i.e}\frac{1-cOs\mathrm{\theta }}{1-cOs\Gamma \theta }$$

Γ is the "telescope magnification" of the "Collimator - Objective OB2" system and results from the focal length ratio: $$\mathrm{\Gamma }=\frac{{ƒ}_{KOllimatO\mathrm{right}}^{\text{'}}}{{ƒ}_{Objektiv2}^{\text{'}}}$$

It can happen that the tuning of the cavity length alone is not sufficient to optimally adjust the path differences between OP1 and OP4. Then, for example, an additional correction element can be inserted into the upstream cavity and/or the mirror SP2 can be curved or made aspheric. The possible types of design are shown below.

The following relationships can serve as an optimization criterion. If a light source point is moved within the light source plane, the interferogram in the camera plane should change by a maximum of ±¼ fringes (±λ/4) for all positions. In other words, it can also be required that the Haidinger interferogram in the light source intermediate image plane 254 should vary at most within half a strip.

A deeper understanding is possible by considering the following theoretical part. The electromagnetic field in the camera plane generated by a coherently radiating monochromatic light source point can be described by: $$E\left(x,y\right)=A\left(x,y\right)e^{i\varphi \left(x,y\right)}={\displaystyle \sum {}_{j=1}^N}{A}_j\left(x,y\right)e^{i{\varphi }_j\left(x,y\right)}$$

Wherex,y = pixel coordinates of the camera; A _j (x,y) = amplitude distribution of wave #j; ϕ _j (x,y) = phase distribution of wave no. j, j = 1, ... , N.

mit λ als Wellenlänge.The optical path of a "ray" with the impact coordinates (x, y) from the light source to the camera is denoted by 0P _j (x,y). What is meant is the sum of all distances s multiplied by the refractive index n of the medium k passing through. Then the phase on the camera is calculated by: $${\varphi }_j\left(x,y\right)=\frac{2\pi }{\lambda }O{P}_j\left(x,y\right)=\frac{2\pi }{\lambda }{\displaystyle \sum {}_{k=1}^K{n}_{jk}\left(x,y\right)s_{jk}\left(x,y\right)}$$

with λ as wavelength.

The intensity distribution on the camera results from the superimposition of N waves $$I\left(x,y\right)={\displaystyle \sum {}_{j=1}^N{\displaystyle \sum {}_{k=1}^N{A}_j\left(x,y\right)A_k\left(\text{x}\text{, y}\right)}\text{cos}\left({\varphi }_j\left(x,y\right)-{\varphi }_k\left(x,y\right)\right)}$$

In the case under consideration, N = 4. The brightness distribution can thus be described by (x,y omitted): $$\begin{array}{l}I=A_{P1}^2+A_{R1}^2+A_{P2}^2+A_{R2}^2+2A_{P1}{A}_{R1}cOs\Delta {\varphi }_{P1R1}+2A_{P1}{A}_{P2}cOs\Delta {\varphi }_{P1P2}+2A_{P1}{A}_{R2}cOs\Delta {\varphi }_{P1R2}\\ ...+2A_{R1}{A}_{P2}cOs{\varphi }_{R1P2}+2A_{R1}{A}_{R2}cOs\Delta {\varphi }_{R1R2}+2A_{P2}{A}_{R2}cOs\Delta {\varphi }_{P2R2}\end{array}$$

• A_P1, A_P2, A_R1 A_R2 die Amplituden der Wellen, die den Weg über Prüfling (P) oder Referenz (R) und Planspiegel (1) oder (2) genommen haben,
• Δϕ_P1R1 = ϕ_P1 - ϕ_R1 die optische Wegdifferenz zwischen der Welle P1 und R1,
• Δϕ_P1P2 = ϕ_P1 - ϕ_P2 die optische Wegdifferenz zwischen der Welle P1 und P2,
• Δϕ_P1P2 = ϕ_P1 - _ϕR2 die optische Wegdifferenz zwischen der Welle P1 und R2,
• Δϕ_R1P2 = ϕ_R1 - ϕ_P2 die optische Wegdifferenz zwischen der Welle R1 und P2,
• Δϕ_R1R2 = ϕ_R1 - ϕ_R2 die optische Wegdifferenz zwischen der Welle R1 und R2,
• Δϕ_P2R2 = ϕ_P2 - ϕ_R2 die optische Wegdifferenz zwischen der Welle P2 und R2.

(8) results in a DC component from the sum of the 4 squared amplitudes and a modulation component with cosine-shaped stripe patterns. designate:

• A _P1 , A _P2 , A _R1 A _R2 the amplitudes of the waves that have taken the path via test object (P) or reference (R) and plane mirror (1) or (2),
• Δϕ _P1R1 = ϕ _P1 - ϕ _R1 the optical path difference between the wave P1 and R1,
• Δϕ _P1P2 = ϕ _P1 - ϕ _P2 the optical path difference between the wave P1 and P2,
• Δϕ _P1P2 = ϕ _P1 - _ϕR2 the optical path difference between the wave P1 and R2,
• Δϕ _R1P2 = ϕ _R1 - ϕ _P2 the optical path difference between the wave R1 and P2,
• Δϕ _R1R2 = ϕ _R1 - ϕ _R2 the optical path difference between the waves R1 and R2,
• Δϕ _P2R2 = ϕ _P2 - ϕ _R2 is the optical path difference between shaft P2 and R2.

in der Lichtquellenebene. Wird über eine leuchtende Fläche integriert, erhält man die Intensitätsverteilung J(x, y) auf der Kamera: $$J\left(x,y\right)={\displaystyle \int {\displaystyle \int {}_{\varrho =\mathrm{0,}\phi =0}^{R\mathrm{,2}\pi }I\left(x,y,\varrho ,\phi \right)\varrho d\varrho }}d\phi$$

The intensity distribution in (8) applies to a light source point with the coordinates $\left(\varrho ,\phi \right)$

in the light source plane. If you integrate over a luminous surface, you get the intensity distribution J(x, y) on the camera: $$J\left(x,y\right)={\displaystyle \int {\displaystyle \int {}_{\varrho =\mathrm{0,}\phi =0}^{R\mathrm{,2}\pi }I\left(x,y,\varrho ,\phi \right)\varrho \mathrm{i.e}\varrho }}\mathrm{i.e}\phi$$

für alle Lichtquellenpunkte $\varrho ,\phi$

The effect of the correctly designed and adjusted ballast cavity is to make the path length difference Δϕ _P1R2 independent of the position of the light source point. This results in the following requirement for the design of an upstream cavity for extended light sources: $$\left|\Delta {\varphi }_{P1R2}\left(x,y,\varrho ,\phi \right)\right|\le kOnst\left(x,y\right)\pm \frac{2\pi }{\mathrm{8th}}$$

for all light source points $\varrho ,\phi$

variieren und nach Integration über die Lichtquelle zu einem Gleichanteil werden, sodass die kosinusförmigen Streifenstrukturen verschwinden.For all five other modulation shares, it is advantageous if they change from $\varrho ,\phi$

vary and become a constant component after integration via the light source, so that the cosine-shaped stripe structures disappear.

für alle Lichtquellenpunkte $\varrho ,\phi$

If broadband light is to be used, the path length difference to be used should lie within the temporal coherence length. The following inequality (11) describes a requirement for the design of the ballast cavity for extended light sources and broadband light: $$\left|\Delta {\varphi }_{P1R2}\left(x,y,\varrho ,\phi \right)\right|\le kOnst\left(x,y\right)\pm \frac{2\pi }{\mathrm{8th}}\le \frac{2\pi }{4}KOH\ddot{\text{a}}\mathrm{right}en\mathrm{e.g}l\ddot{\text{a}}nGe$$

for all light source points $\varrho ,\phi$

In order to determine the shape deviation of the test object, a series of interferograms is preferably required, the fringe patterns of which have been changed in a specific sequence. For this purpose, at least the phase difference Δφ _P1R2 must be changed over time in a suitable manner by so-called phase shifting.

Various techniques can be used for targeted phase changes (constant component, additional tilting if necessary), eg: a) changing the position of the test object and/or reference; b) a change in the position of the plane mirror SP1 and/or SP2 of the upstream cavity; c) a change in the refractive indices of optical media in reference arm and/or test arm; d) a change in the refractive indices of the optical media in the two arms of the ballast cavity; e) a change in wavelength.

The temporal phase change for each of the four waves is described in (12). It is not clear which of the methods a) to e) will be used. With a phase change that is continuous over time, a phase shift frequency ω _j with j = 1,2,3,4 results for each of the four waves, which leads to a phase change in the time element dt: $$\mathrm{i.e}{\varphi }_j\left(x,y,t\right)=\frac{2\pi }{\lambda }\left(\frac{\partial O{P}_j\left(x,y,t\right)}{\partial \text{t}}-\frac{O{P}_j\left(x,y\right)}{\lambda }\frac{\mathrm{i.e}\lambda \left(t\right)}{\partial t}\right)\mathrm{i.e}t={\omega }_j\mathrm{i.e}t$$

Taking into account the optical path changes (OPD) in the six interference terms in (8), there are six frequencies in the interference signal: _ωP - _ωR fringe frequency of wave interference P1 with R1, ω1 - _ω2 fringe frequency of wave interference P1 with P2, ω _P - ω _R + ω ₁ - ω ₂ fringe frequency of wave interference P1 with R2, ω) _R - ω _P + ω ₁ - ω ₂ fringe frequency of wave interference R1 with P2, _ω1 - _ω2 fringe frequency of wave interference R1 with R2, _ωP - _ωR Fringe frequency of wave interference P2 with R2.

The term P1 with R2 carries the information about the specimen deformation that is to be detected. In principle, each of the five phase shift options mentioned can be used (both arms of the upstream cavity, test object, reference, wavelength).

In an example, the test piece and the plane mirror SP2 are fixed, the reference and the plane mirror SP1 move differently. It is expedient to shift the reference and the plane mirror SP1 of the front cavity with different frequencies ω _R ≠ ω ₁ and leave the test object and plane mirror 2 fixed ω _P = ω ₂ = 0.

As a consequence, there are four different frequencies in the interference pattern, namely ω ₁ , ω _R , ω ₁ +ω _R and ω ₁ -ω _R , the latter of which carries the information sought and differs from all other frequencies. If ω ₁ and ω _R are selected appropriately, crosstalk from any undesired interference terms that may still be present can be avoided.

It would also be advantageous to form ω _R and ω ₁ as high-frequency signals that cannot be detected by the camera. The low-frequency signal ω ₁ -m _R is then the only interference signal that the camera can detect.

der ausgedehnten Lichtquelle erfüllt wird. Dazu sind zu beachten:

1. 1. Der durch die Kavitätslänge bedingte Weglängenunterschied sollte ausgeglichen werden,
2. 2. Der durch die Bauformen der optischen Elemente im Interferometer und der Vorschaltkavität bedingte Weglängenunterschied sollte ausgeglichen werden.

There are numerous possible configurations or embodiments for practicing the invention. The upstream cavity should provide for an adjustment of the optical paths, so that the requirement (10) for each pixel (x,y) with variation of the light source point $\left(\varrho ,\phi \right)$

of the extended light source is met. Please note:

1. 1. The path length difference caused by the cavity length should be compensated,
2. 2. The path length difference caused by the design of the optical elements in the interferometer and the upstream cavity should be compensated.

1. A) Michelson-Interferometer (reflektive Korrektur),
2. B) Mach-Zehnder-Interferometer (Durchlichtkorrektur).

Two different design types of the upstream cavity can be used for compensation:

1. A) Michelson interferometer (reflective correction),
2. B) Mach-Zehnder interferometer (transmitted light correction).

1. 1) Kompensation durch Feldwinkelkorrektur,
2. 2) Kompensation durch Bildebenenkorrektur.

In addition, two different compensation principles are applicable:

1. 1) Compensation by field angle correction,
2. 2) Compensation by image plane correction.

All designs should produce Haidinger rings at the output of the ballast cavity, which correspond to those of the Fizeau interferometer without a ballast cavity.

Examples of Michelson interferometers with field angle correction follow.

In the example of 2 the upstream interferometer is constructed as a Michelson interferometer. 3A until 3F show variants. The path difference required for the Haidinger rings is generated after the first objective OB1 has imaged the light source LQ to infinity. Thus, the correction takes place in Fourier space. A point of the light source corresponds to a tilt of the infinity ray. Each tilt must be given a phase corresponding to the corresponding light source point in Haidinger's rings. This can be done in a number of ways.

The 2 and 3A indicate versions in which only the second plane mirror SP2 is placed at the appropriate distance.

If this is not sufficient, the second plane mirror SP2 can be replaced by a combination of a compensation lens and a curved mirror ( 3B) Compensation lens and mirror can have hollow or raised surfaces with spherical, planar or aspherical shape.

Under certain circumstances, the compensation lens can also be designed as a so-called Mangin lens, in which the rear surface is designed as a mirror ( 3C ). A system with several lenses or a computer-generated hologram (CGH) can also be used as a compensation element.

The variants in the 3A until 3C additionally require an objective OB2, which images the light source images from infinity into its focal plane, which coincides with the light source plane 132 of the Fizeau interferometer.

With the variants in 3D until 3F the lenses OB1 and OB2 are omitted. The light source LQ is in the centers of curvature of the concave mirrors SP1 and SP2 or the systems that replace the concave mirror SP2.

In 3D two concave mirrors with the radii of curvature r ₁ and r ₂ are used. The cavity length of the upstream cavity is given by r ₂ -r ₁ .

In In 3E the concave mirror SP2 is replaced by the combination of a compensating lens (or compensating system or CGH) and plane mirror (or concave or raised spherical or aspheric mirror).

In 3F the concave mirror SP2 is replaced by a Mangin lens, which has a raised transparent surface and a hollow, raised or flat reflecting surface. The surface type can be spherical or aspherical.

The following are examples of Michelson interferometers with image plane correction.

The upstream cavity is again constructed as a Michelson interferometer ( 4A , 4B) . The path difference necessary for Haidinger's rings is generated directly in the plane of the light source images. For this purpose, the light source is imaged with a lens OB1 onto a plane mirror SP1 and onto a mirror SP2. The images are split up by a divider element ST as in the previous examples. The shape of the mirror SP2 corresponds to half the phase deviation in the Haidinger rings. For example, with a number of 32 rings at the wavelength λ, the shape of the mirror SP2 at the edge of the light source image deviates by 16* λ from the planar surface.

In 4A the light source LQ is imaged onto a plane mirror SP1 and a curved mirror SP2. The curved mirror SP can be hollow or raised and spherical or aspherical. A lens OB2 images both light source images in the light source plane 132 of the Fizeau interferometer.

At the in 4C The variant shown is lens OB1 designed so that images of the light source LQ arise on the first mirror SP1 and on the second mirror SP2. There is no second lens between the beam splitter ST and the light source plane 132, so that the beam bundles leading from the light sources to the collimator diverge in the light source plane. The light source plane is here virtually from the ers th mirror SP1 relocated. The measuring light propagating to the collimator contains a superimposition of the two light source images lying on the different mirrors.

In 4B the mirror SP2 is replaced by the combination of compensating lens (or compensating system or CGH) and plane mirror (or hollow or raised mirror with spherical or aspheric shape).

The following are examples of Mach-Zehnder interferometers with field angle correction

In the examples of 5A and 5B the upstream cavity is constructed as a Mach-Zehnder interferometer. The path difference required for Haidinger's rings is generated after an objective OB1 has imaged the light source LQ to infinity. Thus, the correction takes place in Fourier space. A point of the light source LQ corresponds to an inclination of the infinity ray. Each tilt must be given a phase corresponding to the corresponding light source point in Haidinger's rings. A splitter mirror ST1 splits the parallel beam path into two arms, which are each deflected via a plane mirror SP1 or SP2 and are recombined at a second splitter mirror ST2. A lens OB2 focuses the two parallel beams in its focal plane, which coincides with the light source plane 132 of the interferometer.

To compensate for Haidinger's rings, the optical paths in the two sub-arms are different.

5A specifies a version in which the plane mirror SP1 and the splitter mirror ST2 are tilted in parallel and decentered in a special way in order to set the required cavity length. In addition, a compensating plane plate element PE is introduced into the beam path in order to compensate for other optical path length differences and to generate the necessary Haidinger rings.

In 5B the plane plate element is off 5A replaced by a combination of converging and diverging lens. Refractive elements or lenses with flat, spherical and aspherical surfaces can be used as compensation elements. Their overall optical effect is largely afocal (= focal length towards infinity).

An example of a Mach-Zehnder interferometer with image plane correction follows.

1. a) und den Planspiegel SP1 auf das Kompensationselement abgebildet und nach Reflexion am Teilerspiegel ST2 mit dem Strahlengang b) zusammengeführt,
2. b) über einen Planspiegel SP2 und nach Durchgang durch den Teilspiegel ST2 mit dem Strahlengang a) zusammengeführt.

The upstream cavity of the variant in 6 is constructed as a Mach-Zehnder interferometer. The path difference necessary for Haidinger's rings is generated by a refractive or diffractive compensation element KE in transmitted light. For this purpose, the light source LQ with a lens OB1 via a splitter mirror ST1

1. a) and the plane mirror SP1 imaged on the compensation element and, after reflection at the splitter mirror ST2, combined with the beam path b),
2. b) merged with the beam path a) via a plane mirror SP2 and after passing through the partial mirror ST2.

The objective OB2 produces two light source images that coincide in the light source image plane 132 of the interferometer. Their coherent superposition reveals the required Haidinger rings. The following are possible as compensation elements KE: a) flat plate; b) single lens spherical or aspherical; c) lens; d) Computer generated hologram (CGH).

The suppression of unwanted reflections is explained below
If one leaves the undesired reflections, which contribute little or nothing to the modulation of the fringes, then the contrast in the interference pattern is reduced to 50% at best. If the interference ability remains, the measurement accuracy is impaired. If you want to maximize the contrast, the unwanted reflections should be suppressed.

As indicated above, only light paths OP1 and OP4 are required. OP2 and OP3 are superimposed as an even brightness background with possibly small interference components. The following shows how to hide OP2 and OP3.

All variants of the field angle correction can be used to hide unwanted reflections. When correcting the aperture, only the Mach-Zehnder interferometers are suitable for the lateral offset of the light source images.

Based on 7 an example with field angle correction is explained. In the case of field angle correction as in the example of 7 the plane mirror SP2 is tilted in such a way that the two light source images are offset laterally with respect to one another without overlapping. In return, the reference 220 in the interferometer is tilted in such a way that the light paths OP1 and OP4 coincide in the light source intermediate image plane 254 in front of the eyepiece. A diaphragm BL is placed in the light source intermediate image plane, which blocks the laterally shifted light source images of OP2 and OP3. The detailed illustration shows how the light source intermediate images created along the optical paths OP1 and OP4 fall into the round aperture BO, while the beam parts belonging to OP2 and OP3 are blocked by the aperture BL.

Based on 8th an example of image plane correction is explained. Only the Mach-Zehnder interferometers come into question for a lateral offset of the light source images. In 8th the lateral offset is generated by (horizontal) decentering of the splitter mirror ST2 or joint decentering of the plane mirror SP1 and the compensation element KE. As a result, two laterally offset light source images LQ1, LQ2 are available in the image plane of objective OB2 (corresponding to light source plane 132).

The use of a monochromatic extended light source in a Fizeau interferometer required the design requirement according to Eq. 10

If one also wants to use a polychromatic extended light source, then the design requirement according to Eq. 11 to be complied with. The optical phase differences are then for all wavelengths used according to Eq. 6 to zero: $${\varphi }_4\left(x,y,\mathrm{e.g}\right)-{\varphi }_1\left(x,y,\lambda \right)=\frac{2\pi }{\lambda }{\displaystyle \sum {}_{k=1}^K\left(n_{4k}\left(x,y,\lambda \right)s_{4k}\left(x,y\right)-n_{1k}\left(x,y,\lambda \right)s_{1k}\left(x,y\right)\right)\to 0}$$

For each light source point, OP1 = OP4 must apply.

Because of the dispersion of the glass materials, the glass paths should also be balanced in the upstream cavity. If glass elements are tested in transmitted light in the Fizeau interferometer, the glass paths in the upstream cavity must be simulated. The 9A and 9B give examples of upstream cavities that adjust the airways of a Fizeau interferometer using refractive elements in the upstream cavity that are matched to each other in both arms (compensation lenses KL1 and KL2).

The examples make it clear that and how the invention eliminates the cavity length limitation of the light source size in a Fizeau interferometer. As a result, high- and medium-frequency coherent error contributions from individual areas of the interferometer are strongly damped. Long-wavelength errors are attenuated depending on the size of the light source. Known methods circumvent the size limitation of the light source by moving a point or line light source and conveying the sequentially determined results. The invention presented is therefore faster and achieves the same effect with only one measurement. The loss of interference contrast caused by the introduction of an additional cavity can be compensated for by deliberately tilting or decentering elements and hiding unwanted reflections.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 10121516 A1 [0007]
• US2003030819A1 [0007]
• DE 102016213237 A1 [0007]
• US2019154427A1 [0007]

Claims (15)

Measuring device (100) for the interferometric determination of a shape of an optical surface (112) of a specimen (110), comprising: an illumination module (120) for generating an effective light source (130) in a light source plane (132) of the illumination module; a collimator (210) for collimating the measurement light emitted from the effective light source (130); a transparent reference element (220) downstream of the collimator (210) with a reference surface (222) facing the surface (112) of the test object (110), with a cavity between the reference surface (222) and the surface (112) of the test object (110). (230) is formed; a beam splitter (240) which is arranged between the illumination module (120) and the collimator (210) in such a way that measuring light emitted by the effective light source (130) passes through to the collimator (210) or is reflected and by the reference surface (222) and the surface (112) of the specimen (110) reflected measuring light is superimposed in the direction of a detector (250) reflected or transmitted; characterized in that the illumination module (120) has an additional module (160) in the form of a ballast interferometer with a ballast cavity (165) which is optically arranged between an extended light source (LQ) and the light source plane (132) and is configured in such a way that from the Light of the extended light source (LQ) two coherent light source images (LQ1, LQ2) can be generated, which have an optical path length difference to one another, the measuring light emitted by the effective light source (130) in the light source plane (132) being a superimposition of light from the two coherent light source images (LQ1, LQ2). measuring device claim 1 , characterized in that the optical path length difference is set in such a way that at least one of the following conditions is met: (i) the optical path length difference is essentially independent of the position of a light source point in the effective light source (130) (ii) at the output of the ballast cavity Haidinger rings are generated that correspond to those of the measuring device without an upstream cavity. (iii) the optical path length difference of the upstream cavity (165) essentially corresponds to the path length difference of Haidinger rings of the interferometer. measuring device claim 1 or 2 , characterized in that the effective light source (130) is an extended light source with a light source diameter ∅ for which the condition $$\varnothing >\frac{\mathrm{8th}ƒ\mathrm{i.e}}{4\mathrm{i.e}-\lambda }\sqrt{\frac{\lambda }{2\mathrm{i.e}}\left(1-\frac{\lambda }{\mathrm{8th}\mathrm{i.e}}\right)}$$

where f is the focal length of the collimator and d is the cavity length of the cavity (230). Measuring device according to one of the preceding claims, characterized by an adjustment device for adjusting the optical path length difference over an adjustment range, the adjustment preferably being able to be carried out steplessly. Measuring device according to one of the preceding claims, characterized in that the additional module (160) has an interferometer arrangement which is configured to receive measuring light from the light source (LQ) in an entry plane (162) and phase-modified measuring light therefrom in an exit plane (132 ) to generate, wherein the additional module (160) comprises the following components: a first beam splitter (ST) in the form of a partially transparent mirror for receiving measuring light from the entrance plane and for splitting the beam path into a first and a second optical path, wherein from the first Measuring light transmitted by the beam splitter (ST) along the first optical path and measuring light reflected by the first beam splitter (ST1) are propagated along the second optical path in the direction of the exit plane; a second beam splitter (ST) in the form of a partially transparent mirror for merging the first and the second optical path in such a way that the first and the second optical path run together between the second beam splitter (ST) and the exit plane; a first mirror (SP1) arranged in the first optical path such that measurement light is reflected to the second beam splitter (ST); a second mirror (SP2) arranged in the second optical path such that measurement light is reflected to the second beam splitter (ST); wherein the first optical path and the second optical path have a predeterminable optical path length difference. measuring device claim 5 , characterized by a first objective (OB1) which is arranged between the entrance plane (162) and the first beam splitter (ST, ST1) and/or a second objective (OP2) which is arranged between the second beam splitter (ST, ST2) and the Exit plane (132) is arranged and / or in that the first mirror and / or the second mirror is designed as a concave mirror. Measuring device according to one of Claims 5 or 6 , characterized in that the first beam splitter and the second beam splitter are formed by the same beam splitter (ST), so that an interferometer arrangement is formed in the manner of a Michelson interferometer, or characterized in that the first beam splitter and the second beam splitter are two separately arranged Beam splitters (ST1, ST2) are such that an interferometer arrangement is formed in the manner of a Mach-Zehnder interferometer. Measuring device according to one of Claims 5 , 6 or 7 , characterized in that the optical path length difference between the first path containing the first mirror (SP1) and the second path containing the second mirror (SP2) is continuously adjustable, with preferably an adjustment device for the axial displacement of the first mirror and/or the second mirror is provided. Measuring device according to one of Claims 5 until 8th , characterized in that the first mirror and/or the second mirror has a curved and/or an aspherical mirror surface. Measuring device according to one of Claims 5 until 9 , characterized in that at least one optical element of the first optical path and/or at least one optical element of the second optical path is misaligned or can be misaligned in a controlled manner such that light source images from the first and the second optical path are laterally offset from one another without overlapping. Measuring device according to one of the preceding claims, characterized in that the optical arrangement can be adjusted in such a way that four light source intermediate images of the light source images can be generated in an intermediate light source plane (254) which is optically conjugate to the sensor surface (252) of the detector (250), two mutually in-phase light source intermediate images overlap one another and phase-shifted light source intermediate images are laterally offset to the in-phase light source intermediate images, with a diaphragm (BL) with a diaphragm opening (BO) preferably being arranged in the intermediate light source plane (254), which is in an operating position can be arranged in such a way that the measuring light components belonging to the in-phase light source intermediate images can pass through the aperture (BO) and the phase-shifted light source intermediate images can be blocked by the aperture (BL). Measuring device according to one of the preceding claims, characterized in that the additional module has at least one additional correction element, which is selected from the group containing the following elements: a lens with refractive power a compensation system with several lenses a computer-generated hologram (CGH) a lens element with a mirror surface Measuring device according to one of claims 3 until 12 , characterized by a primary light source, which is a broadband light source, wherein the path length difference is within the temporal coherence length of the primary measurement light. Measuring device according to one of the preceding claims, characterized in that the measuring device is designed to adjust phase shifts, at least one of the following devices being present: a device for generating a change in position of the test object and/or reference, a device for generating a change in position of the first mirror and/or the second mirror of the upstream cavity, a device for changing the refractive indices of the optical media in the reference and/or test arm, a device for varying the refractive indices of the optical media in the two arms of the upstream cavity, a device for changing the wavelength. measuring device Claim 14 , characterized in that it is configured to carry out the following steps: a) recording a series of interferograms for determining wavefronts with simultaneous shifting of the phase position of the test object, the reference and the two mirrors of the upstream cavity with the respective fringe frequencies ω _P , ω _R , ω ₁ , ω ₂ ; b) recording a series of interferograms according to claim a), wherein ω _R and ω ₁ or ω _P and ω ₂ are in the form of high-frequency signals which cannot be temporally resolved by the receiver.

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