DE102017001524B4 - Arrangement for measuring at least partially reflective surfaces - Google Patents

Abstract

Arrangement for measuring at least partially reflecting surfaces of a measuring object (8) comprising at least one illumination source (1), at least four collecting optics (3, 5, 7, 10), at least two spatial filters (6, 9), at least one optical grating (4 ) and an image pickup unit (11), wherein the illumination source (1), the collecting optics (3, 5, 7, 10), the spatial filters (6, 9), the optical grating (4) and the image pickup unit (11) along a common optical axis are positioned.

Description

The present invention relates to an arrangement for measuring at least partially reflecting surfaces, with which even complex free-form surfaces with large inclinations can be measured and reconstructed with great accuracy without additional zero elements.

An exact shape measurement of surfaces is indispensable in very different application areas of technology, but above all in optical measurement technology. The rapid development in these areas, especially for freeform surfaces, requires new advanced methods and sensors for these measurements with high resolution and accuracy with short measurement time [ 1 ] [ 2 ].

Free-form surfaces are non-rotationally symmetric surfaces, which have some advantages over conventional rotationally symmetric surfaces. For example, they allow a greater number of degrees of freedom in the design and a greater variety of functions for use in an optical system [ 3 ]. This allows better control of aberrations and the development of compact and lightweight optical systems. Therefore, these free-form surfaces are already used in various applications such as in compact projection systems, head-up displays and in lithography [ 4 ].
In addition to these advantages, however, there are still many challenges in the production and characterization of these free-form surfaces, which are produced according to the current state of the art with increasingly accurate and expensive CNC production machines [ 5 ] [ 6 ]. For a timely detection of the resulting manufacturing defects, a direct control of the manufacturing quality is still desirable during production, with a reduction in the dimensions of the test equipment and equipment significantly simplifies their direct integration into a CNC machine [ 7 ].

A variety of methods for the three-dimensional measurement of optically smooth surfaces by means of the detection of their shape or wavefront are known from the prior art.

The most widely used methods are realized with contact or tactile coordinate measuring machines with which even complex surfaces can be measured. The disadvantage here is that these methods require a vibration-free environment, they are limited in their spatial resolution and relatively time consuming. Therefore, these measuring methods are used in production only for the characterization of individual samples. An integration of the required measuring technology into, for example, a CNC machine is only possible to a limited extent [ 8th ].

Other known measuring methods use the properties of light as an electromagnetic wave in order to measure surfaces or wavefronts. The object to be measured is illuminated and the light reflected from its surface is detected and analyzed using different methods and measuring instruments. However, the measuring devices required for this purpose or their assemblies must not be positioned in the beam path of the illumination, since otherwise the measurement object is completely or partially shaded by the optical components (cf. 1 ).

The problem of unwanted shadowing according to the prior art with the use of beam splitters ( 14 ) or an off-axis illumination of the reflective surface of the measurement object is bypassed (s. 2 ). However, this leads to a disadvantageous increase in the dimensions of the entire measuring devices or to very complex structures.

The most important optical methods known from the prior art for measuring surfaces or wavefronts are listed below.

For most optical components with slightly curved surfaces in conventional design and with uncomplicated, simple geometries, such as flat, slightly curved or spherical surfaces (lenses, mirrors, etc.), interferometric methods are often used [ 9 ]. For the illumination of these surfaces mostly coherent or partially coherent light (laser or LED as light source) is used, which is divided and continues on two different paths: on a first path on which the object to be measured is arranged (object wave) and on a second path on which the object to be measured is not arranged (reference wave). Subsequently, the light components of both paths are superimposed and the resulting interference pattern is recorded as an intensity distribution.

In order to achieve a high measuring accuracy, these methods require a high optomechanical precision of the measuring device. This is associated with a very high adjustment effort and a low-noise environment (extraneous light, mechanical vibration, temperature stability). These measures require a high expenditure on equipment with corresponding costs. Another problem is the low dynamic range (measuring range) of these methods of only a few wavelengths of light. To remove this limitation, a reference element becomes a reference wavefront generated (zero test) and compared with the object wave of the measurement object. However, the production of a zero-test component represents a major hurdle both technologically and economically, especially since this must first be tested before it is used. Another method of slightly but not always increasing the measurement range is the Tilted Wave Interferometry [ 10 ] [ 11 ] [ 12 ] [ 13 ].
For the reasons mentioned above, interferometric measuring methods for surface and roughness testing are generally used. However, they are unsuitable as surface reconstruction measuring methods.

For components with sophisticatedly shaped surfaces, such as aspheres, the wavefront measuring methods known from the prior art, for example the Hartmann method or the Shack-Hartmann test, are used (eg patent US Pat. No. 7,525,076 B1 ). There, a so-called differential Shack-Hartmann curvature sensor (SHS) is used to measure major curvatures and inclinations of a wavefront. However, surfaces with large inclinations or curvatures can not be measured (limited by the size of the imaging optics or the microlens arrays). In addition, there are limitations with regard to the measurable height difference in the surface profile and the lateral resolution. In addition, in the case of free-form surfaces, the problem of uniqueness of the measurement occurs, therefore, these measuring methods are used as long as the local slope of the free-form is smaller than the dynamic range of the SHS.

With the publication DE 10 2013 018 569 A1 is a new measurement method for the optical shape detection of glossy smooth freeform surfaces, with which even freeform optics with large inclinations can be absolutely measured and their surface reconstructed. The invention is based on the principle of raster reflectometry, in which at least one light pattern is reflected by the surface to be examined, at least partially reflecting, and is propagated back in space with the aid of a three-dimensional observation unit. The process is absolute and well suited for freeform surfaces with large slopes. However, this method has the disadvantage that it is very time-consuming, since the surface to be measured has to be rastered.

Furthermore, in the relevant patent and non-patent literature a confocal scanning of surfaces to be measured is proposed (eg DE 197 49 974 C2 . DE 10 2005 043 627 A1 ). The disadvantage here is the complicated adjustment of the structure. In addition, these methods are not suitable for surfaces with large inclinations.

Other methods measure a grating pattern which is reflected or transmitted at the specular surface. The grating has a variable grating period adapted to the expected surface shape according to its gradients / inclinations. These methods can be summarized under the term of the deflektometric method. Numerous patents and publications have been published (eg WO 97/40 367 A1 . WO 2005/031 251 A1 . WO 01/23 833 A1 . US 2008/225 303 A1 . US 4,912,336 A1 [ 14 ], [ 15 ] and [ 16 ]). However, the deflektometric methods are always ambiguous without additional expenditure on software and hardware and are not suitable for surfaces with large inclinations.

Another measuring method using the grid reflection method is described in [ 17 ]. In order to be able to determine a surface geometry, underdetermined systems of differential equations must be solved, which can lead to an increased measurement uncertainty. Based on this model assumption, the method can not be used reliably and unambiguously for the measurement of complex measurement objects of unknown geometry.

In the DE 10 2005 007 244 A1 Another refinement method is presented with a pattern of illumination that enables the shape detection of reflective surfaces. However, it is also not suitable for measuring objects with strongly curved surfaces, since not all reflected rays hit the diffuse screen used. In addition, this method has a limited lateral resolution.

A designated as Holographic Wavefront sensor measuring device is in the US Pat. No. 7,268,937 B1 presents. This sensor was developed for the measurement of optical components in transmission. However, it can not be used for reflective surfaces. In addition, this method only works with coherent illumination and consequently with the disadvantages known from the interferometric methods.

In the patent US Pat. No. 6,396,588 B1 introduces a Hybrid Curvature-Tilt Wavefront Sensor. This sensor is highly accurate, but has large dimensions. These are due to the fact that several beam splitters are used in this sensor. So far, this type of sensor has been designed only for transmitting components. An adaptation for the measurement of reflective components would result in a further enlargement of its spatial dimensions.

For use in ophthalmology, an ophthalmic talbot-moire wavefront sensor has been developed (patent US Pat. No. 6,736,510 B1 ). This sensor is based on the so-called self-imaging. It comprises a lateral illumination unit and a beam splitter. This again results in a large spatial extent of the entire measuring arrangement.

In summary, it can be stated that the measurement of reflecting surfaces can not be dispensed with beam splitters or lateral incident illumination. The overall dimensions of the required measurement arrangements are therefore large and the measurement setups are not compact. Consequently, a high adjustment effort between the lighting arm and the pickup arm is necessary. In addition, additional unwanted effects such as multiple propagation and reflection at and between the surfaces of the beam splitters may occur. Furthermore, the cost of high quality beam splitters is often very high.
All these disadvantages and the high demands on a low-noise environment (mechanical vibrations, temperature stability) complicate a direct integration of the presented measuring methods in current manufacturing processes, eg with a CNC machine.

The object of the present invention is to overcome the disadvantages of the prior art.

According to the invention, the solution of this problem succeeds with the features of the first claim. Further embodiments of the solution according to the invention are specified in the subclaims.

The invention is based on the principle of wavefront measurement with Fourier optics, in which the measurement object with the surface to be measured is combined with at least one light source, two imaging systems and a camera, the optical elements simultaneously being used both as a lighting unit and as a modulation unit or evaluation unit become. According to the invention, all optical elements are arranged on an optical axis, without shading taking place between the illumination and the measurement object. Consequently, it is possible to dispense with a lateral illumination or beam splitter and to realize a compact measurement setup with small dimensions.
Another aspect of the present invention is the use of partially coherent illumination. Coherent or partially coherent monochromatic light sources (eg lasers, LEDs) are used, whereby LEDs are much better suited for illumination than laser light sources. The dimensions of inexpensive LEDs are already smaller than one millimeter. This allows the design of new lighting systems in the smallest space.
The partially coherent nature of the light avoids the unwanted interference at the reflective surfaces of the devices of the experimental setup. In addition, the so-called speckle problems can be drastically reduced with partially coherent illumination. For the most part, the LEDs used are not harmful to health compared to a laser light. This leads to the fact that a use of goggles is not absolutely necessary.

• 1- die Abschattung eines Messobjektes durch optische Bauteile (Problemstellung)
• 2- aus dem Stand der Technik bekannte Problemlösungen
  1. a - die Verwendung von Strahlteilern
  2. b - die Verwendung einer seitlichen Beleuchtung
• 3- der Beleuchtungspfad einer erfindungsgemäßen Anordnung
• 4- der Reflexionspfad einer erfindungsgemäßen Anordnung
• 5- Ausführungsbeispiele für eine zusätzliche außeraxiale parallele Beleuchtung des Messobjektes
• 6- weitere Ausführungsbeispiele für eine zusätzliche außeraxiale parallele Beleuchtung des Messobjektes
• 7- Ausführungsbeispiele für eine wellenlängenkodierte Beleuchtung des Messobjektes

In the following, the compact arrangement according to the invention will be explained in more detail with reference to the accompanying drawings. It shows:

• 1 - the shading of a measuring object by optical components (problem definition)
• 2 - Problem solutions known from the prior art
  1. a - the use of beam splitters
  2. b - the use of side lighting
• 3 - The illumination path of an inventive arrangement
• 4 - The reflection path of an arrangement according to the invention
• 5 Embodiments for an additional extra-axial parallel illumination of the measurement object
• 6 - Further embodiments of an additional extra-axial parallel illumination of the measurement object
• 7 - Embodiments for a wavelength-coded illumination of the measurement object

In 3 the illumination path is shown in an arrangement according to the invention. A lighting source ( 1 ) is placed directly in front of a pinhole ( 2 ). This pinhole ( 2 ) is in the object-side focal length of a first collecting optics ( 3 ) (System consisting of eg one or more lenses, aspheres, achromats or lenses or the like) arranged. The pinhole ( 2 ) provides for the first collecting optics ( 3 ) is a point light source. Behind the first collecting optics ( 3 ) a plane wave (= parallel beam) is propagated. In focal length of the first collecting optics ( 3 ) plus any small distance s, an optical grating ( 4 ). This is to bend the plane wave in different directions (diffraction orders). It can be designed as a phase or amplitude grating. The optical grating ( 4 ) becomes the leadership of the Illuminating wave and used to modulate the intensity of the illumination light. After this optical grating ( 4 ) is a second collecting optics ( 5 ) is positioned so that the optical grating ( 4 ) is located in the object-side focal length. Furthermore, in the image-side focal plane (= Fourier plane) of the second collecting optics ( 5 ) a first spatial filter ( 6 ) used. This first spatial filter ( 6 ) leaves only those of the optical grating ( 4 ) generated zeroth diffraction order of the illumination light happen. A subsequent third collecting optic ( 7 ) is adjusted so that its object-side focal plane with the plane of the first spatial filter ( 6 ) coincides. The optical grating ( 4 ) generated zeroth diffraction order of the illumination light, which in Achpunkt the image-side focal plane ( 6 ) of the second collecting optics ( 5 ), in turn serves as a point light source, the light from which is collected by the third collecting optics ( 7 ) on an at least partially reflective surface of the measurement object ( 8th ) is collimated. This at least partially reflective surface of the measurement object ( 8th ) is located in the image-side focal plane of the third collecting optics ( 7 ) and is ultimately illuminated with a parallel beam.

In 4 the reflection path is shown in an arrangement according to the invention. The at least partially reflective surface of the measurement object ( 8th ) reflects the plane wave, being curved by propagation time differences (double optical path length difference) depending on the shape of this surface. The reflected light passes through the third collecting optic ( 7 ) and hits the first spatial filter ( 6 ) in the object-side focal plane of the third collecting optics ( 7 ), which at the same time as a Fourier plane with respect. The plane in which the at least partially reflecting surface of the measurement object ( 8th ) is allowed to pass only a limited portion of the angular spectrum of the reflected light. About the second collecting optics ( 5 ), the reflected light is in the plane of the optical grating ( 4 ) Fourier transformed. Consequently, the plane of the at least partially reflecting surface of the measurement object ( 8th ) and the plane of the optical grating ( 4 ) are optically conjugate to each other (linked together by the optical imaging). At the grid ( 4 ) is thus a complex conjugate and low-pass filtered distribution of the surface to be measured by the measured object ( 8th ) reflected light. This is done on the optical grating ( 4 ) bent. The first collecting optic ( 3 ) focused in a second spatial filter ( 9 ) all diffraction orders: While the zeroth diffraction order of the reflected light through the pinhole ( 2 ) is blocked, they pass higher diffraction orders (eg ± 1 diffraction order) the second filter ( 9 ) and by a fourth collecting optics ( 10 ) guided. In the object-side focal plane of the fourth collecting optics ( 10 ) is an image acquisition unit ( 11 ), with which the distribution of the intensity of the incident light is detected. The plane of the image acquisition unit ( 11 ) is optically conjugate to the image-side focal plane of the first collecting optics ( 3 ), at whose distance s the optical grating ( 4 ) is located. In the case of a surface of the object to be measured which is to be ideally planned ( 8th ) is received by the image acquisition unit ( 11 ) the distribution of the distance s from the optical grating ( 4 ) propagated light (eg structure of the amplitude grating). A deformed surface of the object to be measured ( 8th ) causes an adequate deformation of the light wave corresponding to twice the optical path length difference in the plane of the surface to be measured and thus influencing the distribution of the reflected light. This in turn allows the measurement of this surface deformation.
A suitable analytical procedure for determining this wavefront deformation was presented in [ 18 ] presented. This is based on diffraction theory and Fourier analysis with a modified angle spectrum propagator, analyzing the propagation of a wavefront behind a two-dimensional cross-grating. Using a 4f telescope system with a suitable filter in the Fourier space, the intensity distribution behind a two-dimensional cross-grating is imaged onto the plane of a CCD camera. In order to reconstruct the wavefront, the recorded intensity image in the spectral range is analyzed and the phase gradients of the measured wavefront are directly extracted.

5 shows two embodiments for an additional extra-axial parallel illumination of the surface to be measured of the measurement object ( 8th ). The off-axis illumination can be used to extend the surface gradient measurement range using the previously described arrangement. One in the plane of the first spatial filter ( 6 ) laterally offset point light source ( 1 . 2 ) is in the plane of the surface to be measured of the measured object ( 8th ) collimates. The reflected light from there, which undergoes the arrangement backwards as described above, is caused by surface gradients, which in case of exclusively axial illumination by the first spatial filter ( 6 ) were blocked. Consequently, a section of the wavefront can be measured with a higher gradient spectrum. These measurements can be combined with the wavefronts previously determined with axial illumination (eg by means of stitching in the Fourier plane with subsequent inverse Fourier transformation or by composition of the individual wavefronts in the local area), whereby more curved wavefronts are reproducible.
It is also within the scope of the invention that for the detection of gradient spectra of other directions as well as even higher surface gradients further point light sources ( 1 . 2 ) in the plane of the spatial filter ( 6 ). In this way, the angle of incidence of the light in the x and y directions can be adapted to the regions of the surface gradients and orientations to be determined.

In 6 are two further embodiments of an additional extra-axial parallel illumination of the surface to be measured of the measurement object ( 8th ). There are additional point light sources ( 1 . 2 ) are positioned at different distances from the optical axis of the device. Thanks to this flexible positioning, all gradient fields of the at least partially reflecting surface of the test object ( 8th ) are sampled and measured.

An exemplary embodiment of a wavelength-coded illumination of the surface to be measured of the measurement object ( 8th ) is in 7 disclosed. It allows simultaneous recording of the intensity distribution of all wavelengths used. The illumination source ( 1 ) and the additional sources of illumination ( 16 . 17 . 18 . 19 ) in the plane of the first spatial filter ( 6 ) are each characterized by a different illumination spectrum. With the help of the different wavelengths of the illumination light, the surface gradients can be coded, the illumination sources preferably being switched on simultaneously. This shortens the measuring times. The image acquisition unit ( 11 ), however, must be designed so that all wavelengths can be recorded and the images can be processed wavelength-selective (eg with the help of a color camera). This allows the different areas of the gradients in the object to be selected.

Finally, depending on the lateral dimensions of the object to be measured ( 8th ) and of the areas of the detectable surface gradients, the focal lengths of the individual collecting optics ( 3 ), ( 5 ), ( 7 ) and ( 10 ) and adapted to the resolution and size of the image sensor used.

LIST OF REFERENCE NUMBERS

1

- Illumination source with illumination spectrum a

2

- pinhole

3, 5, 7, 10

- collecting optics

4

- optical grating

6, 9

- room filter

8th

- DUT with at least partially reflective surface

11

- Image acquisition unit

13

- Shading

14

- Beam splitter

16

- Illumination source with illumination spectrum b

17

- Illumination source with illumination spectrum c

18

- Illumination source with illumination spectrum d

19

- Illumination source with illumination spectrum e

Bibliography

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Claims (10)

Arrangement for measuring at least partially reflecting surfaces of a measuring object (8) comprising at least one illumination source (1), at least four collecting optics (3, 5, 7, 10), at least two spatial filters (6, 9), at least one optical grating (4 ) and an image pickup unit (11), wherein the illumination source (1), the collecting optics (3, 5, 7, 10), the spatial filters (6, 9), the optical grating (4) and the image pickup unit (11) along a common optical axis are positioned. Arrangement according to Claim 1 , wherein the illumination source (1) radiates coherent or at least partially coherent light and forms a point light source with the pinhole diaphragm (2). Arrangement according to Claim 1 or 2 wherein the fourth collecting optics (10), the second spatial filter (9) and the first collecting optics (3) comprise a first Fourier optics having a 4f structure and the second collecting optic (5), the first spatial filter (6) and the third collecting optics (7) form a second Fourier optics with a 4f structure. Arrangement according to one of the preceding claims, wherein: The point light source (1, 2) is arranged in the plane of the second spatial filter (9) of the first Fourier optics and the recording unit (11) is arranged in the object-side focal plane of the first Fourier optics; • the optical grating (4) is arranged between the first Fourier optics (10, 9, 3) and the second Fourier optics (5, 6, 7) in the object-side focal length of the second collecting optics (5) of the second Fourier optics and from the first collecting optics Optics (3) of the first Fourier optics has a greater distance than its focal length f, and • the measuring object (8) is arranged with the surface to be measured in the image-side focal plane of the third collecting optics (7) of the second Fourier optics. Arrangement according to Claim 4 , wherein with the aid of the optical grating (4) the light of the illumination source (1) can be divided into different diffraction orders and the light reflected by the surface of the measurement object (8) to be measured can be modulated. Arrangement according to one of the preceding claims, wherein the first spatial filter (6) is adapted to let the zeroth diffraction order of the light of the illumination source (1) and the Fourier spectra reflected by the surface of the measurement object (8) to be measured pass. Arrangement according to one of the preceding claims, wherein the second spatial filter (9) is designed to block the zeroth order of diffraction of the Fourier spectra reflected by the surface to be measured of the measuring object (8) and the non - zero diffraction orders of the surface to be measured by the Test object (8) reflected to let Fourier spectra pass. Arrangement according to one of the preceding claims, wherein the arrangement has at least one additional in the plane of the first spatial filter (6) of the second Fourier optics off axis point light source (1, 2). Arrangement according to Claim 8 wherein the illumination sources (1, 16, 17, 18, 19) each provide light of different wavelengths (illumination spectra a, b, c, d, e). Arrangement according to one of the preceding claims, wherein the focal lengths of the used collecting optics (10, 3, 5, 7) to the lateral dimensions of the measurement object (8) to be measured, the detectable surface gradients and / or the characteristics of the image recording unit (11) used can be adapted.

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