SYSTEM AND METHOD FOR CHARACTERIZING THE ROUGHNESS OF A SURFACE OF A TEST SUBJECT

Description

[0001] The present invention relates to a system for characterizing the roughness of a test specimen's surface. Such a system comprises: a radiation source, wherein the radiation source is configured such that, during operation of the system, the radiation source generates and emits coherent first electromagnetic radiation with a first wavelength and coherent second electromagnetic radiation with a second wavelength, and wherein the first wavelength differs from the second wavelength; an illumination optic, wherein the illumination optic is configured and arranged such that, during operation of the system, the illumination optic illuminates an illumination area with the first electromagnetic radiation and illuminates the same illumination area with the second electromagnetic radiation, and that, during operation of the system, the surface of the test specimen can be positioned within the illumination area; and a detection optic.an image-generating detector, wherein the detection optics and the detector are designed and arranged such that, during operation, the detection optics project a first speckle pattern generated on the surface of the test object in the illumination area by the first electromagnetic radiation and a second speckle pattern generated on the surface of the test object in the illumination area by the second electromagnetic radiation onto the detector, and an evaluation device, wherein the evaluation device is connected to the detector such that, during operation of the system, the evaluation device receives a first image signal representing the first speckle pattern and a second image signal representing the second speckle pattern from the detector, and wherein the evaluation device is configured such that, during operation of the system, the evaluation device determines a measure of similarity between the first image signal and the second image signal.where the measure of similarity characterizes the roughness on the surface.

[0002] Furthermore, the present invention relates to a method for characterizing the roughness of a surface of a test specimen. Such a method comprises the steps of: generating and emitting coherent first electromagnetic radiation with a first wavelength and coherent second electromagnetic radiation with a second wavelength, wherein the first wavelength differs from the second wavelength; illuminating exactly one illumination area on the surface of the test specimen with the first electromagnetic radiation and the second electromagnetic radiation; and imaging a first speckle pattern generated on the surface of the test specimen in the illumination area by the first electromagnetic radiation and a second speckle pattern generated on the surface of the test specimen in the illumination area by the second electromagnetic radiation onto an image-generating detector.Generating a first image signal representing the first speckle pattern and a second image signal representing the second speckle pattern, and determining a measure of similarity between the first image signal and the second image signal, wherein the measure of similarity characterizes the roughness of the surface.

[0003] Surface roughness is a characteristic parameter of all surfaces, especially technical functional surfaces. The roughness of a surface influences the function, wear, and appearance of an object with that surface. If the object is a semi-finished product that requires further processing, the surface roughness can provide information about the suitability of the semi-finished product for further processing. Examples include the sliding behavior and friction properties during the forming of sheet metal or the monitoring of the adhesion of coatings and paint systems to the surface.

[0004] A fast, comprehensive, and spatially resolved testing method for characterizing the surface roughness of a test specimen is therefore desirable. Comprehensive in this sense is a testing method that covers the entire relevant surface. Only then can it be guaranteed, through a 100% inspection, that the object possesses the required roughness across its entire surface for further use or processing.

[0005] Several measurement methods for determining surface roughness are known from the prior art. Tactile measurement, white light interferometry, and confocal measurement methods have proven suitable. In tactile measurement, the surface is mechanically scanned with a measuring tip. Tactile measurement methods only record a two-dimensional profile along a line on the surface of the test piece. White light interferometers and confocal measurement systems are expensive and can only be used in laboratory environments and therefore not in production lines.

[0006] However, the aforementioned measurement methods always require an interruption of the respective production process and are unsuitable or only conditionally suitable for inline measurements. Furthermore, these methods require the surface to be scanned serially, which is time-consuming. Parallel measurement of a large number of measuring points on the surface is not possible.

[0007] Optical measurement methods, which are based on the evaluation of a speckle pattern generated on the respective surface using coherent electromagnetic radiation, have proven to be promising for the complete characterization of the roughness of a test specimen's surface.

[0008] However, all previous approaches based on a speckle pattern are either not spatially resolved, i.e., only an average value is provided for a larger surface, they have a comparatively small measurement area, or they also require serial, slow scanning of the entire surface.

[0009] US Patent 2014 / 0049779 A1 discloses a system for determining a property of a material. The material is illuminated with a light beam having controlled spectral and coherence properties. A stack of speckle-field images is recorded from speckle fields reflected by the illuminated material in multiple spectral channels. The stack of speckle-field images comprises several speckle-field images, each recorded in a different spectral channel. Statistical properties of the speckle-field images in the stack are analyzed to determine at least one property of the illuminated material.

[0010] From the publication by Goch et al.: "Requirements for the Application of Speckle Correlation Techniques to On-Line Inspection of Surface Roughness", CIRP ANNALS , ELSEVIER BV, Vol. 48, No. 1, pages 467-470, January 1, 1999 (ISSN: 0007-8506) a system for determining the surface roughness of a test specimen based on a correlation of several speckle patterns generated at different wavelengths is known.

[0011] US patent 2018 / 0067327 A1 discloses an optical imaging system and an associated method that captures images of a distant object in different spectral ranges using only one camera.

[0012] In contrast, the object of the present invention is to provide a system and a method for characterizing the surface roughness of a test specimen which is sufficiently robust to be used in a production environment. Furthermore, it is an object of the present invention to provide a system and a method for characterizing the surface roughness of a test specimen which is sufficiently fast to be used in a production environment.

[0013] At least one of the aforementioned problems is solved by a system according to the attached independent claim 1. For this purpose, the detector of the system has a sensor surface in exactly one sensor plane, wherein the detection optics are configured such that the detection optics map the first speckle pattern onto a first subsurface of the sensor surface and the second speckle pattern onto a second subsurface of the sensor surface, the first and the second subsurface being different from each other. Furthermore, the detection optics comprise a beam-path-dividing element, wherein the beam-path-dividing element is configured and arranged such that, during operation of the system, the beam-path-dividing element splits electromagnetic radiation reflected from the surface of the test specimen in the illumination area into a first beam path to the first subsurface and a second beam path to the second subsurface.According to the invention, at least one path length compensation element is arranged in at least the first beam path or in the second beam path, wherein the path length compensation element is designed such that a first optical path length of the first beam path and a second optical path length of the second beam path (30) between the beam path dividing element and the sensor surface are of the same length.

[0014] The detection optics can image the electromagnetic radiation reflected or scattered from the surface of the test specimen in such a way that the sensor plane lies in the focal plane of the detection optics or, preferably, outside of it. The imaging can take place in the focal plane, but preferably outside of it.

[0015] The system used to characterize roughness is based on the fact that when coherent electromagnetic radiation is reflected or scattered at a rough surface, interference generates a seemingly random speckle pattern of lighter and darker spots. This speckle pattern is characteristic for a given measurement geometry and for the same illumination or measurement area on the surface of the test specimen. However, the observed speckle pattern changes when the wavelength of the electromagnetic radiation changes. This change in the speckle pattern as a function of wavelength is more significant the rougher the surface.

[0016] In other words, if the first speckle pattern recorded at a first wavelength and the second speckle pattern recorded at a different wavelength of a single surface section with identical measurement geometry show a higher similarity when the surface is relatively smooth, i.e., has low roughness, and a lower similarity when the surface has greater roughness, then a measure of the similarity between the first speckle pattern at the first wavelength and the second speckle pattern at the second wavelength characterizes the surface roughness. This also applies to the similarity between a first image signal representing the first speckle pattern and a second image signal representing the second speckle pattern.

[0017] In order for a first speckle pattern and a second speckle pattern, or the first and second image signals representing them, to show any similarity at all, the optical path lengths between the illumination area on the surface of the test object and the sensor area of ​​the detector for the first and the second electromagnetic radiation must be almost identical.

[0018] There is no prior art method suitable for production environments that can fulfill this condition. Therefore, the fundamental idea of ​​the present invention is to detect the first and second speckle patterns generated on the surface of the test specimen within a single illumination area by the first and second electromagnetic radiation using a detector with a sensor area in a single sensor plane, wherein a first sub-area of ​​the sensor area detects the first speckle pattern and a second sub-area of ​​the sensor area detects the second speckle pattern. The first sub-area is distinct from the second sub-area.Despite the stringent requirements for the optical path lengths of the first and second electromagnetic radiation beams from the surface of the test object in the illumination area to the first and second sub-areas of the sensor surface, this design enables the realization of a robust and easy-to-assemble system. Due to its robustness, this system can also be used in demanding production environments. Furthermore, its simple optical design makes the system inexpensive to manufacture.

[0019] In one embodiment of the invention, the radiation source generates and emits the first and second electromagnetic radiation simultaneously. The illumination surface is also illuminated by both the first and second electromagnetic radiation at the same time. In particular, the ability to simultaneously generate the first and second image signals, which represent the first and second speckle patterns at the two different wavelengths, makes the system very fast and allows for a comprehensive measurement of the test object's surface.

[0020] The radiation source is configured to generate and emit the first electromagnetic radiation and the second electromagnetic radiation during operation of the system. In one embodiment, the radiation source comprises a single emitter, for example, a bicolor laser, which generates the first electromagnetic radiation with the first wavelength and the second electromagnetic radiation with the second wavelength. In an alternative embodiment, the radiation source comprises two emitters, for example, two lasers, each of which generates one of the first and second electromagnetic radiations.

[0021] In one embodiment of the invention, an emitter of the radiation source is a diode laser, preferably a diode laser in the red or infrared wavelength range.

[0022] While the invention is described in detail for the first and second wavelengths in a simplified manner, in one embodiment the system has a radiation source for more than two wavelengths. It is understood that in this case, the number of distinct sub-areas of the sensor surface is equal to the number of wavelengths, and each sub-area detects electromagnetic radiation with one wavelength.

[0023] Crucially, the radiation source must generate and emit the first and second electromagnetic radiation with the necessary coherence to produce the first and second speckle patterns upon reflection or scattering of the electromagnetic radiation at the surface of the test specimen. The required minimum coherence length that the first and second electromagnetic radiation must each exhibit to generate a speckle pattern depends on the maximum surface roughness expected for the test specimens to be characterized. In principle, the coherence length must be at least long enough for the reflected electromagnetic radiation to form a speckle pattern. The electromagnetic radiation reflected from two adjacent surface sections must be able to interfere constructively or destructively with each other for a speckle pattern to be formed.

[0024] In one embodiment of the invention, the first and second wavelengths are each element of a first and a second wavelength range, respectively, wherein the first and second wavelength ranges are completely different from each other, i.e., they do not overlap.

[0025] In one embodiment of the invention, the first wavelength and the second wavelength have a wavelength difference in a range of 1 nm to 15 nm.

[0026] In order for the similarity between the first image signal of the first speckle pattern and the second image signal of the second speckle pattern to characterize the roughness of the surface, the first and second electromagnetic radiations must illuminate the same illumination area, whereby in the operation of the system the surface of the test object or a section of the surface of the test object can be arranged in the illumination area, i.e. the same section of the surface of the test object can be illuminated with the first and the second electromagnetic radiations.

[0027] In one embodiment of the invention, the illumination optics are designed and arranged such that, during operation of the system with the first and second electromagnetic radiation, they generate a strip-shaped illumination area. Such an embodiment is particularly advantageous when the test object is a moving, planar object, for example, a ribbon-shaped semi-finished product. If the illumination area extends in a strip-like pattern, similar to line-shaped illumination, across the entire width of the test object, with the line extending essentially perpendicular to the direction of movement of the test object, the entire surface of the ribbon-shaped test object can be completely captured and characterized by the system.

[0028] In one embodiment of the invention, the evaluation device is a computer with a processor and software executed on the processor for evaluating the first and second image signals.

[0029] In one embodiment, the measure of similarity between the first image signal and the second image signal is a correlation between these two first and second image signals.

[0030] The sensor area, and thus each of the first and second sub-areas, comprises a plurality of image points or pixels. A sensor area with a plurality of image points makes it possible to capture the respective speckle pattern as an image with spatial resolution. In one embodiment of the invention, the image points comprise at least one row of image points. In another embodiment of the invention, the image points of the sensor area are arranged in a rectangular matrix.

[0031] In one embodiment of the invention, the sensor area is formed by two image sensors arranged side by side in the sensor plane. Each of the two image sensors then forms one of the first and second sub-areas of the sensor area. In another embodiment of the invention, the first and second sub-areas of the sensor area are formed by two image sensors mounted side by side on the same substrate or circuit board. It is understood that in this case, too, both sub-areas of the sensor area lie in one and the same sensor plane. In this application, the term "image sensor" is used synonymously with a chip for image acquisition (camera chip).

[0032] In one embodiment of the invention, the image sensor is a CCD chip or a CMOS chip.

[0033] In an alternative embodiment of the invention, the first and second sub-areas of the sensor surface are sub-areas of a single image sensor. Because the first and second sub-areas are sub-areas of a single chip, they necessarily lie in a single sensor plane. An embodiment of the invention in which different areas of the same image sensor are used for the first and second sub-areas of the sensor surface has the advantage that no additional assembly effort is required to ensure that the first and second sub-areas of the sensor surface lie in the single sensor plane.

[0034] In one embodiment of the invention, the detection optics comprise at least one spectral filter, wherein the at least one spectral filter is designed and arranged such that, during operation of the system, the first electromagnetic radiation reaches the first sub-surface but not the second sub-surface, and the second electromagnetic radiation reaches the second sub-surface but not the first sub-surface.

[0035] In one embodiment of the system, such a spectral filter is a filter that reflects or transmits only one of the two wavelengths. In another embodiment of the invention, the spectral filter is a transmission filter and allows only one of the two wavelengths to pass through, so that only this wavelength reaches the corresponding sub-area of ​​the sensor surface. In one embodiment of the invention, the detection optics comprise two spectral filters, each of which allows exactly one of the two wavelengths to reach exactly one of the two sub-areas.

[0036] According to the invention, the detection optics comprise a beam path dividing element, wherein the beam path dividing element is designed and arranged such that, during operation of the system, the beam path dividing element splits the electromagnetic radiation from the surface of the test object in the illumination area into a first beam path to the first sub-area and into a second beam path to the second sub-area.

[0037] In one embodiment of the invention, the beam-path-dividing element is a beam splitter that divides the power of the electromagnetic radiation between the first and second beam paths, regardless of its wavelength. A beam splitter within the meaning of the present application is an optical element that transmits one beam of electromagnetic radiation and reflects another. The beam splitting can be in a 50:50 ratio, but other splitting ratios are also possible and may be advantageous.

[0038] In an embodiment in which the beam path dividing element is a wavelength-independent beam splitter, the at least one spectral filter comprises a first spectral filter and a second spectral filter, wherein the first spectral filter is arranged in the first beam path and the second spectral filter is arranged in the second beam path, wherein the first spectral filter is designed and arranged such that, during operation of the system, the first electromagnetic radiation reaches the first sub-area but not the second sub-area, and wherein the second spectral filter is designed and arranged such that the second electromagnetic radiation reaches the second sub-area but not the first sub-area.

[0039] In an alternative embodiment, the beam path dividing element comprises at least one spectral filter. In one embodiment, the beam path dividing element is a dichroic beam splitter that transmits one of the first and second wavelengths and reflects the other wavelength. In an embodiment where the beam path dividing element comprises at least one spectral filter, the arrangement of further spectral filtering in the first and / or the second beam path is not strictly necessary, but can be added to prevent crosstalk between the two beam paths.

[0040] According to the invention, at least one path length compensation element is arranged in at least the first beam path or in the second beam path, wherein the path length compensation element is configured such that a first optical path length of the first beam path and a second optical path length of the second beam path between the beam path dividing element and the sensor surface are of equal length. In this way, the respective optical path lengths for the first and the second electromagnetic radiation between the surface of the test specimen in the illumination area and the sensor surface can be equalized, and path length differences, which result, for example, from the use of a beam splitter, can be compensated.While both electromagnetic radiations share the same beam path between the surface of the test object and the beam path dividing element and therefore have the same optical path length, after the beam path dividing element they travel on spatially separated first and second beam paths, whose optical path lengths must be of equal length.

[0041] In one embodiment of the invention, the path length compensation element is a glass block.

[0042] In one embodiment, the beam-path dividing element comprises a first beam-deflecting element and a second beam-deflecting element, wherein the first and the second beam-deflecting elements are arranged such that a Fourier plane of the detection optics intersects the first and the second beam-deflecting elements. The first beam-deflecting element is configured and arranged such that, during system operation, it deflects the electromagnetic radiation reflected from the surface of the test specimen in the illumination area onto the first beam path to the first sub-area, and the second beam-deflecting element is configured and arranged such that, during system operation, it deflects the electromagnetic radiation reflected from the surface of the test specimen in the illumination area onto the second beam path to the second sub-area.

[0043] In this embodiment, the beam splitting onto the first and second beam paths addressing the first and second sections of the sensor area is based on beam deflection in the region of the Fourier plane of the detection optics. Crucially, for such a beam splitting to function, each first and second beam-deflectoring element must contain the complete information of both the first and second speckle patterns.

[0044] It is understood that even with such beam splitting in Fourier space, spectral filtering of the first and second beam paths must occur during or after the splitting so that the first and second sub-areas each capture only one of the first and second speckle patterns. In one embodiment of the invention, a spectral filter can be provided directly on or at the beam-deflectoring element. In an alternative embodiment, the spectral filter is arranged in the first and / or second beam path, as previously described for the beam splitter.

[0045] In one embodiment of the invention, the beam-deflectoring element is a reflective element or a diffractive element.

[0046] In one embodiment of the invention, the first and second beam-deflectoring elements are rotationally symmetric with respect to the optical axis in at least one section plane perpendicular to an optical axis of the first and second electromagnetic radiation imaged by the detection optics in a beam direction in front of the beam-path-dividing element. In another embodiment, the first and second beam-deflectoring elements are rotationally symmetric with respect to the optical axis when viewed in all section planes perpendicular to the optical axis. In one embodiment, the rotational symmetry is twofold, i.e., a rotation of the first beam-deflectoring element maps it onto the second beam-deflectoring element.

[0047] In one embodiment of the invention, the first and the second beam-deflectoring element are point-symmetric about a point on the optical axis in at least one section plane perpendicular to an optical axis.

[0048] In such an arrangement, for reasons of symmetry, both the first electromagnetic radiation and the second electromagnetic radiation contain the complete information of the first and second speckle patterns, respectively, on both the first and second beam-deflectoring elements.

[0049] According to one embodiment of the present invention, the sensor surface is arranged in front of the sensor surface at an angle other than 90° to a beam axis of at least the first or the second beam path.

[0050] By tilting the sensor surface relative to at least one of the beam axes of the first or second beam path, there is always, with a corresponding arrangement of the sensor surface, an area in the first subsection and an area in the second subsection of the sensor surface for which the optical path lengths of the first beam path and the second beam path are identical.

[0051] In one embodiment of the invention, the first beam path and the second beam path are essentially parallel to each other in front of the sensor surface, preferably parallel to each other.

[0052] In one embodiment of the invention, an asymmetric aperture is arranged in a beam path of the first electromagnetic radiation and the second electromagnetic radiation reflected or scattered from the surface of the test specimen. In this way, with a moving test specimen and appropriate orientation of the aperture, the mean speckle size is larger in the direction of movement than perpendicular to it, in order to reduce the effects of motion blur.

[0053] In one embodiment of the invention, the sensor plane is arranged outside the focus of the detection optics. In this way, the influences of the surface can be reduced and the accuracy of the measurement increased.

[0054] At least one of the aforementioned tasks is also solved by a device, wherein the device comprises a system as previously described in embodiments thereof, as well as a motion device. The motion device is designed and arranged such that, during operation of the device, it moves the surface of the test specimen relative to the illumination surface.

[0055] In one embodiment of the invention, such relative movement between the surface of the test specimen and the illumination surface is linear. In another embodiment of the invention, the relative movement between the surface and the illumination surface is continuous.

[0056] At least one of the aforementioned problems is also solved by a method for characterizing the roughness of a surface of a test specimen according to the independent claim directed thereto. For this purpose, the detector used in the method of the type mentioned at the outset has a sensor surface in exactly one sensor plane, and the first speckle pattern is mapped onto a first subsurface of the sensor surface and the second speckle pattern is mapped onto a second subsurface of the sensor surface, wherein the first and the second subsurface are different from each other.

[0057] Insofar as aspects of the invention relating to the method are described herein, these also apply to the corresponding system for characterizing the surface roughness of the test specimen and vice versa. Insofar as the method is carried out with a system according to this invention, the system shall include the necessary components for this purpose.

[0058] In particular, embodiments of the system are suitable for carrying out the method and its embodiments described herein.

[0059] In one embodiment of the method according to the invention, the surface of the test specimen is moved at least continuously or linearly relative to the illumination surface.

[0060] Further advantages, features, and applications of the present invention will become clear with reference to the following description of embodiments and the accompanying figures. In the figures, identical elements are designated by the same reference numerals. Figure 1 is a schematic representation of a system for characterizing the surface roughness of a test specimen. Figure 2 is a more detailed representation of the detector. Figure 1 according to a first variant. Figure 3 is a more detailed representation of the detector from Figure 1according to a second variant. Figure 4 is a more detailed representation of a first embodiment of the detector and the detection optics of the system. Figure 1 Figure 5 is a more detailed representation of an unclaimed example of the detector and the detection optics of the system from Figure 1 .

[0061] Figure 1 Figure 1 schematically shows the setup of a system 1 for characterizing the roughness of a surface 2 of a test specimen 3. System 1 is based on a representation of a speckle pattern of lighter and darker spots generated by interference during the reflection or scattering of coherent electromagnetic radiation at the rough surface 2. The observed speckle pattern has an appearance that depends on the wavelength of the electromagnetic radiation.

[0062] Therefore, system 1 has a radiation source 4 with two diode lasers 5 and 6 as emitters. The first diode laser 5 emits a first electromagnetic radiation 7 with a first wavelength of 633 nm, and the second diode laser 5 emits a second electromagnetic radiation 8 with a second wavelength of 638 nm. The first and second electromagnetic radiation are generated and emitted simultaneously. The first and second wavelengths each denote the center wavelength of the emitted radiation within a first and second wavelength band, respectively. The two wavelength bands are completely distinct and do not overlap.

[0063] The first and second electromagnetic radiations are spatially superimposed by a fiber coupler 9. A lens 10 collimates the two superimposed radiations 7, 8, and a beam splitter 11 reflects them onto the surface 2 of the test object 3. On the surface 2, they illuminate a strip-shaped illumination area 12, i.e., a measurement field with 100 x 100 mm². In the illustrated embodiment, the test object 3 is moved linearly at a constant speed of up to 0.6 m / s. The illumination area 10 extends in a line perpendicular to the direction of movement 13 of the test object 3. The lens 10 and the beam splitter 11 are collectively referred to as the illumination optics 14.

[0064] The speckle patterns generated by the first and second electromagnetic radiations 7, 8 are visible in transmission through the beam splitter 11 and an aperture 15. The aperture 15 serves to adjust the properties of the speckle fields in order to optimize the measurement.

[0065] A detection optic 16 projects the two speckle patterns onto a detector 17. The detection optic 16 comprises a lens 18 with a focal length of 50 mm and other components. The task of the detection optic 16 is to project the speckle patterns spatially separately onto a first and a second sub-area 19, 20 of a sensor surface 21.

[0066] Crucial for realizing the advantages associated with the invention is that the two sub-areas 19, 20 of the sensor surface 21 lie in a single sensor plane 22. In this way, the system 1 is easy to adjust and robust against environmental influences such as those occurring in production environments.

[0067] The Figures 2 and 3 Figure 17 shows two variants of a detector 17 with sub-areas 19, 20 of the sensor area 21 arranged in the same sensor plane 22. In the variant from Figure 2 A CMOS chip 23 is mounted on a circuit board 24 as the sole image sensor of the detector 17. The two sub-areas 19, 20 of the sensor area 21 are formed by two sub-areas of this CMOS chip 23. In this embodiment, the sub-areas 19, 20 inherently lie in the same sensor plane 22. Thus, the two speckle patterns of the first and the second electromagnetic radiation 7, 8 are detected by the same image sensor.

[0068] The variant from Figure 3 In contrast, a detector 17 is used in which two separate CMOS chips 25, 26 are mounted side by side on a circuit board 24. In this variant, the two CMOS chips 25, 26 together form the two sub-areas 19, 20 of the sensor area 21. These also lie in a single sensor plane 22.

[0069] Figure 4 shows one embodiment and Figure 5 An example of the further components of the detection optics 18, not claimed as such. The figures illustrate how the first and second speckle patterns imaged by the lens 18 are spatially separated from each other and directed onto the adjacent sub-areas 19, 20 of the sensor area 21.

[0070] In both embodiments, the detection optics 16 comprises a beam-path-dividing element 27, which divides the electromagnetic radiation 28 reflected from the surface 2 of the test specimen 3 in the illumination area 12 into a first beam path 29 to the first sub-area 19 and into a second beam path 30 to the second sub-area 20. Furthermore, in both embodiments, the detection optics 16 includes a spectral filter, which causes the first electromagnetic radiation 7 to reach only the first sub-area 19 and not the second sub-area 20, and the second electromagnetic radiation 8 to reach the second sub-area 20 and not the first sub-area 19.

[0071] In the embodiment from Figure 4The beam path dividing element and the spectral filter are formed by a dichroic beam splitter 27. This transmits the first wavelength of the first electromagnetic radiation 7 to the first sub-surface 19 and reflects the second wavelength of the second electromagnetic radiation 8. A mirror 31 is provided for beam deflection so that the second electromagnetic radiation reaches the second sub-surface 20.

[0072] In order to enable comparability between the first speckle pattern at the first wavelength and the second speckle pattern or the first and second image signals representing them, the optical path lengths between the illumination area 12 on the surface 2 of the test object 3 and the sensor area 21 of the detector 17 must be identical for the first and the second electromagnetic radiation 7, 8.

[0073] In the embodiment, the optical path lengths of the first and second beam paths 29, 30 are aligned to allow for comparison. Figure 4 In the first beam path 29, a path length compensation element in the form of a glass block 32 is provided.

[0074] In the example from Figure 5The beam-path dividing element 27 is formed by two beam-deflectoring elements in the form of a first and a second reflective surface 33, 34. The first and the second reflective surfaces 33, 34 are arranged such that the Fourier plane 35 of the lens 18 intersects the first and the second reflective surfaces 33, 34. The first and second reflective surfaces 33, 34, arranged around the Fourier plane 35, are rotationally symmetric about the optical axis 36 in each intersection plane perpendicular to an optical axis 36 of the first and second electromagnetic radiation 7, 8 imaged by the lens 18 in a beam direction in front of the beam-path dividing element 27, for example, the Fourier plane 35, by 180°. This ensures that the complete information of both the first and the second speckle pattern is present on the first and the second reflective surfaces 33, 34.

[0075] In this beam splitting in Fourier space, the first and second beam paths 7, 8 are spectrally filtered so that the first and second sub-areas 19, 20 each capture only one of the first and second speckle patterns. In the embodiment from Figure 5 is each comprising a beam deflection in the first or second beam path 29, 30 and a spectral filter in the form of a wavelength-matched dielectric mirror 37, 38.

[0076] The first speckle pattern generated at the first wavelength and the second speckle pattern recorded at the second wavelength of the same surface section 2 of the test specimen 3 exhibit a similarity whose degree characterizes the roughness of the surface 2. Therefore, in the illustrated embodiments, an evaluation device in the form of a computer 39 establishes a correlation between a first image signal, representing the first speckle pattern captured by the first sub-surface 19 at the first wavelength, and a second image signal, representing the second speckle pattern captured by the second sub-surface 20 at the second wavelength. This correlation, in turn, provides a measure of the surface roughness. A comparatively smooth surface 2 with low roughness results in a greater similarity between the two speckle patterns, and a rougher surface 2 results in a lesser similarity.The described system 1 enables a roughness measurement range from 0.8 µm Sa to 3.5 µm Sa.

[0077] For the purposes of the original disclosure, it is pointed out that all features as they can be deduced by a person skilled in the art from the present description, the drawings, and the claims, even if they are specifically described only in connection with certain other features, can be combined individually or in any combination with other features or groups of features disclosed herein, unless this has been expressly excluded or technical circumstances render such combinations impossible or pointless. A comprehensive, explicit description of all conceivable combinations of features is omitted here solely for the sake of brevity and readability.

[0078] While the invention has been illustrated and described in detail in the drawings and the preceding description, this illustration and description are merely exemplary and are not intended to limit the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

[0079] Variations of the disclosed embodiments are obvious to a person skilled in the art from the drawings, the description, and the accompanying claims. In the claims, the word "have" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude multiple features. The mere fact that certain features are claimed in different claims does not preclude their combination. Reference numerals in the claims are not intended to limit the scope of protection. Reference symbol list

[0080] 1 System 2 Surface 3 Test object 4 Radiation source 5, 6 Diode laser 7 First electromagnetic radiation 8 Second electromagnetic radiation 9 Fiber coupler 10 Lens 11 Beam splitter 12 Illumination area 13 Direction of movement 14 Illumination optics 15 Aperture 16 Detection optics 17 Detector 18 Objective 19 First sub-area 20 Second sub-area 21 Sensor area 22 Sensor plane 23 CMOS chip 24 Circuit board 25, 26 CMOS chip 27 Beam path dividing element 28 Electromagnetic radiation 29 First beam path 30 Second beam path 31 Mirror 32 Glass block 33 First reflective surface 34 Second reflective surface 35 Fourier plane 36 Optical axis 37, 38 Dielectric mirror 39 Computer

Claims

1. A system (1) for characterizing a roughness of a surface (2) of a test object (3), the system (1) comprising a radiation source (4), wherein the radiation source (4) is configured such that the radiation source (4), in an operation of the system (1), generates and emits a coherent first electromagnetic radiation (7) having a first wavelength and a coherent second electromagnetic radiation (8) having a second wavelength, and wherein the first wavelength is different from the second wavelength, an illumination optical unit (14), wherein the illumination optical unit (14) is configured and arranged such that the illumination optical unit (14), in the operation of the system (1), illuminates an illumination area (12) with the first electromagnetic radiation (7) and illuminates the same illumination area (12) with the second electromagnetic radiation (8), and such that, in the operation of the system (1), the surface (2) of the test object (3) is arrangeable in the illumination area (12), a detection optical unit (16), an imaging detector (17), wherein the detection optical unit (16) and the detector (17) are configured and arranged such that the detection optical unit (16), in the operation, images a first speckle pattern generated at the surface (2) of the test object (3) in the illumination area (12) by the first electromagnetic radiation (7) and a second speckle pattern generated at the surface (2) of the test object (3) in the illumination area (12) by the second electromagnetic radiation (8) onto the detector (17), and an evaluation device (39), wherein the evaluation device (39) is connected to the detector (17) such that the evaluation device (39), in the operation of the system (1), obtains from the detector (17) a first image signal representing the first speckle pattern and a second image signal representing the second speckle pattern, and wherein the evaluation device (39) is set up such that the evaluation device (39), in the operation of the system (1), determines a measure of a similarity between the first image signal and the second image signal, wherein the measure of the similarity characterizes the roughness of the surface (2), characterized in that the detector (17) has a sensor surface (21) in exactly one sensor plane (22), the detection optical unit (16) is configured such that the detection optical unit (16) images the first speckle pattern onto a first sub-area (19) of the sensor surface (21) and the second speckle pattern onto a second sub-area (20) of the sensor surface (21), wherein the first and the second sub-area (20) are different from one another, the detection optical unit (16) comprises a beam-path-splitting element (27), wherein the beam-path-splitting element (27) is configured and arranged such that the beam-path-splitting element (27), in the operation of the system (1), splits electromagnetic radiation (28) reflected by the surface (2) of the test object (3) in the illumination area (12) onto a first beam path (29) to the first sub-area (19) and onto a second beam path (30) to the second sub-area (20), and at least in the first beam path (29) or in the second beam path (30), at least one path-length compensation element (32) is arranged, wherein the path-length compensation element (32) is configured such that a first optical path length of the first beam path (29) and a second optical path length of the second beam path (30) between the beam-path-splitting element (27) and the sensor surface (21) are equal in length.

2. The system (1) according to the preceding claim, wherein the sensor surface (21) is part of exactly one image sensor (23).

3. The system (1) according to any one of the preceding claims, wherein the detection optical unit (16) comprises at least one spectral filter, wherein the at least one spectral filter (27) is configured and arranged such that, in the operation of the system (1), the first electromagnetic radiation (7) reaches the first sub-area, but not the second sub-area (20), and the second electromagnetic radiation (8) reaches the second sub-area, but not the first sub-area (19).

4. The system (1) according to claim 3, wherein the beam-path-splitting element (27) comprises the at least one spectral filter.

5. The system (1) according to claim 3 or 4, wherein the at least one spectral filter comprises a first spectral filter (37) and a second spectral filter (38), wherein the first spectral filter (37) is arranged in the first beam path (29) and the second spectral filter (38) is arranged in the second beam path (30), wherein the first spectral filter (37) is configured and arranged such that, in the operation of the system (1), the first electromagnetic radiation (7) reaches the first sub-area, but not the second sub-area (20), and wherein the second spectral filter (38) is configured and arranged such that the second electromagnetic radiation (8) reaches the second sub-area, but not the first sub-area (19).

6. The system (1) according to any one of the preceding claims, wherein the beam-path-splitting element (27) is a beam splitter.

7. The system (1) according to claim 6, wherein the beam splitter is a dichroic beam splitter.

8. The system (1) according to any one of claims 1 to 5, wherein the beam-path-splitting element (27) comprises a first beam-deflecting element (33) and a second beam-deflecting element (34), wherein the first and the second beam-deflecting element (33, 34) are arranged such that a Fourier plane (35) of the detection optical unit (16) intersects the first and the second beam-deflecting element (33, 34), wherein the first beam-deflecting element (33) is configured and arranged such that the first beam-deflecting element (33), in the operation of the system (1), deflects the electromagnetic radiation (28) reflected by the surface (2) of the test object (3) in the illumination area (12) onto the first beam path (29) to the first sub-area (19), and wherein the second beam-deflecting element (34) is configured and arranged such that the second beam-deflecting element (34), in the operation of the system (1), deflects the electromagnetic radiation (28) reflected by the surface (2) of the test object (3) in the illumination area (12) onto the second beam path (30) to the second sub-area (20).

9. The system (1) according to the preceding claim, wherein the first and the second beam-deflecting element (33, 34), in at least one sectional plane perpendicular to an optical axis (36) of the first and second electromagnetic radiation (7, 8) imaged by the detection optical unit (16), are rotationally symmetric with respect to the optical axis (36) in a beam direction before the beam-path-splitting element (27).

10. The system (1) according to any one of the preceding claims, wherein the sensor surface (21) is arranged at an angle different from 90 degrees with respect to a beam axis of at least the first or the second beam path before the sensor surface (21).

11. Device comprising a system (1) according to any one of the preceding claims and a motion device, wherein the motion device is configured and arranged such that the motion device, in an operation of the device, moves the surface (2) of the test object (3) relative to the illumination area (12).

12. A method for characterizing a roughness of a surface (2) of a test object (3), the method comprising the steps of generating and emitting a coherent first electromagnetic radiation (7) having a first wavelength and a coherent second electromagnetic radiation (8) having a second wavelength, wherein the first wavelength is different from the second wavelength, illuminating exactly one illumination area (12) on the surface (2) of the test object (3) with the first electromagnetic radiation (7) and the second electromagnetic radiation, imaging a first speckle pattern generated at the surface (2) of the test object (3) in the illumination area (12) by the first electromagnetic radiation (7) and a second speckle pattern generated at the surface (2) of the test object (3) in the illumination area (12) by the second electromagnetic radiation (8) onto an imaging detector, generating a first image signal representing the first speckle pattern and a second image signal representing the second speckle pattern, and determining a measure of a similarity between the first image signal and the second image signal, wherein the measure of the similarity characterizes the roughness of the surface (2), characterized in that the detector (17) has a sensor surface (21) in exactly one sensor plane (22), the first speckle pattern is imaged onto a first sub-area (19) of the sensor surface (21) and the second speckle pattern is imaged onto a second sub-area (20) of the sensor surface (21), wherein the first and the second sub-area (20) are different from one another, the detection optical unit (16) comprises a beam-path-splitting element (27), wherein the beam-path-splitting element (27) is configured and arranged such that the beam-path-splitting element (27), in the operation of the system (1), splits electromagnetic radiation (28) reflected by the surface (2) of the test object (3) in the illumination area (12) onto a first beam path (29) to the first sub-area (19) and onto a second beam path (30) to the second sub-area (20), and at least in the first beam path (29) or in the second beam path (30), at least one path-length compensation element (32) is arranged, wherein the path-length compensation element (32) is configured such that a first optical path length of the first beam path (29) and a second optical path length of the second beam path (30) between the beam-path-splitting element (27) and the sensor surface (21) are equal in length.

13. The method according to claim 12, wherein the surface (2) of the test object is moved, preferably at least continuously or linearly, relative to the illumination area (12).