METHOD FOR OPTICAL MEASUREMENT OF TECHNICAL SURFACES AND DEVICE FOR PERFORMING THE METHOD

Description

[0001] The invention relates to a method for the optical measurement of technical surfaces using a confocal sensor, wherein light from a light source (11) is directed via an optical system onto a sample surface to be measured, wherein the optical system comprises an illumination mask (13), a sensor matrix (15), a beam splitter (14) for combining the illumination beam path and the detection beam path, and an imaging optic (6), wherein the illumination mask (13) consists of transparent areas (1) and non-transparent or slightly transparent areas (2) arranged in a checkerboard pattern, wherein the pitch (3) of the pattern on the illumination mask (13) corresponds to the pixel pitch (24) of the sensor matrix (15), and wherein the illumination mask (13) and the sensor matrix (15) are adjusted to each other such that the transparent areas (1) and the pixels of the sensor matrix (15) are simultaneously sharply imaged onto the sample (7) by means of the imaging optic (6).which then sharply projects the image of the illumination mask (13) onto the sensor matrix (15), creating a checkerboard-like pattern of brightly and darkly illuminated pixels.

[0002] The invention also relates to a device for carrying out the method. background

[0003] The conventional method for achieving area-wide confocal imaging involves combining illumination and detection using a beam splitter, followed by the insertion of the confocal filter. This filter can be, for example, a rotating multi-pinhole disk (Nipkow disk), a fixed pinhole pattern with pinholes, a microlens array, or a combination of a microlens array and a pinhole pattern, or a rotating multi-pinhole disk. While microlenses can selectively increase the transparency of the confocal filter, the manufacturing process for this solution is technologically demanding, and the microlenses can negatively impact the overall optical imaging quality of the system.

[0004] Confocal filters without microlenses typically have a transparency of less than 5%, which necessitates the use of very powerful light sources and often results in disruptive light reflections within the device in front of the confocal filter. The system's speed is limited not only by the camera's frame rate but also by the required illumination intensity for sufficient camera signal drive and the maximum rotational speed of a synchronously rotating multipin hole disk. State of the art

[0005] Methods according to the preamble of claim 1 and corresponding devices are known from the following publications: WO 2014 / 125 037 A1 WO 2012 / 083 967 A1 WO 2010 / 145 669 A1 M. Noguchi M., SK Nayar SK, "Microscopic shape from focus using a projected illumination pattern", Mathematical and Computer Modeling, Volume 24, Issues 5-6 (1996), Pages 31-48,ISSN 0895-7177, https: / / doi.org / 10.1016 / 0895-7177(96)00114-8 US 6 229 913 B1 DE 10 2015 209 410 A1.

[0006] In this state of the art, the transparent areas of the lighting mask are angular.

[0007] If a pinhole pattern with rectangular transparent areas is used as a confocal filter, Fig. 2 When used as an illumination mask, direction-dependent diffraction effects occur in the illumination. These lead to local asymmetries in light intensity when focusing through the surface during image acquisition of textured surfaces. This, in turn, promotes the formation of spikes and increased roughness of the measured 3D surface, depending on the orientation. This can be experimentally verified by performing multiple measurements of the same surface area at different rotation angles around the optical axis and comparing the results. Problem and solution of the invention

[0008] The invention is based on the objective of eliminating the aforementioned direction-dependent diffraction effects.

[0009] This problem is solved in the method of the type mentioned at the outset according to the invention by the fact that the transparent areas (1) of the lighting mask (13) are round. Advantages of the invention

[0010] Among other things, the following advantages are achieved: To eliminate direction-dependent artifacts, pinhole patterns with round transparent areas are used. Fig. 3 This approach eliminates the directional dependence of the diffraction effects at the individual pinhole, resulting in the desired symmetrical, round Airy patterns on the sample.

[0011] This invention reduces the implementation of confocal area detection to the essential elements, significantly increases the optical transparency of the overall system, and eliminates the previous limitation of the practically achievable measurement speed.

[0012] Advantageous embodiments of the invention are set out in the dependent claims.

[0013] It is proposed that there is no imaging optics between the illumination mask (13) and the beam splitter (14) or between the camera sensor and the beam splitter (14).

[0014] It is further proposed that the imaging optics (6) focus through the sample (7) during the acquisition of an image stack of confocal images, with the position of the respective focus position being taken into account in the determination of the z-positions of the intensity maxima.

[0015] To acquire a 3D image, a stack of images (typically containing 20 to 1000 images) is acquired, depending on the desired resolution. This is done while the focus is continuously moved through the sample in the Z-direction, or conversely, the sample is moved through the focus. The subsequent acquisition of the image stack is performed either in the reverse scan direction or in the same direction, with the focus being returned to its original position as quickly as possible beforehand. The intensity profile of each pixel is then evaluated from the acquired image stacks.

[0016] It is further proposed that the camera sensor is a monochromatic sensor, wherein the intensity values ​​of the "dark" pixels corresponding to the non-transparent or slightly transparent areas (2) of the illumination mask (13) are first inverted and then the Z-position of the intensity maxima is determined, or the height values ​​for the slightly transparent areas of the illumination mask (13) are interpolated from the height values ​​of the neighboring pixels.

[0017] When a monochromatic sensor matrix (black and white camera) is used, adjacent pixels in the captured image stack each generate an intensity signal with an intensity maximum or minimum at the point of focus. In both cases, their vertical position z0 can be determined algorithmically, for example, using the center-of-mass algorithm. In the case of pixels with intensity minimum at the point of focus, the intensity values ​​are first inverted, and the vertical position z0 is determined analogously to the pixels with intensity maximum.

[0018] The additional analysis of pixels with intensity minimums has the advantage that data normally discarded is also used for calculating the 3D result. This means twice as much raw data is included in the calculation of the overall 3D result compared to the usual analysis of only pixels with intensity maximums. With appropriate post-processing, this can reduce the noise figure to 70.7% of the original value (1 / √N) with identical hardware. Since both partial images, each derived from intensity maxima / minima, are based on fundamentally different underlying information due to their different origins, a suitable combination of both results from adjacent pixels can yield more valid measurement data, particularly in sample areas with low reflectivity. This improves the quality and data density of the 3D results.

[0019] Alternatively, the height positions z0 of the pixels with minimum intensity can also be determined by interpolating the height values ​​of neighboring pixels. The determined height position is output for each individual pixel.

[0020] This can almost double the speed of data analysis.

[0021] Fig. 4a shows the intensity signal of a pixel for pixels, on which the highly transparent areas of the illumination mask are applied, and Fig. 4b the intensity signal of a pixel, onto which the low-transparency areas of the illumination mask are mapped.

[0022] It is further proposed that the camera sensor is a color sensor with a Bayer pattern, wherein the "bright" pixels corresponding to the transparent areas (1) of the illumination mask (13) are the green pixels (21) and the z-position of the intensity maximum is determined for these pixels, and that the height values ​​for the red pixels (23) and blue pixels (22) are interpolated from the height values ​​of the neighboring green pixels (21).

[0023] If a white light source, e.g. a white LED as a light source, and a color sensor matrix are now included in the Fig. 5Using the Bayer pattern shown, the device is adjusted so that the transparent areas of the illumination mask correspond to the green pixels of the sensor matrix. Thus, only the green pixels are illuminated at focus, while the other-colored pixels are "dark." The height position of the intensity maxima is then determined for the green pixels and interpolated for the pixels of other colors in between.

[0024] A known problem in the 3D evaluation of neighboring pixels with different colors is a vertical shift in the determined height position depending on the wavelength. This leads to a checkerboard-like pattern in the 3D result when all pixels are displayed.

[0025] The exclusive use of the "green" pixels for 3D evaluation has the advantage that neither wavelength-dependent interactions between light and sample surface nor chromatic aberrations of the imaging optics lead to visible artifacts in the 3D result.

[0026] It is further proposed that when generating the colored intensity image, the color information for the red pixels (23) and blue pixels (22) is determined from the intensity values ​​just outside the focus.

[0027] The focus for the red and blue pixels is on the intensity due to the confocal effect. Fig. 4b reduced. To still achieve an accurate color determination, the intensity is measured just outside the focus, where the blur is only a few pixels and the intensity signal is not yet reduced by the confocal effect.

[0028] The intensity value in the respective focus is used for the green pixels.

[0029] It is further proposed that the calculation of the Z-position of the intensity maxima begins during the measurement data acquisition, with the calculation of the Z-position of the intensity maxima being carried out using parallelized algorithms.

[0030] Confocal processing is performed during the acquisition of the image stack. Each image is transferred to the graphics card and processed there in parallel. This means that the resulting 3D image is already available, displayable, and storable while the next image stack is being acquired, so the latency for obtaining the result after the image stack has been acquired is typically less than the time required to acquire it.

[0031] Similarly, the resources used for data evaluation are free again after the 3D image has been calculated, so that the next 3D calculation can be started immediately afterwards and thus the image acquisition does not have to be interrupted.

[0032] Thus, when using a fast camera system with, for example, a frame rate of 800 Hz and capturing 40 images per image stack, a 3D frame rate of 20 Hz can be achieved. This allows 3D images to be continuously displayed and stored at video frequency without any waiting time between capturing two 3D images. If the sample surface being measured is moved continuously at a constant speed during data acquisition, a defined distortion of the 3D image is produced, which can be corrected by velocity-related 3D calibration in a post-processing algorithm. Example of implementation

[0033] An embodiment of the invention is described in more detail below with reference to the drawings. In all drawings, identical reference numerals have the same meaning and are therefore explained only once, if necessary.

[0034] They show Figure 1 shows the basic confocal beam path, Figure 2 an illumination mask with checkerboard-like arranged rectangular transparent areas (comparative example), Figure 3 an illumination mask with checkerboard-like arranged round transparent areas, according to the invention, Figure 4 the intensity when focusing through the sample and Figure 5 the Bayer pattern of a color sensor matrix with Bayer pattern.

[0035] Fig. 1The diagram illustrates the basic confocal beam path: A light source (11) illuminates an illumination mask (13) via a collimator (12). This mask is imaged through the beam splitter (14) onto a sample (7) to be measured by means of an imaging optic (6). The sample is imaged through the beam splitter onto a sensor matrix (15) by means of the imaging optic. When the sample is in focus, the pattern is sharply imaged onto the sample, and this image is sharply imaged onto the sensor matrix.

[0036] Fig. 2 shows an illumination mask with rectangular transparent areas arranged in a checkerboard pattern: The transparent areas (1) with edge length (4) are arranged between the less transparent areas (2) with edge length (5) as in a checkerboard pattern with a pitch (3) corresponding to the pixel pitch (24) of the sensor matrix.

[0037] Fig. 3shows an illumination mask with round transparent areas arranged in a checkerboard pattern: The transparent areas (1) with diameter (25) are arranged between the less transparent areas (2) as in a checkerboard pattern with a pitch (3) corresponding to the pixel pitch (24) of the sensor matrix.

[0038] Fig. 4 This shows the intensity when focusing through the sample: a) For the "bright" pixels, the height scan shows an increase in intensity in the area of ​​focus (z0), and b) for the other pixels, the intensity decreases in focus (z0). The Z-position z0 for each extreme value is determined.

[0039] Fig. 5 The Bayer pattern of a color sensor matrix with Bayer pattern is shown: Between the checkerboard diagonally arranged green pixels (21) with a pixel pitch (24) which corresponds to the pitch (3) of the illumination mask, lie the blue pixels (22) and the red pixels (23). Working method of the inventive method for the optical measurement of technical surfaces

[0040] The following section explains in detail the working method of the inventive method for the optical measurement of technical surfaces: Fig. 1 Figure 1 shows the basic structure of the invention. A light source (1) is collimated by means of a collimator and illuminates a permanently installed illumination mask (13) serving as a confocal filter with a pinhole pattern (see Figure 1). Fig. 2 and Fig. 3The pitch of the pinhole pattern corresponds to the pixel pitch of the sensor matrix (15). Using a beam splitter (14), the illumination light is reflected towards the sample and focused onto the surface of the sample (7) by means of imaging optics (6). When the sample is in focus, the illumination mask (13) is sharply imaged onto the sample. In the detection beam path, the surface of the sample (7) is imaged onto the sensor matrix (15) by means of imaging optics (6) through the beam splitter (14). When the sample is in focus, the pinhole pattern of the illumination mask, which is sharply imaged onto the sample, is sharply imaged onto the sensor matrix. If the sample surface is outside the focus area, the pinhole pattern becomes so blurred that it is no longer visible there.

[0041] This setup can also be implemented by exchanging the lighting optics (11) to (13) and the sensor matrix (15), so that the lighting occurs in transmission through the beam splitter, while the sensor matrix is ​​arranged in reflection. Reference symbol list

[0042] 1 Transparent area 2 Low transparency area 3 Pitch of illumination mask 4 Edge length 5 Edge length 6 Imaging optics 7 Sample 11 Light source 12 Collimator 13 Illumination mask 14 Beam splitter 15 Sensor matrix 21 Green pixels 22 Blue pixels 23 Red pixels 24 Pixel pitch of sensor matrix 25 Diameter

Claims

1. Method for optically measuring technical surfaces using a confocal sensor, light from a light source (11) being directed via an optical system onto a sample surface to be measured, the optical system containing an illumination mask (13), a sensor matrix (15), a beam splitter (14), for combining an illumination beam path and a detection beam path, and imaging optics (6), the illumination mask (13) consisting of transparent regions (1) and non-transparent or slightly transparent regions (2) arranged in a checkerboard pattern, the pitch (3) of the pattern on the illumination mask (13) corresponding to the pixel pitch (24) of the sensor matrix (15) and the illumination mask (13) and the sensor matrix (15) being adjusted relative to one another such that the transparent regions (1) and the pixels of the sensor matrix (15) are simultaneously sharply imaged onto the sample (7) by means of the imaging optics (6), whereby the sharp image of the illumination mask (13) is then sharply imaged onto the sensor matrix (15) so that a checkerboard pattern of light and dark illuminated pixels is produced on the sensor matrix, characterized in that the transparent regions (1) of the illumination mask (13) are round.

2. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that there are no imaging optics either between the illumination mask (13) and the beam splitter (14) or between the camera sensor and the beam splitter (14).

3. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that the imaging optics (6) focus through the sample (7) during the acquisition of an image stack of confocal images, the position of the relevant focus position being included in the determination of the z-positions of the intensity maxima.

4. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that the camera sensor is a monochromatic sensor, the intensity values of the "dark" pixels corresponding to the non-transparent or slightly transparent regions (2) of the illumination mask (13) being first inverted and then the Z-position of the intensity maxima being determined, or the height values for the slightly transparent regions of the illumination mask (13) being interpolated from the height values of the adjacent pixels.

5. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that the camera sensor is a color sensor with a Bayer pattern, the "bright" pixels corresponding to the transparent regions (1) of the illumination mask (13) being the green pixels (21), and the z-position of the intensity maximum being determined for these pixels.

6. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that the height values for the red pixels (23) and blue pixels (22) are interpolated from the height values of the adjacent green pixels (21).

7. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that when generating the colored intensity image, the color information for the red pixels (23) and blue pixels (22) is determined from the intensity values just outside the focus.

8. Method for optically measuring technical surfaces, and device for carrying out the method according to claim 1, characterized in that the calculation of the Z-position of the intensity maxima already begins during the measurement data acquisition, the calculation of the Z-position of the intensity maxima being carried out using parallelized algorithms.

9. Device for carrying out the method according to any of claims 1 to 8, the device comprising a confocal sensor, a light source (11) and an optical system, the optical system containing an illumination mask (13), a sensor matrix (15), a beam splitter (14), for combining an illumination beam path and a detection beam path, and imaging optics (6), the illumination mask (13) consisting of transparent regions (1) and non-transparent or slightly transparent regions (2) arranged in a checkerboard pattern, the pitch (3) of the pattern on the illumination mask (13) corresponding to the pixel pitch (24) of the sensor matrix (15) and the illumination mask (13) and the sensor matrix (15) being adjusted relative to one another such that the transparent regions (1) and the pixels of the sensor matrix (15) can be simultaneously sharply imaged onto a sample (7) by means of the imaging optics (6), the device containing a beam splitter plate, a beam splitter cuboid or a beam splitter cube as a beam splitter (14), characterized in that the transparent regions (1) of the illumination mask (13) are round, that the beam-splitting coating has a polarization-neutral splitting ratio or has a polarizing effect, a lambda-quarter retardation plate being located between the beam splitter (14) and the sample (7) in the case of a polarizing effect, which plate rotates the polarization direction of the reflected light by 90 degrees.