Method and apparatus for measuring the thickness of a transparent body

Abstract

Method for measuring the thickness of a transparent body (4), comprising the steps - Arranging the transparent body to be measured (4) on a measuring template, - optical scanning of the measurement pattern with a camera (5) at a predetermined viewing angle (γ), - Determining an offset (V) of the measurement pattern relative to a real or fictitious reference scan without a transparent body under the same predetermined observation angle (γ), and - Calculating the thickness (d) of the transparent body (4) based on the determined offset (V) and the observation angle (γ).

Description

[0001] The present invention relates to a method and a device for measuring the thickness of a transparent body.

[0002] DE 199 42 244 A1 discloses a method for measuring the thickness of transparent materials using a camera. An illumination device is positioned beneath the transparent material, generating a measurement pattern. A camera positioned at an angle to the material to be examined films the measurement pattern through the material, which is represented, among other things, as two colored areas.

[0003] The dividing line between the two colored fields is used to measure the virtual change in the position of this dividing line, caused by the parallel offset of the light through the material, compared to the true position of the dividing line without material. This offset is a measure of the thickness of the transparent material.

[0004] JP H11-257 925 A also describes a method for measuring the thickness of a transparent material. For this purpose, a light cone is created beneath the material to be measured, which shines through it. The material is positioned above it at an angle. Due to refraction, the transmitted light is distorted, and if it is received by a sensor, conclusions can be drawn about the material's thickness.

[0005] DE 42 13 601 A1 discloses a method for thickness measurement in which a light spot is directed obliquely through the transparent layer to be measured. Behind this is a sensor that measures the position of this light spot. Alternatively, the light spot can be reflected again through the transparent layer via a reflective layer, so that the sensor is located on the same side as the origin of the light spot. Based on the offset of the light spot on the sensor, which is caused by the introduction of a transparent material, conclusions can be drawn about the thickness of the transparent material.

[0006] There are two methods for measuring the thickness of a body: contact and non-contact. Thickness measurements can be performed either on one side against a reference or on both sides using two sensors. In contact thickness measurements, one sensor is held in contact with the body to be measured.

[0007] Non-contact thickness measurement can be performed using a laser or an ultrasonic source. The advantages of non-contact measurement are that there is no material contact, the measurement is generally independent of the color of the material, the measurement is independent of the material's composition, high spatial resolution is achieved with a small measuring point, the body can be scanned at a high measurement frequency, a large distance from the body is possible, and the measurement of hot surfaces is possible.

[0008] Optical measurement methods for measuring the thickness of a body are based on triangulation. For transparent bodies, a special measuring device is required that allows direct reflection, meaning that the light source and the camera are positioned at a glancing angle with respect to the body being scanned.

[0009] The invention is based on the object of creating a method and a device for measuring the thickness of a transparent body, which can be carried out simply and inexpensively and yet provides precise measurement results.

[0010] Another object of the present invention is to provide a method and a device for measuring the thickness of a transparent body, which allow a rapid scanning of a line and / or an area of ​​the transparent body.

[0011] Another object of the present invention is to provide a method and apparatus for measuring the thickness of a transparent body which can be reliably carried out in a production environment such as that found in the manufacture of glass plates and can be reliably carried out over a long period of time.

[0012] One or more of the aforementioned objects are achieved by the subject matter of the independent patent claims. Advantageous embodiments are specified in the respective subclaims.

[0013] The method according to the invention for measuring the thickness of a transparent body comprises the steps - Arranging the transparent body to be measured on a measuring pattern, - optical scanning of the measurement pattern with a camera at a predetermined observation angle, - Determining an offset of the measurement pattern compared to a real or fictitious reference scan without a transparent body at the same predetermined observation angle, and - Calculate the thickness of the transparent body based on the determined offset and observation angle.

[0014] When a light beam passes between media with different optical refractive indices, the light beam is refracted, changing its direction of propagation. This results in a pattern located beneath a transparent body being displaced by the refraction of the light at the interfaces between the transparent body and the environment (usually air).

[0015] With the method according to the invention, this offset is determined and, based on this offset and knowledge of the observation angle, the thickness of the transparent body is calculated.

[0016] A reference measurement can be performed without a transparent body, whereby the same measurement pattern is scanned during the reference measurement and the reference measurement is related to the measurement with the transparent body. The reference measurement can be related to the measurement with the transparent body in such a way that the position of the measurement pattern relative to the camera is recorded, and the two measurements are compared with each other when the position of the measurement pattern matches.

[0017] In the following, the term “location of the measurement pattern” refers to the location of the measurement pattern with respect to the camera.

[0018] During optical scanning, the location of the measurement pattern is preferably recorded and the offset is determined based on the pattern information contained in the pattern. If, during optical scanning, the location of the measurement pattern and the pattern information contained in the pattern are known, i.e. if a description of the measurement pattern is available in an evaluation device, then a fictitious scan without a transparent body can be carried out without a reference measurement. This fictitious scan can be compared with a measurement image obtained during optical scanning of the transparent body on the measurement pattern to determine the offset. Fictitious scanning means that, with knowledge of the measurement pattern and the location of the measurement pattern with respect to the camera, it is clearly determined how the camera sees the measurement pattern if there is no body deflecting or refracting the light rays between the camera and the measurement pattern.The measurement pattern can thus be transferred into a reference image, which can be compared with a measurement image captured of the measurement pattern with the camera. The conversion of the measurement pattern into a reference image is carried out using the imaging properties of a camera lens. The imaging properties include, for example, the focal length, the direction of view, the distance to the measurement pattern, and, if applicable, the distortion of the lens. A measurement image is usually generated with a transparent body placed on the measurement pattern. To calibrate a device for measuring the thickness of a transparent body, it can also be useful to generate a measurement image of the measurement pattern without a transparent body and to compare this with the reference image according to a fictitious scan.

[0019] The location of the measurement pattern can be detected using a location sensor, such as an incremental encoder. The measurement pattern can also be only partially covered by the transparent body. By scanning an area of ​​the measurement pattern not covered by the transparent body, the location of the measurement pattern relative to the camera can be determined. The measurement pattern then contains a pattern in the uncovered area that is characteristic of the respective locations, which is detected by the camera.

[0020] The measurement pattern can be arranged in a predetermined relative position to the camera, or the relative position of the measurement pattern to the camera can be detected using a position sensor, so that the arrangement of the scanned measurement pattern resulting from a fictitious scan without a transparent body can be determined based on this relative position. This fictitious scan corresponds to a reference image that can be compared with the measurement image to determine the offset.

[0021] When the transparent body is scanned with the camera, a measurement image is generated which can be compared with a reference image to determine the offset. Matching feature points are extracted from the measurement image and the reference image and their distance is determined as the offset.

[0022] The distance between the matching feature points can also be measured only along a specific preferred direction.

[0023] This preferred direction can correspond to a direction of movement with which the measurement image is generated by relative movement of the measurement pattern and the transparent body arranged thereon with respect to a line scan camera aligned transversely to the direction of movement.

[0024] The feature points can be extracted from the measured image and the reference image using cross-correlation. In principle, other methods for extracting feature points are also suitable. In this regard, reference is made to the review article "Image Registration Techniques: A Comprehensive Survey; Guido Bartholi; Università degli Studi di Siena, June 2007 (available for download on February 14, 2017: (http: / / citeseerx.ist.psu.edu / viewdoc / download?doi=10.1.1.471.1134&rep=rep1&type=pdf.) Chapter 2 describes various methods that can be used for the automatic extraction of image features. Chapter 3 explains the mapping of features from different images. These well-known methods can be used to extract and map feature points from the measurement image and the reference image.

[0025] In principle, it is also possible for the camera to scan the measurement pattern with the transparent body on it from at least two different observation angles. From the position of at least two characteristic points of the measurement pattern in the measurement image thus obtained, the offset of at least one of these characteristic points of the measurement pattern can be determined compared to a fictitious scan without a transparent body at the same observation angles. If, for example, the distance between these two characteristic points in the measurement pattern is known, the thickness of the transparent body can be determined from the distance between the corresponding characteristic points in the measurement image, provided the transparent body is a plane-parallel plate. With this method, only the presence of two characteristic points and their distance in the measurement pattern must be known as reference information.With this reference information, the thickness of the transparent body can be determined from the distance between these characteristic points in the measurement image.

[0026] The arrangement, consisting of the transparent body to be measured and the measuring pattern, can be moved relative to the camera in a direction of movement, whereby the predetermined observation angle can be the angle that the viewing direction of the camera makes with the transparent body, either in the direction of movement or perpendicular to the direction of movement. When using a line scan camera, the angle that the viewing direction of the camera makes with the transparent body in the direction of movement is preferably used as the predetermined observation angle, since this angle is the same for all points along a line to be scanned if the line scan camera is arranged perpendicular to the direction of movement. This results in the same offset along the line to be scanned for a constant thickness of the transparent body.

[0027] The methods explained above are preferably used for measuring transparent bodies, which are approximately plane-parallel plates. These plane-parallel plates can have varying thicknesses to some extent.

[0028] According to a further aspect of the present invention, there is provided an apparatus for measuring the thickness of a transparent body, comprising - a measuring pattern on which a transparent body to be measured can be placed, - a camera for scanning the body located on the measuring pattern at a predetermined observation angle, - an evaluation device for determining an offset of the measurement pattern compared to a real or fictitious scan without a transparent body under the same predetermined observation angle, and for calculating the thickness of the transparent body based on the determined offset and the observation angle.

[0029] The camera is preferably a line scan camera.

[0030] A moving device may be provided for relatively moving the measuring pattern and the transparent body arranged on the measuring pattern with respect to the camera.

[0031] Preferably, an illumination device is provided for illuminating the measurement pattern or the transparent body located on the measurement pattern. The illumination device preferably has a diffuse light source.

[0032] The camera can be equipped with an object-side, telecentric lens. When using a line-scan camera, the object-side, telecentric lens can be telecentric at least along one scanning line. When using an area-scan camera, the object-side, telecentric lens is preferably telecentric over the entire scanning area. The use of an object-side, telecentric lens has the advantage that only parallel light rays from the scanning area are imaged onto the camera. This ensures the same predetermined viewing angle at all points within the scanning area. This simplifies the analysis.

[0033] When using a line camera which is designed transversely to the direction of movement of a relative movement between the line camera and the transparent body to be scanned, and when using the angle enclosed by the viewing direction of the camera and the transparent body to be scanned exclusively in the direction of movement as the predetermined observation angle, this predetermined observation angle is also constant over the entire scanning area along a predetermined scanning line, without the use of an object-side, telecentric lens being necessary.

[0034] The measurement pattern preferably includes markers that can be used to detect the offset. The markers can be any points or markings in the measurement pattern that are recognizable and clearly identifiable in the captured image.

[0035] The measurement pattern is preferably a random pattern. The measurement pattern is expediently a two-dimensional pattern, with the individual points of the measurement pattern corresponding to a random distribution. The points of the measurement pattern can also correspond to a sequence of maximum length or maximum sequence (MLS: Maximum Length Sequence). For a clear assignment of features of the pattern, it is expedient if reference sections of a predetermined length are as unique as possible, i.e., they do not repeat, or only very rarely, at any other location in the pattern. The individual points of the measurement pattern form such markers or feature points.

[0036] The device for measuring the thickness of a transparent body preferably comprises a control device which is designed to carry out one of the methods explained above.

[0037] The invention is explained in more detail below with reference to an embodiment illustrated in the drawings. The drawings show: Fig. 1a deflection of a light beam in a transparent medium, Fig. 2 schematically shows the structure of a device for measuring the thickness of a transparent body in a block diagram, Fig. 3 two diagrams showing the measured displacement for transparent bodies of different thicknesses, with the upper diagram showing the displacement in the transport direction and the lower diagram showing the displacement transverse to the transport direction, Fig. 4a and Fig. 4b schematically shows the scanning of an approximately flat transparent body by means of a line camera with a viewing direction inclined relative to the transparent body, and Fig. 5the assignment of feature points of a reference image to a measurement image using a correlation method.

[0038] The method and device according to the invention are based on Snell's law of refraction, which describes the transition of light rays from a medium with a first refractive index n1 to another medium with a second refractive index n2. A light ray 1 crossing an interface 2 between the two media with refractive indices n1 and n2 is refracted at the interface 2 according to the following formula ( Fig. 1) sinβsinα=n1n2, where α is the angle of incidence in the medium with the refractive index n1 with respect to a normal 3 on the interface 2 and β is the angle of reflection with respect to the normal 3.

[0039] The observation angle γ is defined as the angle subtended by the incident light beam 1 and the interface 2. The medium with the refractive index n1 is usually air, and the medium with the refractive index n2 is a transparent body 4 whose thickness d is to be measured.

[0040] The thickness d is determined from the following formula: d=Vtan(α)−n2sin(α)n12−n22sin2(α)=kV k=1tan(α)−n2sin(α)n12−n22sin2(α) where V is the offset at the bottom of the transparent body 4 with respect to a light beam 1' which is not deflected with respect to the real light beam 1.

[0041] If a camera 5, which is located in the medium with the refractive index n1, is viewed in the direction of the light beam 1 at the medium with the refractive index n2, which is the transparent body 4, then the camera 5 sees a point P1 on the underside of the transparent body. Since the light beam captured by the camera 5 is refracted at the interface between the transparent body 4 and the environment with the refractive index n1, which is usually air, the camera 5 sees the point P1 offset by the offset V from a point P2 ( Fig. 1). If the transparent body 4 were not present, the light beam would not be refracted and the camera 5 would see the point P2, which is offset from the point P1 by the distance V.

[0042] If a pattern is placed on the underside of the transparent body 4, the pattern can be captured with the camera 5 offset by a distance V relative to a state in which the transparent body with the refractive index n2 is not present. From the above formula, it follows that the thickness d of the transparent body 4 is proportional to this offset V. The proportionality constant is k. It is constant for a specific angle of incidence α or for a specific angle of observation γ.

[0043] This observation angle γ is linked to the angle of incidence via the following formula: γ=90°−α

[0044] Thus, the thickness of the transparent body can be determined directly from the measured offset V and with knowledge of the observation angle γ or the angle of incidence α.

[0045] This applies to transparent bodies 4 in the form of approximately plane-parallel plates. Glass plates represent such plane-parallel plates for which the relationships between thickness and offset explained above apply. During the manufacture of glass plates, there is a significant need for a simple way to check their thickness.

[0046] A device 6 for measuring the thickness of a transparent body 4 according to a first embodiment comprises a moving or conveying device 7 for conveying a transparent body 4 to be measured in the direction of movement 8 ( Fig. 2). In the present embodiment, the conveyor device 7 is a conveyor belt 9, which is supported by rollers 10. In Fig. 2 shows only a section of the conveyor belt 9. The conveyor belt 9 is coupled to a drive device (not shown) that drives the conveyor belt 9 in the direction of movement 8.

[0047] A measurement pattern 11 is printed on the top side of conveyor belt 9. The measurement pattern is a two-dimensional black / white pattern with a large number of randomly distributed black dots on a white background. However, the dots can also be arranged according to a different distribution, with preference given to distributions that have as many and as large as possible unique dot segments. Such a distribution can, for example, correspond to a maximum length sequence (MLS).

[0048] A position sensor, which in the present embodiment is an incremental encoder 11, is coupled to the underside of the conveyor belt 9. The incremental encoder 11 is coupled to the conveyor belt 9 via a roller, so that the roller rotates when the conveyor belt 9 moves. The rotation of the roller is converted into electrical pulse signals, with an electrical pulse being output at a specific angle of rotation of the roller and thus a specific path of the conveyor belt 9.

[0049] The incremental encoder 11 is connected to a central control device 12, which receives the pulses from the incremental encoder 11. The pulses thus represent a position signal for the location of the conveyor belt 9 or the measurement pattern printed on it.

[0050] The central control device 12 is connected to a camera 5, which is directed at a predetermined observation angle γ with its viewing direction onto the conveyor belt 9 or the measurement pattern printed thereon.

[0051] The observation angle γ is not equal to 90°. Preferably, the observation angle is greater than 20°, in particular greater than 25°, and preferably greater than 40°. The observation angle may be less than 70°, in particular less than 65°, and preferably less than 60°.

[0052] The camera 5 is a line-scan camera, which is arranged with its line-shaped scanning area transverse to the direction of movement 8. The line-scan camera 5 thus scans the conveyor belt 9 or the measuring pattern with a scanning line 14 running transversely to the direction of movement 8 ( Fig. 4a).

[0053] A lighting device 13 is provided, which illuminates the conveyor belt 9 in the area of ​​the scanning line. The lighting device preferably has a diffuse light source.

[0054] The camera 5 is connected to the central control device 12, which can trigger the capture of a one-dimensional image at a time. These one-dimensional images can be combined to form a two-dimensional measurement image. To distinguish between the two, the one-dimensional images are referred to below as "frames," and the two-dimensional images generated from them are referred to as measurement images.

[0055] The camera 5 is preferably controlled such that the individual frames scan several lines of the conveyor belt, each of which is equidistant from the other. This can be achieved, for example, by moving the conveyor belt 9 at a constant speed in the direction of movement 8 and recording the frames at a constant frequency. The speed at which the conveyor belt 9 is moved can be monitored by the central control device 12 using the incremental encoder 11.

[0056] On the other hand, it is also possible to control the camera 5 using the pulses generated by the incremental encoder 11 in such a way that a frame is captured each time the conveyor belt 9 travels a predetermined distance. Thus, the conveyor belt 9 can be moved at different speeds, yet a measurement image consisting of several individual frames is generated, in which the individual frames are equally spaced from one another.

[0057] Since the line camera 5 is aligned transversely to the direction of movement 8, the line camera 5 looks along the entire scanning line with the same observation angle γ in the direction of movement 8 onto the conveyor belt 9. If a different observation angle were chosen that would not include the angle between the light rays 1 and the conveyor belt 9 in the direction of movement 8, but for example the angle transversely to the direction of movement 8, then different angles would result for the individual light rays 1 along the scanning line 14, as can be seen from Fig. 4a. If the camera 5 is arranged centrally above the conveyor belt 9, then the light beam in the middle has no inclination relative to the conveyor belt 9 transverse to the direction of movement, whereas the light rays that strike the conveyor belt 9 at the edge are most inclined relative to the conveyor belt 9 transversely with respect to the direction of movement 8. In principle, these angles could also be used as observation angles for measuring the thickness of a transparent body. However, the different observation angles would have to be taken into account during the evaluation. Therefore, the arrangement with a line scan camera, in which the scanning line 14 is arranged transversely to the direction of movement 8 and the use of the observation angle γ, which includes the angle between the light rays 1 and the conveyor belt 9 or the corresponding measurement pattern in the direction of movement 8, is preferred ( Fig. 4b).

[0058] The central control device is connected to an evaluation device 15, with which the thickness d of a transparent body is determined based on the measurement image captured by the camera 5.

[0059] The measurement of the thickness d of a transparent body 4 using the device 6 explained above is explained in more detail below.

[0060] A transparent body 4 is conveyed by the conveyor belt 9 in the direction of movement 8. The transparent body 4 is scanned by the camera 5 and a measurement image is generated, as explained above. The measurement image contains an image of the measurement pattern located on the conveyor belt 9, wherein the measurement pattern is offset by the distance V due to the refraction of light by the transparent body 4. The size of the offset depends on the setting of the observation angle γ. The angle γ is preferably less than 85°, in particular less than 80°. The angle γ can also be less than 75° or less than 70°. The smaller the observation angle γ, the greater the measured offset. The smaller the observation angle, the greater the resolution of the method. However, the brightness of the light decreases as the observation angle γ decreases.At the interface between an optically dense and an optically thin medium, total internal reflection occurs above a certain critical angle. The observation angle is therefore large enough to detect light that is not totally reflected. The critical angle depends on the refractive index of the material being examined. For a refractive index of two (n1 = 2.0) and a refractive index of the ambient air of one (n2 = 1.0), the critical angle is approximately 30°. The observation angle γ is therefore preferably greater than 60°, in particular greater than 65°, and preferably greater than 70°.

[0061] The measurement image thus represents a representation of the measurement pattern, whereby the measurement pattern in the measurement image is, on the one hand, slightly offset in the direction of movement 8 due to the presence of the transparent body 4, and, on the other hand, the measurement image is slightly compressed in the transverse direction, especially at the edge area. Due to the inclination of the light rays relative to the conveyor belt 9 in the transverse direction, which is more pronounced at the edge area than in the center ( Fig. 4a), the individual pixels of the measurement pattern are offset slightly inwards in the measurement image, with the offset being greater the further outwards the pixels are located. Since the inclination of the light rays in the transverse direction with respect to the conveyor belt or the measurement pattern is considerably lower than in the direction of movement 8, this "compression" of the measurement image in the transverse direction can often be neglected. On the other hand, it is also possible to "de-compress" the measurement image in the transverse direction accordingly, for example by detecting certain feature points and comparing them with the corresponding feature points in the undistorted measurement pattern, so that based on this comparison the measurement image in the transverse direction can be correspondingly corrected. Such methods are known, for example, from the above-mentioned document Image Registration Techniques: A Comprehensive Survey by Guido Bartoli.

[0062] In the present embodiment, however, no such rectification of the measurement image is carried out, since the distortion or compression is so small that it can be neglected.

[0063] The following explains the determination of an offset V for a specific point F0(x, y) of the transparent body 4. This point F0(x, y) is a specific point on the measurement pattern at whose location the thickness of the transparent body 4 is to be determined. The measurement pattern is available as a reference image. This reference image is an image of the reference image that has been converted such that it represents an image of the measurement pattern as if it had been recorded with the camera 5 without the presence of a transparent body 4. The conversion by means of a "fictitious scan" has already been explained above. The reference image can also be generated by actually capturing the measurement pattern with the camera 5 without the presence of a transparent body 4.

[0064] The measurement image, which shows the measurement pattern with the transparent body 4 located thereon, is compared with the reference image. For this purpose, a reference section 16 is selected in the reference image around the pixel F0(x, y). In the present exemplary embodiment, the reference section 16 is square with a side length of b. This reference section 16 is superimposed in a search window 17 of the measurement image at several positions such that the central pixel F0(x, y) of the reference section 16 is superimposed on a pixel F1(x, y). The pixels F1(x, y) of the measurement image are selected such that, starting from a pixel which is offset by one pixel from the pixel F0(x, y) of the reference image in the direction of movement 8, they are successively superimposed.

[0065] In the measurement image and the reference image, the X-axis runs transversely to the direction of movement 8 or parallel to the scanning line 14 and the Y-axis runs in the direction of movement 8 or transversely to the scanning line 14. The reference section 16 of the reference image or its central pixel is thus successively aligned with several points of the measurement image in the direction of the Y-axis.

[0066] Since the position of the measurement pattern was also detected by the position sensor or incremental encoder 11 when the measurement image was captured, the position of the measurement pattern relative to the camera is known when the individual frames were captured. This position information is stored together with the measurement image. When comparing the measurement image with the reference image, these images are assigned to one another based on this position information. In the present exemplary embodiment, this means specifically that the point F0(x, y) of the reference image corresponds to the point F1(x, y) in the measurement image, which was recorded at the moment at which the point F0(x, y) of the measurement pattern would have been in the field of view of the camera 5 if no transparent body 4 had been arranged on the measurement pattern. In other words, this means that the point F0(x, y) of the measurement pattern is at the point P2 according to Fig. 1, whereby the camera saw the point P1 due to the presence of the transparent body 4, which is shown in the measurement image at the point F1(x, y).

[0067] Since the point F0(x, y) was already recorded at an earlier point in time due to the deflection of the light beam 1, when the measuring pattern with its point F0(x, y) was located at the point P1, the image of this point F0(x, y) of the measuring pattern in the measuring image is offset by the offset V in the Y direction (i.e. at F1(x, y + V)).

[0068] Starting from point F1(x, y) in the measurement image, the number w of pixels of the measurement image are successively aligned with the central point of the reference section 16 within the search window 17 in the Y direction or direction of movement 8. The length of the search window is thus w + b - 1 and is selected such that the maximum possible thicknesses d of the transparent body 4 can be reliably detected. The width of the search window 16 is b.

[0069] A correlation value is calculated for each position of the reference section 16 in the search window 17. The correlation value is a measure of how well the pixels of the reference section 16 of the reference image match the pixels of the measurement image with which they are overlaid. The sum of squared differences (SSD), for example, can be used as a correlation value. The lower this value, the better the match. Any other metric that describes the difference between two vectors can also be used as a correlation value. This could be, for example, the Euclidean distance or a correlation value. Other suitable metrics are ZNCC (zero-mean-normalized cross-correlation), adaptive binary window, POC (phase-only correlation), free-mode census, guided image filters, or a minimum spanning tree.

[0070] The position of the reference section 16 in the search window 17 that has the best correlation value is selected to determine the offset V. The offset V (x, y) of the reference section 16 at this position represents the corresponding offset value that is assigned to the corresponding pixel F0(x, y) in the reference image.

[0071] This determination of the offset V is repeated for all pixels of the reference image within a predetermined, relevant range.

[0072] The search window 17 can be supplemented at the edge by further fictitious pixels, in particular black pixels, so that the center of the reference section 16 can be aligned with each pixel of the search window.

[0073] Using the offset V and the observation angle γ, the thickness d can be calculated at all points F0(x, y). Since the thickness is proportional to the constant k specified above at a constant observation angle γ, the thickness d can be determined in real time for all points in the measurement range.

[0074] This method is therefore suitable for measuring the thickness of a transparent body over the entire surface in a very narrow grid and thus for continuously monitoring the production of such transparent bodies in real time.

[0075] This method can be used, for example, to determine the areas of a transparent body where a transparent coating is applied, with the area where the coating is present having a different thickness than the area where no coating is present. This thickness difference can be determined for individual points, allowing this thickness measuring device to continuously monitor in real time where the layer is applied or not applied.

[0076] The measuring device is very simply designed. In the preferred embodiment according to the present example, which uses a line-scan camera whose scanning line is oriented transversely to the direction of movement, the viewing angle is constant along the entire scanning line 14, so that the same calculation can be performed along the entire scanning line or across the entire width of the measuring area to calculate the thicknesses.

[0077] In Fig. 3 shows two diagrams. The upper diagram shows the displacement of the pixels Pi of the measurement image for a thin glass plate L1, a medium-thick glass plate L2, and a thick plate L3 in the direction of movement and in the Y-direction, respectively. The observation angle here is aligned along the direction of movement. This diagram is based on a measurement setup in which, unlike in Fig. 1 the camera does not look at the measurement pattern in the direction of movement but opposite to the direction of movement 8. This results in a negative offset of the pixels.

[0078] The lower diagram in Fig. Figure 3 shows a diagram in which the observation angle is oriented perpendicular to the direction of movement. This diagram shows the displacement of the pixels Pi of the measurement image for a thin glass plate L4, a medium-thick glass plate L5, and a thick plate L6, perpendicular to the direction of movement or in the X-direction. There is no offset in the center, and the offset increases the further outward the pixels are scanned. In this diagram, the lines L4, L5, and L6 intersect in the center. The slope of these lines is a measure of the thickness d of the respective glass plate.

[0079] An embodiment of the invention using a line scan camera is explained above. Within the scope of the invention, it is also possible to use an area scan camera. When using an area scan camera, it is expedient to provide a telecentric lens on the object side, as this allows the camera to capture only parallel light rays. This ensures that the same observation angle is present across the entire scanning range. Such a device for measuring the thickness of a transparent body is particularly useful when individual transparent bodies are not moved along a conveyor line, but are positioned sequentially in the measuring area.

[0080] Within the scope of the invention, it is also possible to identify only certain characteristic points in the measurement image instead of a reference image. Based on the respective distances between these characteristic points, the offset can then be determined, knowing the distance between the respective characteristic points in the measurement pattern. This is particularly suitable for measuring plane-parallel plates that have a constant thickness across the measurement range. List of reference symbols 1 light beam 2 Interface 3 lots 4 transparent body 5 Camera 6 Device for measuring the thickness 7 Conveyor system 8 Direction of movement 9 Conveyor belt 10 rolls 11 incremental encoders 12 central control device 13 Lighting device 14 scan lines 15 Evaluation device 16 Reference section 17 search windows

Claims

[1] Method for measuring the thickness of a transparent body (4), comprising the steps - arranging the transparent body (4) to be measured on a measuring pattern, - optically scanning the measurement pattern with a camera (5) at a predetermined observation angle (γ), - determining an offset (V) of the measurement pattern compared to a real or fictitious reference scan without a transparent body at the same predetermined observation angle (γ), and - Calculating the thickness (d) of the transparent body (4) based on the determined offset (V) and the observation angle (γ). [2] Method according to claim 1, characterized by that a reference measurement without a transparent body is carried out as a real reference scan, wherein the reference measurement is related to the measurement with the transparent body (4). [3] Method according to claim 2, characterized bythat the reference measurement is linked to the measurement with the transparent body (4) by detecting the location of the pattern with respect to the camera (5) in both measurements and comparing the two measurements with the same location of the measurement pattern with respect to the camera (5). [4] Method according to one of claims 1 to 3, characterized by that during optical scanning the location of the measuring pattern is recorded and the offset is determined based on the pattern information contained in the measuring pattern. [5] Method according to claim 3 or claim 4, characterized by that the location of the measuring pattern is detected by means of a location sensor (11) or in that the measuring pattern is only partially covered with the transparent body (4) and the location of the measuring pattern with respect to the camera (5) is determined from the scanning of the area of the measuring pattern not covered by the transparent body (4). [6] Method according to one of claims 1 to 5, characterized bythat the measuring pattern is arranged in a predetermined relative position to the camera (5), or the relative position of the measuring pattern to the camera (5) is detected by means of a location sensor (11), so that on the basis of this relative position the arrangement of the scanned measuring pattern resulting from a fictitious scan without a transparent body can be determined, so that the offset of the measuring pattern can hereby be determined. [7] Method according to one of claims 1 to 6, characterized by that during scanning with the camera (5) a measurement image is generated which is compared with a reference image to determine the offset, wherein matching feature points are extracted from the measurement image and the reference image and their distance is determined as the offset. [8] Method according to claim 7, characterized by that the distance between matching feature points is measured only along one preferred direction. [9] Method according to claim 8, characterized by that the camera (5) is a line scan camera and the preferred direction corresponds to a direction of movement (8) with which the measurement image is generated by relative movement of the measurement pattern and the transparent body (4) arranged thereon with respect to the line scan camera (5) aligned transversely to the direction of movement (8). [10] Method according to one of claims 7 to 9, characterized by that the feature points are extracted using correlation. [11] Method according to one of claims 1 to 10, characterized bythat the camera (5) scans the measuring pattern with the transparent body (4) located thereon at least two different observation angles, wherein the offset (V) of at least one of these characteristic points of the measuring pattern is determined from the position of at least two characteristic points of the measuring pattern in the measurement image generated by the camera compared to a fictitious scan without a transparent body at the same observation angles (γ). [12] Method according to claim 11, characterized by that the distance between the characteristic points in the measurement pattern is taken into account when determining the offset. [13] Method according to one of claims 1 to 12, characterized by , that the arrangement consisting of the transparent body (4) to be measured and the measuring pattern is moved relative to the camera (5) in the direction of movement (8), wherein the predetermined observation angle (γ) is the angle which the viewing direction of the camera (5) encloses with the transparent body (4) either along the direction of movement (8) or transversely to the direction of movement (8) and the offset (V) of the measuring pattern in the direction of movement (8) or transversely to the direction of movement (8) is determined accordingly. [14] Device for measuring the thickness of a transparent body, comprising - a measuring pattern on which a transparent body (4) to be measured can be arranged, - a camera (5) for scanning the body (4) located on the measuring pattern at a predetermined observation angle (γ), - an evaluation device (15) for determining an offset (V) of the measurement pattern compared to a real or fictitious scan without a transparent body (4) at the same predetermined observation angle (γ), and for calculating the thickness (d) of the transparent body (4) based on the determined offset (V) and the observation angle (γ). [15] Device according to claim 14, characterized by that the camera (5) is a line scan camera. [16] Device according to claim 14 or 15, characterized by that a movement device (7) is designed for the relative movement of the measuring pattern and the transparent body (4) arranged on the measuring pattern with respect to the camera (5). [17] Device according to one of claims 14 to 16, characterized by that an illumination device (13) is provided for illuminating the measurement pattern, wherein the illumination device (13) preferably has a diffuse light source. [18] Device according to one of claims 14 to 17, characterized by that the camera (5) has an object-side telecentric lens. [19] Device according to one of claims 14 to 18, characterized by that the measurement pattern is a random pattern. [20] Device according to one of claims 14 to 19, characterized by that a central control device (12) is provided which is designed to carry out a method according to one of claims 1 to 13.

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