DE102017104766B4 - Method and device for measuring the thickness of a transparent body - Google Patents

Abstract

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 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

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

The DE 199 42 244 A1 discloses a method for measuring the thickness of transparent materials using a camera, wherein an illumination device is provided underneath the transparent material, which generates a measurement pattern. A camera positioned at an angle to the material to be examined films the measurement pattern through this material, which is represented, among other things, as two colored areas.

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.

The JP H11-257 925 A also describes a method for measuring the thickness of a transparent material. To do this, a cone of light is generated under the material to be measured, which shines through this material. The material is arranged above it at an angle. The light passing through is distorted by refraction, and if it is received by a sensor, conclusions can be drawn about the thickness of the material.

The DE 42 13 601 A1 discloses a method for measuring thickness in which a point of light is sent at an angle through the transparent layer to be measured. Behind this is a sensor that measures the position of this point of light. Alternatively, the point of light can be reflected again through the transparent layer via a reflective layer so that the sensor is on the same side as the origin of the point of light. Based on the offset of the point of light on the sensor, which is caused by the introduction of a transparent material, conclusions can be drawn about the thickness of the transparent material.

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

A non-contact thickness measurement can be carried out using a laser or an ultrasound source. The advantages of a 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 composition of the material, a high spatial resolution is achieved by a small measuring point, the body can be scanned with a high measuring frequency, a large distance from the body is possible and the measurement of hot surfaces is possible.

Optical measuring methods for measuring the thickness of a body are based on triangulation. For transparent bodies, a special measuring device is necessary that allows direct reflection, i.e. the light source and the camera are arranged at the glancing angle with respect to the body to be scanned.

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.

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.

Another object of the present invention is to provide a method and an 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 carried out reliably over a long period of time.

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.

• - Anordnen des zu messenden transparenten Körpers auf einem Messmuster,
• - optisches Abtasten des Messmusters mit einer Kamera unter einem vorbestimmten Beobachtungswinkel,
• - Bestimmen eines Versatzes des Messmusters gegenüber einer realen oder fiktiven Referenzabtastung ohne transparentem Körper unter dem gleichen vorbestimmten Beobachtungswinkel, und
• - Berechnen der Dicke des transparenten Körpers anhand des bestimmten Versatzes und des Beobachtungswinkels.

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 under the same predetermined observation angle, and
• - Calculate the thickness of the transparent body based on the determined offset and observation angle.

When a light beam passes between media with different optical refractive indices, the light beam is refracted so that it changes its direction of propagation. This causes a pattern located underneath a transparent body to be displaced by the refraction of the light at the interfaces between the transparent body and the environment (usually air).

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

A reference measurement can be carried out 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 location of the measurement pattern is recorded with respect to the camera and the two measurements are compared with each other when the location of the measurement pattern matches.

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

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 is known and the pattern information contained in the pattern is 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, which 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, knowing 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 between the camera and the measurement pattern that deflects or refracts the light rays. The measurement pattern can thus be transferred to a reference image that can be compared with a measurement image that is recorded from the measurement pattern with the camera. The measurement pattern is converted into a reference image using the imaging properties of a camera lens. The imaging properties are, 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 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.

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 with the transparent body. By scanning an area of the measurement pattern that is not covered by the transparent body, the location of the measurement pattern can be determined with respect to the camera. The measurement pattern then contains a pattern in the uncovered area that is characteristic of the respective locations and is recognized by the camera.

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 by means of a location 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.

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, whereby matching feature points are extracted from the measurement image and the reference image and their distance is determined as the offset.

The distance between the matching feature points can also be measured only along a certain preferred direction.

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.

• http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.471.1134&rep=rep1&type=pdf .) In Kapitel 2 sind unterschiedliche Verfahren beschrieben, die zur automatischen Extraktion von Bildmerkmalen verwendet werden können. In Kapitel 3 ist die Zuordnung von Merkmalen unterschiedlicher Bilder zueinander erläutert. Diese bekannten Verfahren können dazu verwendet werden, Merkmalspunkte des Messbildes und des Referenzbildes zu extrahieren und einander zuzuordnen.

The feature points can be extracted from the measurement image and the reference image using cross-correlation. In principle, other methods for extracting feature points are also suitable. In this regard, please refer to the overview article " Image Registration Techniques: A Comprehen sive survey; Guido Bartholi; Universitä degli Studi di Siena, June 2007 (downloadable on February 14, 2017 :

• http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.471.1134&rep=rep1&type=pdf .) Chapter 2 describes different methods that can be used to automatically extract image features. Chapter 3 explains how to assign features from different images to each other. These known methods can be used to extract feature points from the measurement image and the reference image and assign them to each other.

In principle, it is also possible for the camera to scan the measurement pattern with the transparent body on it at at least two different observation angles. From the position of at least two characteristic points of the measurement pattern in the measurement image obtained in this way, 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 that 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 pattern information, which form 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.

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, wherein the predetermined observation angle can be the angle that the viewing direction of the camera encloses with the transparent body either in the direction of movement or transversely to the direction of movement. When using a line camera, the angle that the viewing direction of the camera encloses 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 camera is arranged transversely to the direction of movement. This results in the same offset along the line to be scanned if the thickness of the transparent body is the same.

The methods explained above are preferably used for measuring transparent bodies, which are, for example, plane-parallel plates. These plane-parallel plates can have different thicknesses to a certain extent.

• - ein Messmuster, auf welchem ein zu messender transparenter Körper angeordnet werden kann,
• - eine Kamera zum Abtasten des sich auf dem Messmuster befindenden Körpers unter einem vorbestimmten Beobachtungswinkel,
• - eine Auswerteeinrichtung zum Bestimmen eines Versatzes des Messmusters gegenüber einer realen oder fiktiven Abtastung ohne transparentem Körper unter dem gleichen vorbestimmten Beobachtungswinkel, und zum Berechnen der Dicke des transparentem Körpers anhand des bestimmten Versatzes und dem Beobachtungswinkel.

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 arranged,
• - 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.

The camera is preferably a line scan camera.

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.

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.

The camera can be designed with an object-side, telecentric lens. When using a line camera, the object-side, telecentric lens can be designed to be telecentric at least along one scanning line. When using an area camera, the object-side, telecentric lens is preferably designed to be 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 projected onto the camera. This means that the same predetermined observation angle is present at all points in the scanning area. This simplifies the evaluation.

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 need for the use of an object-side, telecentric lens.

The measurement pattern preferably has 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.

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 predetermined length are as clear as possible, i.e. they are not repeated or only very rarely repeated at another point in the pattern. The individual points of the measurement pattern form such markers or feature points.

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

• 1eine Ablenkung eines Lichtstrahles in einem transparenten Medium,
• 2 schematisch den Aufbau einer Vorrichtung zum Messen der Dicke eines transparenten Körpers in einem Blockschaltbild,
• 3zwei Diagramme, mit welchen der gemessene Versatz bei unterschiedlich dicken transparenten Körpern dargestellt ist, wobei am oberen Diagramm der Versatz in Transportrichtung und im unteren Diagramm der Versatz quer zur Transportrichtung gezeigt ist,
• 4a und 4b schematisch die Abtastung eines in etwa ebenflächigen transparenten Körpers mittels einer Zeilenkamera mit einer gegenüber dem transparenten Körper geneigten Blickrichtung, und
• 5die Zuordnung von Merkmalspunkten eines Referenzbildes zu einem Messbild mittels eines Korrelationsverfahrens.

The invention is explained in more detail below using an embodiment shown in the drawings. The drawings show:

• 1a Deflection of a light beam in a transparent medium,
• 2 schematically shows the structure of a device for measuring the thickness of a transparent body in a block diagram,
• 3two Diagrams showing the measured displacement of 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,
• 4a and 4b schematically 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
• 5the Assignment of feature points of a reference image to a measurement image using a correlation method.

wobei α der Einfallswinkel im Medium mit dem Brechungsindex n₁ gegenüber einem Lot 3 auf der Grenzfläche 2 und β der Ausfallswinkel gegenüber dem Lot 3 ist.The method and the 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 n ₁ into another medium with a second refractive index n _2. A light ray 1 that crosses an interface 2 between the two media with the refractive indices n ₁ and n ₂ is refracted at the interface 2 according to the following formula ( 1 ) $$\frac{\text{sin}\beta }{\text{sin}\alpha }=\frac{n_1}{n_2},$$

where α is the angle of incidence in the medium with the refractive index n ₁ with respect to a perpendicular 3 on the interface 2 and β is the angle of reflection with respect to the perpendicular 3.

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 n ₁ is usually air and the medium with the refractive index n ₂ is a transparent body 4 whose thickness d is to be measured.

$$k=\frac{1}{\text{tan}\left(\alpha \right)-\frac{n_2\text{sin}\left(\alpha \right)}{\sqrt{n_1^2-n_2^2{\text{sin}}^2\left(\alpha \right)}}}$$

wobei V der Versatz an der Unterseite des transparenten Körpers 4 bezüglich eines Lichtstrahls 1', der gegenüber dem realen Lichtstrahl 1 nicht abgelenkt ist.The thickness d is determined from the following formula: $$d=\frac{V}{\text{tan}\left(\alpha \right)-\frac{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102017104766B4\_0005.png,DE102017104766B4\_0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2\text{sin}\left(\alpha \right)}{\sqrt{{\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="DE102017104766B4\_0005.png,DE102017104766B4\_0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_1^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102017104766B4\_0005.png,DE102017104766B4\_0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2{\text{sin}}^2\left(\alpha \right)}}}=kV$$

$$k=\frac{1}{\text{tan}\left(\alpha \right)-\frac{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102017104766B4\_0005.png,DE102017104766B4\_0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2\text{sin}\left(\alpha \right)}{\sqrt{{\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="DE102017104766B4\_0005.png,DE102017104766B4\_0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_1^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102017104766B4\_0005.png,DE102017104766B4\_0007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2{\text{sin}}^2\left(\alpha \right)}}}$$

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.

If one looks with a camera 5, which is located in the medium with the refractive index n ₁ , in the direction of the light beam 1 at the medium with the refractive index n ₂ , which is the transparent body 4, then the camera 5 sees a point P ₁ on the underside of the transparent body. Since the light beam that the camera 5 captures is refracted at the interface between the transparent body 4 and the environment with the refractive index n ₁ , which is usually air, the camera 5 sees the point P ₁ offset by the offset V from a point P2 ( 1 If the transparent body 4 were not present, the light beam would not be broken and the camera 5 would see the point P ₂ , which is offset from the point P ₁ by the distance V.

If a pattern is arranged on the underside of the transparent body 4, the pattern can be captured with the camera 5 offset by the distance V compared to a state in which the transparent body with the refractive index n ₂ 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 certain angle of incidence α or for a certain angle of observation γ.

This observation angle γ is related to the angle of incidence via the following formula: $$\gamma =90°-\alpha$$

Thus, the thickness of the transparent body can be determined directly from the measured offset V and knowing the observation angle γ or the angle of incidence α.

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. When producing glass plates, there is a considerable need to check their thickness in a simple manner.

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

A measuring pattern 11 is printed on the top of the conveyor belt 9. The measuring 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 distributions being preferred that have as many and as large as possible unique dot sections. Such a distribution can, for example, correspond to the sequence of maximum length or maximum sequence (MLS: Maximum Length Sequence).

A position sensor is coupled to the underside of the conveyor belt 9, which in the present embodiment is an incremental encoder 11. The incremental encoder 11 is coupled to the conveyor belt 9 with a roller, so that the roller is rotated 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 certain angle of rotation of the roller and thus a certain path of the conveyor belt 9.

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

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 measuring pattern printed thereon.

The observation angle γ is not equal to 90°. The observation angle is preferably greater than 20°, in particular greater than 25° and preferably greater than 40°. The observation angle can be less than 70° and in particular less than 65° and preferably less than 60°.

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

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.

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

The camera 5 is preferably controlled in such a way that the individual frames scan several lines of the conveyor belt, each of which is at the same distance from one another. This can be achieved, for example, by moving the conveyor belt 9 at a constant speed. direction of travel 8 and the frames are picked up at a constant frequency. The speed at which the conveyor belt 9 is moved can be monitored by the central control device 12 by means of the incremental encoder 11.

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 recorded each time the conveyor belt 9 has covered a predetermined path. In this way, the conveyor belt 9 can be moved at different speeds and a measurement image is still generated from several individual frames in which the individual frames are equally spaced from one another.

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 did 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 4a can be recognized. 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 beams that hit the conveyor belt 9 at the edge are most inclined relative to the conveyor belt 9 in the transverse direction 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 in the evaluation. Therefore, the arrangement with a line camera in which the scanning line 14 is arranged transverse to the direction of movement 8 and the use of the observation angle γ, which includes the angle between the light beams 1 and the conveyor belt 9 or the corresponding measuring pattern in the direction of movement 8, is preferred ( 4b) .

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.

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

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 that is on the conveyor belt 9, whereby the measurement pattern is offset by the distance V due to the refraction of the 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 reflection occurs from 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. With a refractive index of two (n ₁ = 2.0) for the material being examined and a refractive index of 1 (n ₂ = 1.0) for the ambient air, the critical angle is approximately 30°. The observation angle γ is therefore preferably greater than 60°, in particular greater than 65°, preferably greater than 70°.

The measurement image thus represents an image of the measurement pattern, whereby the measurement pattern in the measurement image is on the one hand offset a little 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 in the edge area. Due to the inclination of the light rays in relation to the conveyor belt 9 in the transverse direction, which is more pronounced in the edge area than in the center ( 4a) , the individual pixels of the measurement pattern are offset a little inwards in the measurement image, whereby the offset is greater the further out the pixels are. Since the inclination of the light rays in the transverse direction with respect to the conveyor belt or the measurement pattern is much 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 "uncompress" the measurement image in the transverse direction by, for example, recording certain feature points and comparing them with the corresponding feature points in the undistorted measurement pattern, so that the measurement image in the transverse direction can be correspondingly undistorted based on this comparison. Such methods are known, for example, from the above-mentioned document Image Registration Techni ques: A Comprehensive Survey by Guido Bartoli.

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.

The determination of an offset V for a specific point F ₀ (x, y) of the transparent body 4 is explained below. This point F ₀ (x, y) is a specific point of 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 in such a way 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.

The measurement image, which shows the measurement pattern with the transparent body 4 located on it, is compared with the reference image. For this purpose, a reference section 16 is selected in the reference image around the pixel F ₀ (x, y). In the present 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 F ₀ (x, y) of the reference section 16 is superimposed with a pixel F ₁ (x, y). The pixels F ₁ (x, y) of the measurement image are selected such that, starting from a pixel that is offset by one pixel from the pixel F ₀ (x, y) of the reference image in the direction of movement 8, they are superimposed successively.

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.

Since the position of the measurement pattern was also recorded by means of the position sensor or incremental encoder 11 when the measurement image was recorded, the position of the measurement pattern with respect to the camera is known when the individual frames were recorded. This position information is stored together with the measurement image. When the measurement image is compared with the reference image, these images are assigned to one another using this position information. In the present exemplary embodiment, this means specifically that the point F ₀ (x, y) of the reference image corresponds to the point F ₁ (x, y) in the measurement image that was recorded at the moment at which the point F ₀ (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 F ₀ (x, y) of the measurement pattern is at the point P ₂ according to 1 was located, whereby the camera, due to the presence of the transparent body 4, saw the point P ₁ which is shown in the measurement image at the point F ₁ (x, y).

Since the point F ₀ (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 F ₀ (x, y) was located at the point P ₁ , the image of this point F ₀ (x, y) of the measuring pattern in the measuring image is offset by the offset V in the Y direction (i.e. at F ₁ (x, y + V)).

Starting from point F ₁ (x, y) in the measurement image, the number w of image points 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 therefore w + b - 1 and is selected so that the maximum possible thicknesses d of the transparent body 4 can be reliably recorded. The width of the search window 16 is b.

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 superimposed. 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 is also suitable as a correlation value. This can 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 filter, or a minimum spending tree.

The position of the reference section 16 in the search window 17 which 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 set value that is assigned to the corresponding pixel F ₀ (x, y) in the reference image.

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

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.

Based on the offset V and the observation angle γ, the thickness d can be calculated at all points F ₀ (x, y). Since the thickness is proportional to the constant k given above at a constant observation angle γ, the thickness d can be determined in real time for all points in the measuring range.

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.

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

The measuring device is very simply designed. In the preferred embodiment according to the present exemplary embodiment, in which a line camera is used whose scanning line is aligned transversely to the direction of movement, the observation angle is constant along the entire scanning line 14, so that the same calculation can be carried out along the entire scanning line or along the entire width of the measuring area in order to calculate the thicknesses.

In 3 Two diagrams are shown. 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 or in the Y direction. The observation angle is aligned along the direction of movement. This diagram is based on a measurement arrangement in which, unlike in 1 the camera does not look at the measuring pattern in the direction of movement but against the direction of movement 8. This means that the offset of the pixels is negative.

The lower diagram in 3 shows a diagram in which the observation angle is aligned perpendicular to the direction of movement. This diagram shows the displacement of the image points 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 is greater the further out the image points 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.

An embodiment of the invention with a line camera is explained above. Within the scope of the invention, it is also possible to use an area camera. When using an area camera, it is expedient to provide a telecentric lens on the object side, since this means that the camera only records light rays that are parallel to one another. This ensures that the same observation angle is present over the entire scanning area. 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 one after the other in the measuring area.

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 of 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

beam of light

2

interface

3

Lot

4

transparent body

5

camera

6

device for measuring thickness

7

conveyor system

8

direction of movement

9

conveyor belt

10

role

11

incremental encoder

12

central control device

13

lighting device

14

scan line

15

evaluation device

16

reference section

17

search window

Claims (20)

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 measuring pattern with a camera (5) at a predetermined observation angle (γ), - determining an offset (V) of the measuring 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). procedure according to claim 1 , characterized in 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). procedure according to claim 2 , characterized in that the reference measurement is linked to the measurement with the transparent body (4) in that the location of the pattern with respect to the camera (5) is recorded in both measurements and the two measurements are compared with the same location of the measurement pattern with respect to the camera (5). Method according to one of the Claims 1 until 3 , characterized in that the location of the measuring pattern is detected during optical scanning and the offset is determined on the basis of the pattern information contained in the measuring pattern. procedure according to claim 3 or claim 4 , characterized in 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). Method according to one of the Claims 1 until 5 , characterized in that 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. Method according to one of the Claims 1 until 6 , characterized in 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. procedure according to claim 7 , characterized in that the distance between matching feature points is measured only along a preferred direction. procedure according to claim 8 , characterized in that the camera (5) is a line 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 camera (5) aligned transversely to the direction of movement (8). Method according to one of the Claims 7 until 9 , characterized in that the feature points are extracted by means of correlation. Method according to one of the Claims 1 until 10 , characterized in that the camera (5) scans the measurement 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 measurement pattern is determined from the position of at least two characteristic points of the measurement pattern in the measurement image generated with the camera compared to a fictitious scan without a transparent body at the same observation angles (γ). procedure according to claim 11 , characterized in that the distance between the characteristic points in the measuring pattern is taken into account when determining the offset. Method according to one of the Claims 1 until 12 , characterized in 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 accordingly the offset (V) of the measuring pattern in the direction of movement (8) or transversely to the direction of movement (8) is determined. 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 measuring 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). device according to claim 14 , characterized in that the camera (5) is a line scan camera. device according to claim 14 or 15 , characterized in 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). Device according to one of the Claims 14 until 16 , characterized in that an illumination device (13) is provided for illuminating the measurement pattern, wherein the illumination device (13) preferably has a diffuse light source. Device according to one of the Claims 14 until 17 , characterized in that the camera (5) has an object-side telecentric lens. Device according to one of the Claims 14 until 18 , characterized in that the measurement pattern is a random pattern. Device according to one of the Claims 14 until 19 , characterized in that a central control device (12) is provided which is designed to carry out a method according to one of the Claims 1 until 13 trained.

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