EP4133260A1

EP4133260A1 - Method and inspection device for optically inspecting a surface

Info

Publication number: EP4133260A1
Application number: EP21715496.2A
Authority: EP
Inventors: Stefan Leute; Koichi Harada
Original assignee: Isra Vision GmbH
Current assignee: Isra Vision GmbH
Priority date: 2020-04-09
Filing date: 2021-03-22
Publication date: 2023-02-15
Also published as: US20230140278A1; CA3179520A1; BR112022020214A2; JP2023521175A; MX2022012556A; DE102020109945A1; WO2021204521A1; CN115398213A; KR20230014686A; IL297164A

Abstract

The invention relates to a method for optically inspecting a surface (10) of an object (1) and to an inspection device (9). In the method, a pattern (13), which is chronologically periodic during an image-capturing sequence and has different illumination patterns (130), is generated on the surface (10) by means of an illumination device (8) of the inspection device (9) and, in the image-capturing sequence, multiple images of the pattern (13) on the surface (10) are captured by means of an image-capturing device (7) of the inspection device (9), the generation of one of the different illumination patterns (130) being synchronised in each case with the capture of one of the images of the pattern (13); the phase pattern (13) is determined from the sequence of known illumination patterns (130) captured in at least one pixel; and defects (4, 5) on the surface (10) are identified from deviations of the illumination pattern (130) captured from the known illumination pattern (130) generated. The illumination device (8) and the image-capturing device (7) are arranged at the reflection angle (α), wherein the object (1) is moved relative to the inspection device (9), and the duration of the image-capturing sequence is selected such that a sequence reflection region (17) can be considered constant (figure 4b).

Description

Method and inspection device for the optical inspection of a surface

The invention relates to a method and an inspection device for the optical inspection of a surface of an object, as well as an advantageous use of the method and the inspection device. When the method is used, an illumination device of the inspection device is used to generate a pattern which is periodic over time during an image recording sequence and has different illumination patterns on the surface. In or during this image recording sequence, several images of the pattern are recorded on the surface by means of an image recording device of the inspection device.

The generation of one of the different lighting patterns is synchronized with the image recording of one of the images of the pattern in such a way that each image from the image recording sequence is recorded with a known lighting pattern of the various lighting patterns. In other words, exactly one lighting pattern is visible in each camera image. The synchronization of image recording and pattern generation in particular ensures that the lighting pattern does not change during the exposure time of an image recording. The phase pattern is determined in at least one image point from the sequence of the recorded known lighting patterns. Because the pattern or the periodically different lighting pattern of the pattern are known, the pixel can be assigned to a point of the known pattern. On the surface, errors from deviations from the lighting pattern recorded in at least one image to the known lighting pattern generated are recognized. Defects on the surface lead to distortions of the known pattern or of the one known illumination pattern recorded in one image. This can be done using an image evaluation known in principle to the person skilled in the art by means of suitable algorithms, for example a suitable computing device, which identify and output errors. By scanning several surface areas one after the other, ie a repeated application to different areas of the surfaces, the entire surface or selected sections of the surface can be inspected.

In surface inspection, one of the most important tasks is to find and classify defects which, due to their top graphic characteristics, cause light deflections. These errors are often not perceived by the eye as topographical errors, but only as changes in brightness or shades on the surface. An inspection in motion is often necessary or at least an advantage. Particularly preferred applications for such upper surfaces in the context of this invention will be described later.

In principle, the method according to the invention is suitable for the optical inspection of reflective surfaces. Reflective surfaces within the meaning of the invention include both ideally reflective (i.e. reflective) surfaces and surfaces which, in addition to reflective properties, also have a certain scattering effect. The criterion for this is that a surface illuminated with a pattern (including a pattern projected onto the surface) can be optically recorded in an image.

A method for inspecting surfaces that has been known for a long time is deflectometry. The image of the reflection of a known pattern on the surface is recorded by a camera and evaluated with a computer. Defects in the surface lead to distortions in the pattern on the surface, which are recognized. If the recording geometry and the pattern geometry are known, it can also be used to determine a 3D topography of the surface. Various methods such as this are known to those skilled in the art is carried out. These are assumed within the scope of the invention and are no longer described in detail.

The basic principle of deflectometry is to determine the deflection of a light beam striking the surface by identifying the point in the pattern on which a visual beam from a camera (recording device) and reflected on the surface is incident. In other words, the deflection of the line of sight on the surface in reflection is determined, which depends on the direction of the surface normal (a straight line perpendicular to the surface in the reflection point) at the corresponding point (reflection point). From the normal field determined in this way, the topography of the surface can be determined e.g. by integration.

Common methods for finding a point are so-called phase shift methods, in which a periodic pattern is used as the pattern and the phase position of the pattern is determined in which the point to be determined is located.

A basic distinction must be made here between processes that get by with one image of the pattern or that require several images.

Processes that manage with one image have the advantage that they can also be used on a moving surface and thus initially appear more suitable for the inspection of, for example, web goods or in production processes. However, they have the disadvantage that they are more prone to failure or require a second, physically present pattern in the beam path. From WO 98/17971 A1, for example, a method is known how the smallest beam deviations can be recognized and determined. Essentially, a stripe pattern is observed there with a camera. The method described manages with a single image because the camera's pixel grid is used as the second pattern will. However, this has the disadvantage that a very precise adjustment of the camera and pattern is required. In the industrial environment, for example, in production processes, this can hardly be achieved or not at a reasonable cost.

Processes that work with several images are much more robust against interference and do not require any complex adjustment. For this purpose, the pattern is displayed and recorded one after the other in several mutually shifted phase positions. A particularly simple evaluation results when a stripe pattern with a sinusoidal brightness curve is used, which is recorded four times with a shift of a quarter of a period length. However, other patterns and sequences of patterns are also possible. The phase position within the pattern then results from the sequence of gray values in each pixel. This method is extensively described in relevant textbooks and articles (e.g. Gorthi and Rastogi, Fringe Projection Techniques: Whither we are ?, Proc. Optics and Lasers in Engineering, 48 (2): 133-140, 2010). The disadvantage here, however, is that several images of the same point on the surface are required. When inspecting foils or other web goods in the produc tion process or basically of surfaces moving relative to the inspection device, however, it is practically impossible to take several pictures of exactly the same place on the surface, since the surface moves continuously. For example, lanes running at high speed cannot be stopped during production. The problem could be solved by an inspection device traveling synchronously with the train. However, this solution is technically complex and therefore expensive, and it takes up a lot of space, which is often not available, especially in production environments.

From EP 2 390656 B1 a method is known in which a running web surface is preferably viewed with a line camera. The lighting is a quickly switchable pattern lighting installed across the track (in front of preferably LED lighting). This lighting consists of individually controllable LEDs or LED modules that can be used to dynamically generate different lighting patterns very quickly. The switchover is synchronized with the image recording so that images with different lighting patterns can be recorded from the surface in quick succession. In particular, scanning and switching can take place so quickly that the distance between the image recordings in the feed direction is very much smaller than the extent of a pixel in the feed direction. Thus, pictures can be taken in almost the same place. However, recordings at exactly the same point cannot be made with it.

The object of the invention is to propose a robust option for inspecting moving surfaces, which can also be easily implemented in an industrial environment, for example in production processes

This object is achieved according to the invention by a method according to claim 1 and an inspection device according to claim 12.

In the method described at the outset, it is provided that the lighting device and the image recording device are arranged at the angle of reflection (each based on the surface normal aligned perpendicular to the surface in the reflection area). The reflection angle means that the marginal rays (ie the visual rays emanating from the edge of the image point) of the image point are reflected in the reflection points on the surface and mark the visible area of the lighting pattern (pattern area) in the image point. In other words, the reflection of the lighting pattern of the pattern on the surface is just imaged in the image points of the image recording device. A camera (as a recording device) therefore looks exactly at the pattern (ie the lighting device, which can be designed, for example, as a lighting line.) In the case of a moving object, the angle of reflection does not change as long as the shape of the surface and its arrangement relative to a stationary inspection device does not change. This applies to a flat surface or a slightly curved surface if the curvature is the same on average and the direction of the surface normals of the surface changes only negligibly (at least in relation to the direction of the lines of sight). This can be the case, for example, with a wavy surface structure in which the change in direction of the surface normals is small. Small means that the change is only so great that the pattern area remains visible in the pixel. The pattern area must therefore be correspondingly wide in the feed direction. As soon as this is no longer possible, the method according to the invention cannot be used with a stationary inspection device. In this case, however, the inspection device can be moved over the curved surface in a reflection arrangement in accordance with the method proposed according to the invention.

If, due to a known periodicity of the change in direction of the surface normals, mechanical tracking of the inspection device is possible in such a way that the reflection conditions are met or, through the use of flat lighting device and recording device, the reflection angle is in the recording area, and the image points correspond to the periodically occurring If the angle of reflection are selected, the method can also be used for curved surfaces.

Independently of this, the method can be used to determine the topology of defects as long as the changes in the pattern can be recorded in a defined manner by the inspection device. In order to be able to inspect larger surface areas or to enable a continuous inspection, e.g. defines controlled direction of movement.

For the phase shift method described at the outset, it is actually necessary that the images that belong to an image sequence each record the same point on the surface. Since a surface moved relative to the receiving device, for example a moving web of material, is inspected here, this is not possible, please include. In order to still be able to use the method and to be able to recognize the phase of the pattern, the duration of the image recording sequence is selected to be so short that a sequence reflection area can be viewed as constant. The sequence reflection area is defined as the total surface area covered or captured by the reflection areas in the respective images from the image recording sequence. Put simply, the images of an image recording sequence are recorded so quickly one after the other that the travel path from the first to the last image of this image recording sequence is so small that the recorded surface area (reflection area) can still be viewed as approximately the same point on the surface.

The total surface area covered by the reflection areas in the respective images from the image recording sequence results from combining all reflection areas of all individual images that were recorded during the image recording sequence into a common area, which is then referred to as the sequence reflection area. This surface area can then be viewed as at least approximately constant if the reflection areas of all images from the image recording overlap at least 40% or more, preferably at least 60%. However, these values are not to be understood as fixed values, but rather typical guide values that the specialist may need. can experimentally adapt to the respective conditions. In principle, the methods can be used well as long as the optical conditions mean that significantly less than one period length of the pattern is imaged on an image point. Particularly critical are concave curvatures of the surface due to the concave mirror effect, which are used to map large pattern areas in one pixel. For the detection of defects, a range of 40% to 70% overlap should be sufficient, with an estimate of the surface normals (ie an estimate of the topology of the surface, a range of 60% to 80% overlap. Depending on the shape of the surface and the type of Occurring errors may also result in other areas that the person skilled in the art can determine and / or specify empirically by test measurements when setting up a corresponding inspection device based on the teaching of the invention. In other words, the invention proposes that the duration of the image recording sequence is chosen so that the images recorded within the image recording sequence are recorded so quickly one after the other that the displacement of the surface due to the movement of the object from the first image to the last image of the image recording sequence is so small that the reflection areas of the first image and the last picture as the same area a can be viewed on the surface. Compared to a measurement when the object is at a standstill, there is a measurement error that is smaller, the better the above condition is met.

The reflection area on the surface that is recorded in the image point (defined in the smallest resolution by a camera pixel or possibly by combining several camera pixels) is predetermined by the recording geometry (distance, recording angle) and the recording optics. Due to the arrangement of the recording device and the lighting device at the angle of reflection with reference to the surface normal, a change in the angle of one of the two devices must also be reproduced in the other device will. This makes changes in the angle of reflection comparatively expensive. The same applies to the changes in the recording optics. According to the invention, the size of the reflection area and / or the pattern area depicted in the reflection area can be varied or adjusted comparatively easily via the distance between the recording device and / or the lighting device. However, this also requires a change to the structure of the inspection device.

According to the invention, it is simpler to influence other parameters when carrying out the method proposed according to the invention. Suitable parameters for carrying out the method are described below. In order to adapt the duration of the image recording sequence as a function of a predetermined speed and direction of the movement of the object so that the sequence reflection area can be viewed as constant, it is possible, according to a preferred embodiment of the invention, to take one or more of the measures listed below .

Thus, when carrying out the method according to one embodiment of the invention, provision can be made for the size of the image point to be set. In the simplest case, the size of the image point can correspond to the pixel resolution of the camera (used as an image recording device). At a given distance from the camera and a given focal length of the camera, this represents the highest possible resolution. The higher the resolution of the camera, the smaller the reflection area assigned to a pixel on the surface and the smaller the defects that can be seen on the surface. One way of changing the size of the image pixel is to change the pixel resolution of the camera. The pixel resolution of the camera is - in the case of the digital image recording preferred according to the invention - predetermined by the photo chip used as the recording sensor of the camera, on which individual pixels (sensor pixels) which detect (integrate) the light falling on this pixel during the exposure time. By reducing the resolution, the size of the image point can also be achieved by combining several sensor pixels of the camera into one image point. An image point can also be referred to as a pixel. Image pixels and sensor pixels are different, however, if several sensor pixels are combined into one image pixel.

According to one embodiment, the setting of the size of an image point can take place by combining several pixels of an exception sensor (sensor pixel) of the recording device to form an image pixel. In one variant, according to the invention, the number of combined pixels in the direction of movement of the object and transversely to the direction of movement of the object can be selected differently. It can be useful to increase the size of the reflection area in the direction of movement of the object, while accepting a lower resolution, in order to achieve a greater over-thinking of the reflection areas of the individual images in a respective image recording sequence. This increases the sequence reflection area in the direction of movement of the object. A higher resolution can be maintained across the board. The resolution transverse to the direction of movement of the object and its surface is determined solely by the recording geometry, ie essentially by the size of the image points (limited by the pixel size of the recording sensor of the image recording device with regard to the smallest possible extent), the focal length of the lens and the viewing distance . The resolution transverse to the direction of movement is not influenced by the movement.

A motion blur occurs along the direction of movement of the object. Because the camera integrates all of the light that occurs during an exposure in an image point (pixel of the image that does not necessarily coincide with a pixel of the recording sensor) during an image recording If this pixel falls, the observed surface is enlarged in the direction of movement that is mapped onto the one pixel. In relation to the moving surface (also referred to as the reflection area assigned to the image point), the image point appears elongated, so to speak. “Longitudinal” and “transverse” refer to the direction of movement and do not necessarily have to match the row and column directions of the cameras. With an oblique viewing angle, each pixel appears stretched at an angle in relation to the direction of the rows and columns of the camera.

In an image sequence (during an image acquisition sequence) that is acquired for a multi-image phase shift process, the same point on the surface must actually be imaged in all images in each image point (image pixel). However, when several images are taken one after the other, they are shifted relative to one another in relation to a moving surface. In order to compensate for this, measures are therefore taken according to the invention which result in the reflection area of the different images still being able to be viewed approximately as the same point on the surface. The change in the size of the image point can contribute to this in the manner described above.

According to the present invention, a further measure can be to set the duration of the image recording sequence when the method is carried out. The duration of the image recording sequence, in other words the time required to record all images of the one image recording sequence, determines - at a given speed of movement of the object or the surface - how far the surface area corresponding to the reflection area of the first image extends to the recording of the last picture. This results in the size of the sequence reflection area and the overlap of the reflection areas of the individual images to be set according to the invention. Basically, the overlap is greater, the shorter the duration of the image recording sequence. Apart from the limits of the maximum scanning frequency of the recording sensor and the shortest possible exposure time of the recording device, the scanning frequency (defined as the frequency of successive image recordings) and / or the exposure time can be adapted. The shorter the exposure time, the sharper the recorded image (reduction of motion blur) and the faster images can be recorded one after the other (sampling rate). The exposure time can be shortened by increasing the brightness of the pattern produced on the surface and / or by opening the aperture of the recording optics. The exposure time can be shortened by increasing the brightness or enlarging the aperture (usually defined in optics by smaller f-numbers). It is therefore sensible to use a lighting device with a high but dimmable light intensity.

Suitable lighting devices can be built from individually dimmable LEDs which, when individually dimmed, enable the creation of a pattern and, when dimmed together, enable the overall light intensity to be set. In principle, it can be preferred to operate the lighting device with maximum light intensity and to reduce the exposure time until suitably exposed images are recorded.

According to the invention, when setting the duration of the image recording sequence, at least one of the variables listed below can be adjusted: Be the exposure time of an image, brightness of the pattern generated on the surface, sampling frequency of the recording sensor and / or the number of images per image recording sequence. It is also possible to customize all or a selection of more of the sizes. Thus, according to the invention, the duration of the image recording sequence can of course also be changed by changing the number of images per image recording sequence, whereby a shortening of the image recording sequence can be achieved by reducing the number of images, and vice versa.

Furthermore, according to the invention, the sensitivity of the measurement can also be influenced by the choice of the illumination distance (at the same time also the viewing distance between the recording device and the sample) and the viewing angle. Larger distances as well as flatter viewing angles (i.e. flatter in relation to the surface; perpendicular to the surface would be maximally steep) lead to a higher sensitivity. Particularly in the case of partially reflective and partially diffusely reflective surfaces, a flat viewing and illumination angle between (e.g. <30 °) and / or the greatest possible illumination distance can be selected. According to the invention, the greatest possible illumination distance can mean that an available space is used for the arrangement of the illumination device. For example, the lighting distance (distance between the lighting device and the surface) can be selected to be greater than the distance between the receiving device and the surface, with typical values in the range between 1 and 10 times, for example. The person skilled in the art will, if necessary, select the values experimentally adapted to the respective application, which corresponds to the basic teaching of the invention that smaller lighting and viewing angles and / or a larger lighting distance (between the recording device and the lighting device) reduce the sensitivity in many Cases increased.

The aim of recording multiple images is to determine the phase of the pattern in order to identify the position of the known illumination pattern in a recorded image point. This can cause errors in the surface can be recognized by distortions of the pattern on the surface. According to one embodiment, three image recordings can be made, for example. For example, it is possible to periodically design the generated pattern asymmetrically so that the phase of the pattern can be clearly determined from three images. Alternatively, the pattern can also be designed periodically symmetrically, and the images can be recorded asymmetrically, for example by varying the scanning or image recording frequency between different images within the same image recording sequence.

An application preferred according to the invention, however, provides for scanning with at least or exactly four images within the same image recording sequence. The pattern itself can, for example, be a brightness sinusoidal distribution that is recorded with the same scanning sequence in four different phase positions. From this, the phase of the pattern in each of the images can be precisely determined in a simple manner. For example, the phase shift between the phases in the image recording sequence of successive images can be just% of the period length of the pattern. However, other phase shifts between the images of an image recording sequence are also possible.

According to a further aspect of the invention, the lighting pattern can be generated by the lighting device in such a way that the area of the lighting pattern that is visible in the image points of the images recorded during each image recording sequence can be viewed as constant.

The area of the illumination pattern (pattern area) visible in the image points during an image recording sequence can be viewed as constant as long as this pattern area still remains visible at all in the image point and the detected intensity of the pattern area does not change significantly. This can be assumed, for example, if the recorded intensity does not change by more than 10%, preferably by no more than 8%, and particularly preferably by no more than 4% during an image recording sequence, or another defined criterion is complied with. In principle, the criteria already explained above also apply here

To this end, according to a preferred aspect of the invention, the period length of the pattern in the lighting pattern can be selected such that, depending on a topology of the surface in the direction of the pattern profile, a change in intensity can be viewed as sufficiently constant, i.e. in other words the change in intensity depending on the respective circumstances does not exceed the appropriate criterion. The person skilled in the art can determine the selection of the criterion experimentally when setting up the system and certain patterns.

The topology of the surface is determined in particular by its curvature, which changes the direction of the surface normal. The direction of the surface normal is correlated with the angle of reflection. Using the topology of surfaces that are to be examined with the method according to the invention, it is therefore possible to use the resulting reflection angle with a known arrangement of the inspection device to determine which pattern area of a lighting pattern is mapped into the image point during a defined duration of the image recording sequence will. By specifying the period length, the lighting pattern can be set so that the aforementioned criteria are met. This means that the process can be used flexibly for defined inspection tasks. According to a further aspect of the method proposed according to the invention, it can be provided that the periodic pattern is generated along the direction of movement of the object, transversely to the direction of movement of the object or alternately along and transversely to the direction of movement of the object.

In the case of a pattern along the direction of movement of the object, with a curved surface to be inspected, the already discussed motion blur and the shift of the reflection area with a shift of the pattern area seen through the image point due to the change in the angle of reflection and associated with a change in intensity, because the intensity of the pattern changes in that direction.

In the case of a pattern transverse to the direction of movement, the reflection area also changes. However, because the pattern has the same intensity along the direction of displacement of the object, a change in the angle of reflection does not necessarily lead to a change in the intensity. The intensity measured in the image point remains the same as long as the image point captures the same pattern area and the pattern area captured in the image point does not shift transversely to the direction of movement due to a curvature of the surface.

According to the invention, this difference can be taken into account, for example, in the previously described adaptation of the period length of the pattern as a function of the orientation of the pattern along or across the direction of movement of the object. In particular, according to the invention, the period length of the pattern for pattern along and across the direction of movement can particularly preferably be different.

In addition, according to the invention, a known curvature of the surface of an object in a defined surface area to be inspected can also be used to establish suitable criteria in order to determine a non-defective surface to distinguish from a defective surface and / or to correct the deviation resulting from the known (expected) surface shape in the evaluation of the recorded images within the scope of the error detection.

By alternately generating patterns along and across the direction of movement, different defects, in particular specifically directed defects on the surface, can be systematically detected more reliably.

In one embodiment of the method proposed according to the invention, the recording device can be focused in such a way that the illumination pattern recorded in the image is out of focus.

This can be achieved, for example, in that the recording device is not focused on the pattern, but on the surface or another defined point. By specifying certain aperture and focus settings, the depth of field or depth of field can also be selected in a targeted manner according to the invention in order to depict the illumination pattern in the image in a blurred manner, but the surface in sharp focus. As a result, a sharp distribution of brightness is washed out. For example, a sharp pattern consisting simply of alternating sharply defined light-dark areas can be mapped approximately as a sinusoidal brightness curve. In this case, a particularly simple lighting device can be used without the need for additional optical elements to generate the desired brightness curve. In addition, the brightness distribution becomes more blurred, which can have a positive effect, in particular in the case of curved surfaces and the associated effects, if displaced pattern areas are mapped onto the images recorded in one of the image recording sequences. In many cases, the surface to be inspected is not ideally reflective, but reflects semi-diffusely. The reflection is directed, but it scatters in a relatively large solid angle, ie the bidirectional reflection distribution function BRDF (English. Bidirectional Reflectance Distribution Function) has a scattering lobe of medium width. This also leads to a very helpful washing out of the brightness distribution of the pattern in the images, as long as the scattering lobe is still so narrow that there is sufficient modulation of the mirrored pattern in the camera image and a reflective, albeit not ideally reflective, surface is assumed can be. Such a property of the surface can also be used to achieve an effect similar to that of the described blurring of the camera on the pattern. Such an (additional) effect must be taken into account when defocusing, because part of the (in this case desired) blurring is already generated by the surface itself.

On the other hand, the surface must reflect enough that a pattern can be observed at all. With relatively few reflective surfaces, it is therefore advantageous to choose a viewing and lighting angle that is as flat as possible and to make the lighting distance large.

It can be particularly advantageous if, during the inspection of the surface carried out according to the invention, the three-dimensional topography of the surface of the object is determined by means of deflectometric methods. If, as in the method proposed according to the invention, the recording geometry and the pattern geometry are known, a 3D topography of the surface can also be determined. There are various known ways in which this can be carried out. Deflectometry primarily determines the deflection of a light beam striking the surface by determining the point of the pattern on which a visual beam emanating from the camera (recording device) and reflected on the surface strikes. So it will be the Deflection of the line of sight determined, which depends on the surface normal at the corresponding point. The topography of the surface can be determined from the resulting normal field of the surface, for example by integration. A particularly preferred use of the method described above or parts thereof and / or the inspection device described afterwards arises when inspecting web goods, for example during a production process or after their manufacture, or of especially treated, curved or flat surfaces.

An important concrete application example is the inspection of an FCCL film, during or after production. FCCL foils (Flexible Copper Clad Laminates) is the core material for the manufacture of flexible printed circuit boards. FCCL foils usually have a thickness of up to about 100-150 μm and have, for example, a polyamide core (or generally a plastic foil) which is laminated on one or both surface sides with a copper foil. During the lamination, wrinkles can arise that should be known by the method proposed according to the invention. During the surface inspection, one would also like to detect lamination errors, in particular so-called lamination folds 4 (as shown schematically in FIG. 1) or inner folds 5 (as shown schematically in FIG. 2). In the case of lamination creases, the material formed slight creases that were pressed flat again during the lamination process. Inner folds are caused by folds in the inner plastic film that have been laminated in. Both defects are very difficult to see with the human eye, as the foils are very thin and the surface is therefore only slightly affected by the folds. The faults can only be recognized if the direct reflection of the light on the surface of the film is viewed. This is made more difficult by the fact that the copper foil reflects half-diffusely. With other laminated foils, this is Appearance is important, which is disturbed by such errors despite the low topographical characteristics.

When inspecting curved surfaces, such as painted containers or car bodies, the inspection device according to the invention is programmed according to a preferred embodiment, for example by means of a corresponding handling device, guided over the curved surface in such a way that the lighting device and the recording device are each at a reflection angle to the surface being held. In this case, the Inspektionseinrich device is moved relative to the mostly stationary object. This generates a relative movement of the object or object surface to the inspection device. This type of relative movement is also meant in this description when speaking of a moving object relative to the inspection device. Often the smallest, flat topographical defects that can interfere with the appearance or function of the surface are to be found on the curved surface. It is often helpful to also measure such defects in three dimensions, i.e. to determine the 3D topology of the surface and the defect.

The invention also relates to an inspection device for the optical inspection of a surface of an object and its use for the applications described above. The inspection device is provided with a lighting device and a receiving device, which are aligned with one another in such a way that a visual ray emanating from the receiving device as a visual ray reflected on the surface hits the illuminating device when a surface normal that is perpendicular to the surface at the point of incidence of the visual ray the angle between the outgoing line of sight and the reflected line of sight just halved. In other words, the recording device and the lighting device of the inspection device are thus arranged at the angle of reflection with respect to the surface. The lighting device is designed to provide a temporally periodic pattern with different lighting patterns to generate, and the recording device is designed to record images of the pattern reflected on the surface synchronously with the generation of the lighting pattern during the image recording sequence. The inspection device also has a computing unit for controlling the inspection device and for evaluating the recorded images, a processor of the computing unit being designed to carry out the above-described method or parts thereof.

According to a preferred embodiment of the inspection device proposed according to the invention, the lighting device has individually controllable light elements in a row or matrix arrangement. The recording device can further preferably have a recording sensor for recording images imaged on the recording sensor via recording optics, the recording sensor having individual sensor pixels (camera pixels) in a row or matrix arrangement.

The lighting device can, for example, be designed as a lighting line, which is preferably arranged transversely or longitudinally to the feed direction (direction of movement of the object or the surface relative to the inspection device). A lighting line as a row arrangement of individually controllable light elements can consist of many LEDs or LED modules arranged next to one another, which can be individually switched synchronously with the image recording. The periodic patterns required for the phase shift method are generated in rapid succession with the lighting device. The recording device can also be designed, for example, as a line camera which, if necessary, can also be constructed from several line camera modules arranged next to one another. In such an arrangement, the composite image field of the line camera is a line on the upper surface (the so-called scan line). This scan line can be perpendicular to the relative Be aligned in the direction of movement of the surface and also has a certain width in the direction of movement, which is very small compared to its length (running transversely to it) and which depends on the pixel resolution of the line camera.

The lighting line can be so long (transverse to the direction of movement) that it covers the entire width of the web to be inspected (or of the desired inspection area on the surface) at the angle of reflection. If the camera and lighting are arranged at the same distance from the surface, the lighting line on each side must be about half a scan line width of a single line camera longer than the scan line observed by all cameras on the surface, with other Ab stand conditions longer or shorter accordingly.

The width of the lighting line (in the direction of movement) can determine the maxima len surface angle that can still be measured with the arrangement. If the surface angle is greater than the maximum surface angle, the camera's line of sight reflected from the surface no longer falls on the lighting and the camera can no longer see anything.

The method can also be used with area cameras (matrix arrangement). Then the scan line becomes the image field because the width in the direction of movement is much larger. The width of the lighting line in the direction of movement can also be increased accordingly. In a variant, an illumination matrix can be used instead of the illumination line. This consists of many individual LEDs or LED modules that are arranged in several seamlessly joined lighting rows, all of which can be switched independently of one another, synchronized with the image recording. This means that the width of a lighting line can also be varied easily by inserting several lighting lines in switched in the same way.

With a lighting matrix, not only can patterns be switched across the direction of the web, but also patterns along the direction of the web. This is advantageous because deflectometry methods primarily measure surface angles or normals, specifically in the direction of the periodic pattern. So if you use a lighting line you can only measure angles perpendicular to the direction of movement, with an illumination matrix you can measure all directions, preferably the two directions along and across the direction of movement.

Show it:

1 shows, in a schematic sectional illustration, an object with a surface to be inspected with a first exemplary defect;

FIG. 2 shows, in a schematic sectional illustration, the object according to FIG. 1 with the surface to be inspected with a second exemplary defect;

3a shows a plan view of the one inspection device according to an embodiment of the invention during the inspection of a flat surface;

3b shows a side view of the inspection device according to FIG. 3a; 1 and 2 is as object 1, the surface 10 of which is to be checked by an inspection device according to the Invention, an FCCL film Darge provides, which serves as the starting material for printed circuit boards. It is a laminated film 1 which consists of three layers, a middle plastic film 3 as the middle layer, onto which copper foils 2 are laminated as outer layers. The upper surface 10 of the object 1 is typically checked for surface defects.

During this surface inspection, lamination errors should also be detected, in particular so-called lamination folds 4 (FIG. 1) and inner folds 5 (FIG. 2). In the case of laminating folds 4, the material has formed slight folds that were pressed flat again during the laminating process. Inner folds 5 arise from the fact that folds have formed in the inner plastic film 3, which folds have been laminated in.

Fig. 3b shows a side view of an inspection device 9 with a lighting device 8 and a recording device 7. On the lighting device 8, a periodic pattern 13 with different lighting patterns 130 is shown, which illuminates or illuminates the surface 10 of the object 1 ( see also top view according to FIG. 3a). The lighting pattern 130 has a brightness distribution 14. As a result, the pattern 13 is also produced on the surface 13. The recording device 7 records the pattern 13 on the surface 1 in an image.

For this purpose, the recording device 7 comprises a recording sensor 11, which generates an image of many image points 12. Optical rays 15 emanating from the (each) image point 12 are reflected on the surface 10 by optics (not shown) of the recording device and strike the pattern 13 generated there as reflected rays 19 on the lighting device 8 Lines of sight 15, 19 are shown. The marginal rays go from the edges of the image point 12 and border on the surface 10 the reflection area 17 from. All lines of sight 15 which hit the surface starting at the pixel 12 in the reflection angle a are located in the reflection area 17 on the surface 10 and are also reflected in the reflection angle a from the surface as reflected lines of sight 19. You meet in the pattern area 18 on the lighting device 8, because according to the fiction, contemporary arrangement, the receiving device 7 and the lighting device 8 are arranged in the reflection angle α based on the surface 10.

The angle of reflection a is defined as the angle between the incident (emanating from the image point 12) or the outgoing (reflected from the surface 10) line of sight 15, 19 and the associated surface normal 16. The surface normal 16 belonging to a line of sight 15, 19 stands in the reflection point 170, at which the lines of sight 15, 19 strike the surface 10, perpendicular to the surface.

Specifically, Fig. 3a shows a line of the recording sensor 11 of the recording direction 7, which along the width of the surface 10, for example. A device 6 moving in the direction of movement web material as an object 1, such as an FCCL film. The recording device 7 can be configured as a line camera with only one sensor line of the recording sensor 11, or as an area camera with several such sensor lines. An image point 12 can be formed from one or more sensor pixels. An image point 12 of the recording device (camera) detects the reflection area 17 on the surface 10 via the optics (not shown) Illumination pattern 130 of pattern 13 at the time of image capture is given. In the example shown in FIGS. 3a and 3b, the lighting device is designed as a lighting line that is oriented transversely to the direction of movement 6 of the surface 10. 3b shows the same arrangement in a side view in which the reflection of the visual rays 15, 19 (as shown in all figures as marginal rays) with the reflection angle α in relation to the surface normal 16 can be clearly seen. The drawn marginal rays of the lines of sight 15, 19 illustrate the size / area of the reflection area 17 on the surface 10 and of the pattern area 18 in the pattern 13.

The. 3a and 3b show the state during an image recording, it being assumed that the movement of the surface 10 moved in the direction of movement 6 can be neglected during the short exposure time of the image recording. If this is not the case, the recorded images show a certain degree of motion blur, which can be counteracted by shortening the exposure time (with sufficiently bright lighting). As already described, in the case of the storage according to the invention, a plurality of images are recorded in chronological order during an image recording sequence. Because the surface moves in the direction of movement 6 during the image recording sequence, the image point 12 in the respective reflection area 17 of the successively recorded images no longer sees the same surface area. Rather, the reflection areas 17 on the surface 10 are advanced towards one another in the successive recorded images.

This is shown in FIGS. 4a and 4b, in which the displacement 61 of the upper surface 10 between the first and the last image acquisition is shown in an image acquisition sequence. The reflection area 17a is the reflection area of the first and the reflection area 17b is the reflection area of the last image recording from the image recording sequence, in each case in hatching rotated by 90 °. The two hatches are superimposed in the overlap area. The entire reflection area 17 over all images of the recording sequence is correspondingly enlarged (based on the total surface 10 covered by reflection areas of the individual recording). The effect is fundamentally similar for the motion blur already explained, with the difference that the entire reflection area is integrated in one image. This leads to a blurring of the picture, provided that a motion blur can be recognized at all.

Because the recording geometry does not change in the case of the flat surface, the displacement of the surface 10 has no effects on the pattern area 18; this remains unchanged during the recording sequence, whereby of course, as already described, the pattern illuminations are generated out of phase. For the sake of clarity, this is not shown in FIG. 4a.

FIG. 4b shows the same situation as FIG. 4a in the side view. The normal surfaces 16a when the image a was recorded were at the same position as the normal surface 16b when the image b was recorded, which is shown here as a snapshot of the arrangement. Because of the flat surface 10, the alignment of the surface normals 16a and 16b is the same, with the result that the pattern area 18 does not change either.

3c and 4c show an arrangement of the inspection device 9, in which the lighting device 8 has a lighting line aligned along the direction of movement 6 of the surface 10. This can be achieved by a line lighting device (with a correspondingly directed line) or by a matrix lighting device that is controlled accordingly. Due to the flat surface, a situation comparable to that shown in FIGS. 3a, 3b and 4a, 4b also results in this arrangement. For a detailed description, reference is made to the description above. In Figs. 3d and 4d an arrangement of the inspection device 9 similar to that of FIGS. 3c and 4c is shown, in which not only the lighting line of the lighting device 8, but also the sensor line of the recording sensor 11 is aligned along the direction of movement 6 of the surface 10 are. The recording device can accordingly be designed as a line camera (with only one sensor line) or as a matrix camera (with several sensor lines arranged next to one another). Due to the flat surface, a situation comparable to that shown in FIGS. 3a, 3b, 3c and 4a, 4b, 4c also results in this arrangement. For a detailed description, reference is made to the description above.

It looks different if the surface is actually not even. This is shown in FIGS. 5a, 5b, 5c and 5d or 6a, 6b, 6c and 6d. The views and arrangements correspond to those with reference to FIGS. 3a, 3b, 3c and 3d and 4a, respectively. 4b, 4c and 4d illustrated views and arrangements. With regard to the general description, reference is therefore made to the above. Due to the curvature of the surface 10, which affects the alignments of the surface normals 16, 16 'and influences the reflections of the visual rays 15, 19, different pattern areas 18a, 18b also result for the different images of a recording sequence.

In FIGS. 5a, 5b, 5c and 5d, the situation is shown for one image each, for example the first image of the image sequences. FIG. 5 a essentially corresponds to FIG. 3 a, the curved sides of the surface 10 showing the curvature of the surface 10 transversely to the direction of movement 6. Because of the curvature of the surface 10, the lines of sight are not reflected in a straight line — in plan view — but rather are also deflected at the reflection point 170, 170 '. Correspondingly, the reflected lines of sight 19 hit the pattern 13 in a pattern area 18 which is at a different point than the mus- terbbereich 18 according to Fig. 3a. 5b accordingly shows that the surface normals 16 and 16 'are directed differently at the reflection points 170, 170' (and have therefore also been identified with different reference symbols). Correspondingly, the angles of reflection a, a 'are different.

6a and 6b show the reflection area 17a (for the lines of sight 15, 19 shown in FIGS. 5a, 5b when recording) and the reflection area 17b (for the lines of sight 15, 19 shown in FIGS. 6a, 6b) together with the overlap area 171. The pattern areas 18a and 18b and their overlap area 181 are shown in a corresponding manner.

The image point 12 is illuminated in the recording sensor 11 by the area 18, 18a, 18b of the pattern delimited by the edge lines of sight 15 (before the reflection on the surface 10) or 19 (after the reflection on the surface) 10, this area 18, 18a, 18b being imaged on the pattern 13 via the reflection areas 17, 17a, 17b of the surface 10 in the receiving device 7. However, each of the line of sight 15 is deflected according to the surface normal 16, 16 ', 16a, 16a' or 16b, 16b 'present at the point.

In Figs. 5a, 5b, 5c and 5d you can see the situation in the first image of the sequence. Again, the camera pixel 12 in the image sensor 11 is illuminated by the area 18 of the pattern delimited by the marginal rays 15 (before the reflection on the surface) or 19a (after the reflection on the surface), this area 18 overlying the pattern 13 the area 17a of the surface 10 is imaged in the camera. Now, however, the line of sight 15 is deflected in accordance with the surface normal 16a or 16b present at the point. The situation in the last image of each image recording sequence is shown in FIGS. 6a, 6b, 6c and 6d. Now the area 18b of the pattern 13 over the area 17b over the shifted surface 10 in the image point 12. The surface normals 16b, 16b 'are now decisive for the reflection of the marginal rays 15 emanating from the camera. Since these are different than in the first image (FIGS. 5a, 5b, 5c and 5d), the area of illumination pattern 130 on the illumination device 8, which is seen or imaged in the pixel 12, is now also shifted. Overall, the image point 12 sweeps over the area 17 of the surface 10 in FIGS. 6a, 6b, 6c and 6d and thus the entire area 18 of the pattern 13 during the image sequence from the first to the last image. The image point 12 sees the area , which is located in both the reflection areas 17a and 17b on the surface 10 and both in the pattern areas 18a and in 18b on the pattern 13. The areas which are only located in 17a or 17b or 18a or 18b are not seen by the image point 12 during the entire image sequence.

It should also be noted that the size relationships in FIGS. 3a, 3b, 3c, 3d, 4a, 4b, 4c, 4d, 5a, 5b, 5c, 5d, 6a, 6b, 6c, 6d are not realistic. The cut areas 171, 181 shown in each case with cross-hatching do not correspond to realistic sizes, but are only used for illustration and understanding. In fact, at least the pattern 13 or the illumination pattern would have to be much longer-wave compared to the size of the image point 12 shown, so that an image point 12 only covers a small fraction of a wavelength. With realistic proportions, however, one would no longer be able to recognize the principle in the drawing.

As already explained, in an image recording sequence that is recorded for a multi-image phase shift method, the same point on the surface 10 must actually be depicted in all images in each image point 12. be det, that is, the same reflection area 17. If you take several pictures in a row, these are based on a moving surface 10 but shifted against each other. The decisive factor for assessing whether what an image point 12 picks up during an image sequence can still be viewed as “approximately the same point” in the sense of the invention ultimately depends on how much the image of the periodic pattern 13 extends over the surface 10 changes in the recording device 7 during an image sequence. This in turn depends on the one hand on the pattern 13 (lighting pattern 130) itself and its distance from the surface 10, on the other hand on the reflection area 17, which is imaged on the image point 13 during the entire image sequence, and how this area (reflection area 17) changes. The area of the reflection area depends on the optical pixel resolution (i.e. the area that is mapped onto a pixel in the viewing plane), the exposure time, the duration of the exposure sequence and the travel speed (i.e. how far the surface 10 moves during a complete image sequence) . What changes in the pattern area 18 depends on the surface topography (in particular the change in the surface normal).

If the phase shift method is to be carried out, the pattern 13 and the image point 12 (even in the case of a stationary surface 10) must be coordinated with one another in such a way that in the part of the lighting pattern 130 that an image point 12 covers on the lighting pattern 130, can see the brightness as almost constant or the average brightness actually represents the brightness measured in the pixel 12 benefits. The brightness may also change so much that the brightness changes sufficiently with the required minimum surface deviation (caused by an error to be detected) that the inspection device 9 can perceive it. The former is the case when the Surface 10, which covers an image point 12 as a reflection area 19, can be viewed as almost flat. If this is not the case, a topographical measurement is no longer possible without further information; it can only be detected that there is a surface deviation. In addition, the lateral resolution (i.e. the size of the area on the surface) must be coordinated in such a way that the smallest surface deviations that are to be detected during the inspection are still resolved. In the case of the moving surface 10, it must also be taken into account that a larger area (total reflection area 17 on the surface 10 of FIGS This influences the lateral resolution. If the surface is additionally curved, a larger pattern area 18 on the pattern 13 is additionally covered by an image point. This influences the depth resolution Moving images in the image recording sequence, the decisive factor is how the respective line of sight 15, 19 of an image point 12 sweeps over the illumination pattern 130 (snapshot of the pattern 13).

In the case of a flat surface 10, according to FIGS. 3 and 4, this effect does not occur at all. This means that there are no errors caused by the mapping of different recorded pattern areas in the images of a recording sequence. However, this is only applicable if there are no measurement errors in the undisturbed case. As soon as there is any disturbance on the surface (or it is curved anyway) this is no longer the case. The case shown in FIGS. 5 and 6 also occurs in the case of measurement errors. The system is designed by the method according to the invention and the corresponding inspection device in such a way that the above-mentioned conditions are also met for exposure times or the entire recording time for a complete image recording sequence. For this purpose, the images of an image recording sequence are recorded so quickly one after the other that the displacement of the surface 10 during the recording is so small that each pixel 12 covers an area (reflection area 17) on the surface 10 that is still viewed as constant can. In addition, the period length of the pattern 31 is designed in such a way that the area which is swept over by a line of sight 15, 19 of the recording device 7 that is mirrored or reflected on the surface during the recording of an image recording sequence can still be seen as constant. that the resulting error is smaller than the required depth resolution.

The more the surface 10 is curved, the faster the images have to be recorded and the longer the wave pattern 13 has to be. However, both conditions only have to be complied with for the areas on the surface 10 that are actually to be inspected. In most cases, these are the structurally flawless surface areas and the areas in which flat, long-wave topographical defects are present. In addition, in most of the surfaces 10 there are small, mostly very steep topographical errors. For these, the conditions can usually no longer be observed, although this usually also applies to the static case. These errors can then only be detected (recognition of an error), but no longer measured (measurement of the topography).

Very high image recording frequencies are necessary for the process, since it also provides the required lateral resolution for the entire image recording sequence is achieved. These in turn require very short exposure times, which in turn require very bright lighting.

For the phase shift method used particularly advantageously in this context, it is most advantageous if the pattern 13 (or each of the illumination patterns 130) is a sinusoidal brightness curve. Typically, screens or patterns projected onto a surface are used for this purpose. This means that the sine curve can be displayed very well to perfectly. However, the brightness that can be achieved economically is often insufficient for these types of lighting, and the possible frame rate is limited so that they can only be used for slow processes.

With an LED row or LED matrix, in which the individual LEDs or individual LED modules, which consist of several individual LEDs, can be controlled separately, both the required brightness and the required switching frequency can be synchronized with the Realize image recording of the cameras. But you can put several lines together to form a matrix.

In the simplest form, the individual LEDs or LED modules can only be switched on or off. This means that only a rectangular brightness curve can be realized which is only a very rough approximation of the sinusoidal brightness curve that is actually desired. The phase shift method can also be carried out with this, but the accuracy is limited. A better approximation of the desired course can be achieved through various measures. The closer you get to a sine curve, the better the accuracy. The lighting line or matrix can be modified in such a way that intermediate brightness levels can also be set for the individual LEDs. Depending on size of the LEDs or LED modules, the sine curve can thus be approximated well. This is possible, for example, because the individual LEDs or LED modules are only switched on temporarily during the actual exposure time. However, this method is complex because you then need extremely fast control electronics. A solution preferred according to the invention provides for the pattern to be depicted in a blurred manner on the camera. This has already been described and will not be repeated here.

It should be noted that the terms camera and image recording device are used synonymously in the context of the above description. All features and functions disclosed in relation to the camera also apply accordingly to the image recording device, and vice versa.

List of reference symbols:

1 object

2 copper foil

3 plastic film

4 first error 5 second error

6 direction of movement

61 displacement

7 Reception facility

8 Lighting device 9 Inspection device

10 surface

11 Pickup sensor

12 pixel

13 patterns 130 lighting patterns

14 Brightness distribution

15 line of sight

16 surface normals

17 Reflection Area

170 reflection point

171 Intersection of the reflection areas of the individual images

18 Pattern area

181 Intersection of the pattern areas in the individual images

19 lines of sight a reflection angle

Claims

Expectations:

1. A method for the optical inspection of a surface (10) of an object (1) by means of an inspection device (9), in which by means of a lighting device (8) of the inspection device (9) a temporally periodic pattern (13) with different during an image recording sequence Lighting patterns (130) is generated on the surface (10) and in the image recording sequence several images of the pattern (13) on the surface (10) are recorded by means of an image recording device (7) of the inspection device (9), the generation of one of the different lighting patterns (130) are each synchronized with the image recording of one of the images of the pattern (13) in such a way that each image from the image recording sequence is recorded with a known lighting pattern (130) of the various lighting patterns (130); the phase pattern (13) is determined from the sequence of the recorded known illumination patterns (130) in at least one image point; errors (4, 5) on the surface (10) are detected from deviations of the illumination pattern (130) recorded in at least one image from the known illumination pattern (130) generated; characterized in that the lighting device (8) and the image recording device (7) are arranged at the reflection angle (a), the object (1) being moved relative to the inspection device (9) during the inspection of the surface (10); the duration of the image recording sequence is chosen so that a sequence reflection area (17), which is defined as the surface area covered overall by the reflection areas (17a, 17b) in the respective images from the image recording sequence, can be viewed as constant.

2. The method according to claim 1, characterized in that the size of the image point (12) is set when the method is carried out.

3. The method according to claim 2, characterized in that the setting of the size of an image point (12) takes place by combining several pixels of an exception sensor (11) of the recording device (7).

4. The method according to any one of the preceding claims, characterized in that the duration of the image recording sequence is set when the method is carried out.

5. The method according to claim 4, characterized in that when setting the duration of the image recording sequence, at least one of the variables listed below is adapted:

- exposure time of an image

- Brightness of the pattern (13) generated on the surface (10)

- Sampling frequency of the recording sensor (11)

- Number of images per image acquisition sequence

6. The method according to any one of the preceding claims, characterized in that the lighting pattern (130) is generated by the lighting device (7) in such a way that the area of the lighting pattern (130) that is visible in the image points (12) of the images recorded during each image recording sequence can be regarded as constant.

7. The method according to claim 6, characterized in that the period length of the pattern (13) in the lighting pattern (130) is selected such that depending on a topology of the surface (10) in the direction of the pattern, an intensity change is viewed as sufficiently constant can be.

8. The method according to any one of the preceding claims, characterized in that the periodic pattern (13) along the direction of movement of the object (10), transversely to the direction of movement of the object (10) or alternately along and transversely to the direction of movement of the object (10) is produced.

9. The method according to any one of the preceding claims, characterized in that the recording device (7) is focused in such a way that the illumination pattern (130) recorded in the image is out of focus.

10. The method according to any one of the preceding claims, characterized in that during the inspection of the surface (10) by means of deflectometric methods, the three-dimensional topography of the surface (10) of the object (1) is determined.

11. Use of a method according to one of claims 1 to 10 for the inspection of web goods or of treated surfaces (10).

12. Inspection device for the optical inspection of a surface (10) of an object (1) with an illumination device (8) and a receiving device (7) which are aligned with one another in such a way that a line of sight (15) emanating from the receiving device (7) ) then hits the lighting device (8) as a line of sight (19) reflected on the surface when a line of sight (15, 19) is perpendicular to the surface (10) at the point of impact of the line of sight (15, 19) Surface normal (16) halves the angle between the outgoing line of sight (15) and the reflected line of sight (19), the lighting device (8) being designed to produce a periodic pattern (13) with different lighting patterns (130) during an image recording sequence. to he testify and the recording device (7) is designed to record images of the pattern (13) reflected on the surface (10) during the image recording sequence synchronously with the generation of the lighting pattern (130), the inspection device (9) having a computing unit for Control of the inspection device (9) and for evaluating the recorded images, characterized in that a processor of the computing unit is designed to carry out the method according to one of Claims 1 to 10.

13. Inspection device according to claim 12, characterized in that the lighting device (8) has individually controllable light elements in a row or matrix arrangement and that the receiving device (7) has a sensor (11) for receiving via a receiving optics on the sensor (11) has imaged images, the sensor (11) having individual sensor pixels in a row or matrix arrangement.

14. Inspection device according to claim 12 or 13, characterized in that the receiving device (7) and the lighting device (8) are arranged in such a way that a flat viewing and lighting angle between the respective line of sight (15, 19) and the surface (10 ) and / or a large lighting distance between the surface (10) and the lighting device (8) is provided.