Surface inspection system and inspection method

Abstract

The present invention relates to a surface inspection system (10) for inspecting the surface of a plate element (4) present in an inspection area (5), the surface inspection system (10) comprising an image evaluation unit (18), two light sources (12, 14) and a camera (16), wherein the two light sources (12, 14) are on opposite sides of an illumination plane (O11) and are oriented to illuminate the inspection area (5), the camera (16) being suitable for acquiring a line image (l12, l14) of the inspection area (5) along a viewing plane (O16), the illumination plane (O11) and the viewing plane (O16) being respectively arranged on opposite sides of a middle plane (M), the middle plane (M) being perpendicular to the inspection plane, the angle (α) between the illumination plane (O11) and the middle plane (M) and the angle (α) between the viewing plane (O16) and the middle plane (M) being equal. The invention also relates to a method for inspecting the surface of a sheet element (4) using the surface inspection system (10) described above, wherein a first light source of two light sources (12, 14) directs light onto the surface of the sheet element (4) to be inspected, and a camera (16) acquires a line image (16) of the inspection area (5). 12 ;1 14 ), then the second of the two light sources (14, 12) directs light to the surface of the plate element (4) to be inspected, and the camera (16) acquires a line image (1) of the inspection area (5) 14 ;1 12 ), wherein the image evaluation unit (18) compares the two acquired line images (1 12 ;1 14 ), specifically subtracting two line images from each other.

Description

Technical Field

[0001] The invention relates to a surface inspection system and a method for inspecting the surface of a sheet metal component. The invention is particularly suitable for implementation in a quality control station of a sheet metal component processing machine.

[0002] The term "board element processing machine" is herein intended to include any machine for processing board elements such as paper, cardboard or other similar materials, in particular printing machines, coating machines, laminating machines and converting machines (e.g. cutting, punching, folding and / or gluing machines). Background Art

[0003] It is well known that the quality of sheet metal components can be controlled using cameras. Typically, a camera captures an image of the sheet metal component as it passes through a quality control station. The image is then analyzed for a number of different parameters to determine whether the sheet metal component meets certain standards.

[0004] Certain types of defects in sheet metal components are difficult to detect. This is especially true when inspecting sheet metal components with highly reflective ("glossy") surfaces for defects such as scratches, etc., while the sheet metal components are passing through the quality inspection system at speeds of several meters per second. Summary of the Invention

[0005] An object of the present invention is to reliably detect defects, such as scratches, etc., on sheet materials having highly reflective surfaces.

[0006] To achieve this object, the present invention provides a surface inspection system for inspecting the surface of a sheet material element present in an inspection area, the inspection system comprising an image evaluation unit, two light sources, and a camera, wherein the two light sources are adjacently arranged on opposite sides of an illumination plane and oriented to illuminate the inspection area, the camera being adapted to acquire a line image of the inspection area along a viewing plane, the illumination plane and the viewing plane being respectively arranged on opposite sides of a middle plane, the middle plane being perpendicular to the inspection plane, the angle between the illumination plane and the middle plane being equal to the angle between the viewing plane and the middle plane. Furthermore, the present invention provides a method for inspecting the surface of a sheet material element using the above-mentioned surface inspection system, wherein a first light source of the two light sources directs light onto the surface of the sheet material element to be inspected, and the camera captures a line image of the inspection area, then a second light source of the two light sources directs light onto the surface of the sheet material element to be inspected, and the camera captures a line image of the inspection area, wherein the image evaluation unit compares the two acquired line images, specifically subtracting the two line images from each other.

[0007] The basic concept of the present invention is to compare two line images of the same surface area of ​​the sheet material being inspected, where the difference lies in the angle at which the light source is directed toward the surface of the surface area. In the first line image, the light comes from a first light source positioned to one side of the illumination plane; in the second line image, the light comes from a second light source. For easier understanding, the light sources are hereinafter referred to as the "upper" and "lower" light sources.

[0008] Assuming the surface of the sheet metal component being inspected is glossy, the reflection of light from both light sources is theoretically specular (assuming the surface is a mirror) and practically close to specular. Therefore, the reflected light from the upper light source passes through the camera from above, while the reflected light from the lower light source passes through the camera from below.

[0009] If the surface of the sheet material component to be inspected is a plane mirror, the camera will "photograph" the space between the two light sources and will not detect any light. When the surface of the sheet material component to be inspected is close to a mirror reflection, the camera will actually capture some light, namely scattered light. In addition, because the camera's aperture is not zero, the camera does not focus on the light source but on the surface of the sheet material component, so the camera will capture a circular surface on the light plane at each pixel. In fact, the camera will capture a line image that includes the lower edge of the image reflected by the upper light source, and a line image that includes the upper edge of the image reflected by the lower light source. Basically, the line image captured by the camera is (almost) the same as the plane surface. If these line images are compared (for example, by subtracting the line images), there will be no difference (and the result of the subtraction is zero).

[0010] Light sources can also overlap. In other words, there doesn't necessarily have to be enough space between them for the camera to "shoot."

[0011] However, if the inspected sheet metal component has surface defects, such as scratches, the situation regarding the acquired line image changes significantly. The surface defect locally changes the orientation of the "mirror," so that light from the upper and lower light sources is no longer reflected symmetrically toward the camera, but rather asymmetrically relative to the camera (i.e., only the portion with the surface defect). For example, if the surface defect causes a localized tilt in the surface, light from the upper light source is reflected directly toward the camera. This inevitably results in light from the lower camera being reflected further away from the camera than it would be without the surface defect.

[0012] Therefore, the two line images captured by the camera differ. If the reflection were specular, the camera would "image" the upper light source (at the surface defect) without capturing the lower light source. If these line images are compared, a clear difference is apparent (at the surface defect).

[0013] The inspection area can be covered by a camera with a viewing area. When inspecting wider sheet elements, a plurality of cameras arranged adjacent to each other can be used, wherein the viewing areas of the plurality of cameras are combined to cover the inspection area.

[0014] A suitable angle between the illumination plane and the intermediate plane is in the range of 15° to 30°, preferably about 20°.

[0015] The two light sources (the "upper" light source and the "lower" light source) may be tilted relative to the illumination plane at an angle in the range of 5° to 10°, preferably about 7.5°.

[0016] The camera can in particular be a line scan camera. This can reduce the amount of data that the image evaluation unit needs to process, thus allowing for a higher processing speed. As an alternative to a line scan camera, the camera can be a two-dimensional (2D) camera.

[0017] When the camera is preferably a line scan camera, the term "line image" is used hereinafter to refer to an image acquired by the camera. When the camera is an area camera, the line image acquired by the camera contains multiple lines rather than just a single line.

[0018] Depending on the evaluation to be performed, if color information is required, the camera is a color camera. However, considering the amount of data to be processed, it may be more advantageous to use a grayscale (monochrome) camera.

[0019] In order to reliably detect surface defects, such as scratches, the resolution of the camera on the surface of the sheet metal element to be inspected is advantageously in the range of 0.05 to 0.6 mm, preferably approximately 0.1 mm.

[0020] When the surface inspection system and method are used on a sheet metal processing machine, the camera is adapted to capture more than 10,000 line images per second, preferably more than 40,000 line images per second. This rate is advantageous because it allows two line images to be captured of the same surface area to be inspected—a first line image captured when the surface is illuminated by a first light source, and a second line image captured when the surface is illuminated by a second light source—without significant movement of the sheet metal component between the two line images. Thus, the two line images can be considered to show the same surface area of ​​the sheet metal component.

[0021] When using high performance cameras capable of acquiring 40,000 or more line images per second, the sheet material component can be moved through the surface inspection system at speeds ranging from approximately 1 to 5 m / s, and even up to 15 m / s.

[0022] A suitable transport system for moving the sheet element through the viewing area of ​​the camera may be part of the surface inspection system, or the surface inspection system may be associated with the transport system.

[0023] The invention can be used for diffusely reflecting surfaces and has proven to be advantageous for inspecting sheet elements which at least partially have a reflective surface, such as cardboard (cartons) with a diffusely reflecting surface containing a transparent glossy paint.

[0024] The image processing unit combines the acquired line images into a complete image. For example, it can reconstruct the entire brightfield illumination image and the entire darkfield illumination image from the interwoven line images. These reconstructed images can be used to perform various subsequent quality inspections. For example, the extent of scratches can be determined from the difference image. This information can be used to determine whether to reject the sheet material. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will now be described with reference to a preferred embodiment, which is shown in the accompanying drawings. In the drawings:

[0026] - Figure 1 Schematically shows a side view of a surface inspection system according to the invention, which is used in a quality control system of a sheet element processing machine;

[0027] - Figure 2 Schematically shows Figure 1 A top view of a surface inspection system;

[0028] - Figure 3 Schematically shows Figure 1 Details of the surface inspection system;

[0029] - Figure 4 Schematically illustrates the lighting situation when inspecting a surface without surface defects;

[0030] - Figure 5 Schematically shows the Figure 4 In the case of , a line image obtained by a camera of a surface inspection system, and the result of comparing the two line images;

[0031] - Figure 6 Schematically illustrates the lighting situation when inspecting a surface with surface defects;

[0032] - Figure 7 Schematically shows the Figure 6 In the case of , a line image obtained by a camera of a surface inspection system, and the result of comparing the two line images;

[0033] - Figure 8 schematically illustrates different line images acquired by a camera; and

[0034] - Figure 9 Schematically shows how to create a reconstructed image based on line images acquired under different illumination conditions. DETAILED DESCRIPTION

[0035] Figure 1 Figure 2 schematically illustrates a quality control station 2 used in a sheet metal component processing machine, including a conveyor table 3. The sheet metal component processing machine processes a sheet metal component 4 as it is conveyed in the direction of arrow A. The sheet metal component 4 can be a sheet of paper, cardboard, or similar material, or a long web. The sheet metal component processing machine can be a printing press, a stamping press, a laminating machine, a folding machine, a gluing machine, or the like.

[0036] The quality control station 2 is used to control the quality of the sheet material element 4. Generally speaking, the quality control station 2 comprises a surface inspection system 10. The surface inspection system 10 comprises an illumination system 11 having two light sources 12, 14 and directing light to illuminate an inspection area 5, which inspects the surface of the sheet material element 4, a camera 16 for acquiring a line image, and an image evaluation unit 18.

[0037] The inspection area 5 is an elongated narrow area extending over the entire width of the channel for conveying the sheet elements 4, the longitudinal axis of the inspection area 5 being preferably perpendicular to the direction A (see Figure 2 ).

[0038] The details of the surface inspection system 10 are as follows Figure 3 shown.

[0039] The upper surface of the conveyor table is denoted by reference numeral 20. This upper surface can be considered uniform or flat (at least in the inspection region 5). Consequently, the upper surface of the sheet metal element 4 to be inspected in the inspection region 5 can also be considered uniform or flat. Reference numeral M denotes a median plane extending perpendicularly relative to the surface 20 (and therefore perpendicular to the upper surface of the sheet metal element 4 in the inspection region 5), and in this embodiment perpendicular to the direction A. The median plane can also have other orientations, as will be discussed later. In any case, the median plane M is not parallel to the direction A.

[0040] Optical plane O of camera 16 16 The angle α is tilted relative to the midplane M. In a preferred embodiment, the angle α is approximately 20°. Other angles may be selected depending on the structural constraints and the specific nature of the inspection to be performed.

[0041] The camera 16 is preferably a line scan camera having a resolution of about 0.05 to 0.3 mm on the surface 20 to resolve details of 0.05 to 0.3 mm on the surface of the sheet element 4 . The sensor lines of the camera 16 are oriented parallel to the longitudinal axis of the inspection area 5 .

[0042] The lighting unit 11 has an optical plane O 11, the optical plane O 11 They are tilted at the same angle α relative to the middle plane M, but are arranged on opposite sides of the middle plane M. Thus, when the lighting unit 11 is upstream of the inspection area 5 and the middle plane M, the camera 16 is downstream of the inspection area 5 and the middle plane M. However, the opposite is also possible.

[0043] The longitudinal axes of the light sources 12 , 14 are parallel to the longitudinal axis of the examination region 5 .

[0044] The two light sources are symmetrically arranged on the optical plane O 11 The angle β represents the angle between the optical planes of the two light sources 12 and 14 and the optical plane O of the lighting unit 11. 11 The angle of inclination, angle β, is here approximately 7.5°. When using a single lighting unit 11 , the light source optical planes of the light sources 12 , 14 are oriented such that the light from the light sources 12 , 14 is directed toward the surface of the sheet metal element 4 in the inspection region 5 and covers its entire width.

[0045] As an alternative to arranging the light sources 12 , 14 , two light sources may be arranged right next to each other without any space in between.

[0046] The light sources 12 and 14 can be composed of multiple columns of closely spaced LEDs. When used with lenses and other optical elements, such as diffuse reflectors, a narrow band of light is generated that is directed to illuminate the surface of the sheet member 4 in the inspection area 5. Furthermore, the light is substantially uniform in the x-direction of the viewing area 5, resulting in uniform light in the z- and y-directions.

[0047] The light sources 12, 14 may share the same diffuse reflector.

[0048] It is also possible to use two or more illumination units arranged laterally adjacent to each other. In a similar manner, it is also possible to use two or more cameras arranged laterally adjacent to each other. Each illumination unit or camera thus "covers" a portion of the width of the inspection area 5.

[0049] Since the optical plane O 11 and optical plane O 16 The two light sources 12, 14 are oriented in a mirror-symmetrical manner relative to the middle plane M, and are arranged relative to the optical plane O. 11 angularly offset so that the camera "photographs" the space between the two light sources (i.e., along the optical plane O, assuming the sheet element surface is a highly reflective "glossy" plane). 11 In the preferred embodiments of the surface inspection method and the surface inspection system according to the present invention, this assumption is feasible.

[0050] Assuming that the light generated by the light sources falls on the glossy surface of the sheet element 4 in the inspection area 5, it can be understood that the light of both light sources is not detected by the camera 16. The light from the light source 12 is reflected along the plane R12 and the light from the light source 14 is reflected along the plane R14.

[0051] The general operation mode of the surface inspection system is as follows Figure 4 and Figure 5 shown.

[0052] The light sources 12, 14 are activated intermittently and the camera 16 acquires a line image of one of the two illumination conditions in the examination area 5. Thus, the camera 16 acquires a line image of the examination area 5 illuminated by the light source 12. 12 , and then obtain the line image l of the inspection area 5 illuminated by the light source 14 14 .like Figure 5 The upper part is schematically shown. For a clearer description, each line image captured by the camera 16 is shown as 10 pixels. In practice, each line image may contain thousands or even tens of thousands of pixels, depending on the resolution and the width of the inspection area 5.

[0053] exist Figure 4 In the embodiment shown, the surface of the sheet element is glossy and flat and has no surface defects, so the reflection of light is specular, and since the camera 16 "photographs" the space between the two light sources 12, 14 (see Figure 4 ), so each pixel gets an intensity of (at least theoretically) zero. This result is obtained by Figure 5 The blank spaces in the image represent pixels.

[0054] The image evaluation unit of the surface inspection system 10 can compare the acquired line images. In a preferred embodiment, the line images are subtracted from each other. Figure 5 The result of the subtraction is shown in the lower half of the image S. The first column is the first line image l 12 Subtract the first line image l 14 The result of subtraction is the image S1. The second column is the second line image l 12 and the second line image l 14 The result of subtracting each other is the image S2 obtained after subtraction.

[0055] When the acquired line images are identical, the result of subtracting the value of each pixel is zero.

[0056] If the subtraction is performed in the opposite form (ie, the line image acquired later is subtracted from the line image acquired earlier), the same result will be obtained.

[0057] Since the inspection principle is based on acquiring two line images of the same surface of the sheet element 4, it is necessary to acquire the line images very quickly in succession. 12, l 14 Because the camera 16 acquires line images at a rate of about 10,000 or more per second, there is no appreciable movement of a particular portion of the surface of the sheet member between the time intervals between successive acquisitions of line images.

[0058] Depending on the specific information that needs to be obtained during surface inspection, the camera can be a color camera or a grayscale camera.

[0059] In the embodiment shown, the reflective properties of the surface of the sheet element 4 to be inspected are assumed to be mirror-like. In practice, very glossy surfaces (surfaces covered with foil or varnish, surfaces with holograms) will have properties close to mirror reflection, but not completely mirror reflection. However, when comparing the line image 1 obtained by illuminating the inspection area 5 with the light source 12, the reflection properties of the line image 1 are similar to those of the surface of the sheet element 4 to be inspected. 12 and the line image 1 obtained by irradiating the inspection area 5 with the light source 14. 14 The result will be the same.

[0060] Assuming that the reflection is close to the mirror, the camera will actually get some scattered light. Figure 3 As shown, the camera 16 will capture some scattered light from the light source 12 and some scattered light from the light source 14, that is, on the line R 12 A portion of the light above (i.e. closer to the optical plane O 16 ) and online R 16 The portion of light below (i.e. closer to the optical plane O 16 ).

[0061] Assuming that the surface of the plate element 4 is flat, the intensity of each pixel light captured by the camera is the same, so the two line images will be the same. Therefore, the result of line image subtraction will be the same as Figure 5 The lower half is the same as shown, that is, zero.

[0062] Detection of defective surfaces such as Figure 6 and Figure 7 shown.

[0063] Generally speaking, surface defects can be considered as areas of the surface where the "mirror" (reflective surface) is misoriented. This misorientation can prevent the camera 16 from "picturing" the space between two adjacent light sources 12, 14. Instead, the symmetry of light reflected toward the camera is disrupted, potentially causing the camera to directly image one of the light sources (thus capturing a line image of maximum brightness) while simultaneously failing to image the other light source (thus capturing a line image of no brightness).

[0064] Figure 6 The inspection area 5 is shown having surface defects therein, causing light from the light source 14 to be reflected into the camera 16 (bright field illumination), while light from the light source 12 is reflected away from the camera (dark field illumination).

[0065] Therefore, Figure 7 As shown, the obtained line image l 12 The intensity of the pixel is zero, and the acquired line image l 14 Some pixels have the highest intensity (assuming here that the surface defect is four pixels wide).

[0066] Because the acquired line images are different, the line image comparison results show that the difference values ​​of some pixels (at the location of surface defects) are not equal to zero (the comparison result is obtained only based on the order in which the two line images are subtracted from each other, and is not related to positive or negative values). When the pixel value of the subtraction result S is not equal to zero, it clearly indicates that a defect exists on the surface.

[0067] Here, a simplified embodiment is used to explain the system and method of the present invention. The distinction between "no surface defect" and "presence of surface defect" requires a specific threshold value to clearly define the intensity of the pixel of the comparison result, because in practice, due to the tolerance value (for example, the light intensity of two light sources), and the difference between each two consecutively acquired line images l 12 , l 14 Since the two line images are taken from slightly different surfaces of the sheet metal component (because the sheet metal component is constantly moving during the inspection), the two line images may not be 100% identical. Any intensity below a suitable threshold is considered "no surface defect", while an intensity above this threshold is considered "presence of surface defects".

[0068] In fact, the detection of surface defects such as scratches is not based on a simple threshold, but on the image S obtained after subtraction of the individual n The difference image created detects local changes. The reconstructed image may be 800mm x 800mm and is created while the sheet element 4 is moved in direction A.

[0069] The method of reconstructing the image is as follows Figure 8 and Figure 9 shown.

[0070] Generally speaking, from the interwoven line images acquired under different illumination conditions, a first reconstructed image (a reconstructed image created from the line images acquired under illumination by the first light source 12) and a second reconstructed image (a reconstructed image created from the line images acquired under illumination by the second light source 14) are created, and then these reconstructed images are analyzed by an image evaluation unit.

[0071] In all embodiments, the illumination for acquiring line images of the sheet element is synchronized with the displacement of the camera and the sheet element (the speed of movement of the sheet element). For example, a line image is acquired while the viewing area is illuminated, and a short time later (e.g., when the sheet element moves 0.05 mm), the viewing area is illuminated again to acquire the next line image, and so on.

[0072] Figure 8 Schematically shows the interwoven line image l captured by the camera 12 (shown as normal lines) and l 14 (shown in dotted lines). Here, only 10 line images are shown for each lighting condition. In practice, thousands of line images can be acquired for each sheet element 4. 12 ,l 14 Assume that 4,000 line images are acquired for a plate element 4. 12 , and 4,000 line images 14 , and the camera 16 can acquire 80,000 line images per second, so ten plate elements 4 can be processed per second.

[0073] Figure 9 The first reconstructed image 40 is schematically shown to be composed of line images 1 12 Created, and the second reconstructed image 50 is composed of the line image l 14 create.

[0074] If the camera 16 is used in more than the two lighting situations described here (two different light sources 12, 14) Figure 8 If line images are acquired as shown, the data acquired by the camera 16 will include three or more types of interleaved line images, and three or more reconstructed images will be created (one reconstructed image for each lighting condition).

[0075] The image evaluation unit 18 processes the reconstructed images 40 , 50 (the entire image or a specific portion thereof) to detect specific items. In this case, scratches on the surface of the sheet metal element 4 can be identified by comparing the reconstructed images 40 and 50 .

[0076] Assume that the "normal" value of the grayscale intensity unit of the image obtained after subtraction is 20. If the grayscale value suddenly changes to 0 or 40, and the change in grayscale intensity corresponds to the geometric shape of the scratch, the image evaluation unit 18 can recognize that this area of ​​the sheet element contains a scratch. When the sensor line is positioned perpendicular to the direction A, there is a type of surface defect that cannot be detected by the method and system: when the scratch extends completely parallel to the direction A, in which the sheet element 4 moves through the quality inspection station 2. Such a surface defect will result in some pixel values ​​in the pixel line of the acquired line image being different from the other pixel values, but the continuously acquired line image l 12 ,l 14 The intensity will not change.

[0077] In order to detect surface defects in different directions, the above-mentioned surface inspection system can be used additionally, and the direction of the additional surface inspection area is different from the direction of the inspection area 5 of the first surface inspection system.

[0078] Likewise, the inspection area 5 of the first surface inspection system can be arranged at +45° relative to the transport direction A, and the inspection area 5 of the second surface inspection system can be arranged at −45° relative to the transport direction A.

[0079] The surface inspection system 10 may include other lighting units to form a more complex inspection system. In particular, the light sources 12 and 14 may form a more complex lighting unit for detecting creases and embossed structures on a sheet material unit.

Claims

1. A surface inspection system (10) for inspecting a surface of a sheet material element (4) present in an inspection area (5), the surface inspection system (10) comprising an image evaluation unit (18), a first light source (12) and a second light source (14), and a camera (16), wherein the first light source (12) and the second light source (14) are arranged adjacent to each other in an illumination plane (O 11 ) on opposite sides and oriented to illuminate the inspection area (5), the camera (16) is adapted to illuminate the inspection area (5) along the viewing plane (O 16 ) obtains a line image (l) of the inspection area (5) 12 , l 14 ), the illumination plane (O 11 ) and the viewing plane (O 16 ) are respectively arranged on opposite sides of the middle plane (M), the middle plane (M) is perpendicular to the inspection plane, the illumination plane (O 11 ) and the angle (α) between the middle plane (M) and the viewing plane (O 16 ) and the median plane (M) are equal, in, The first light source (12) and the second light source (14) direct light at different angles relative to the inspection area (5) toward a same surface area of ​​the sheet material element (4) to be inspected, so that the camera acquires a first line image of the same surface area illuminated by the first light source and a second line image of the same surface area illuminated by the second light source, The image evaluation unit (18) compares the first line image and the second line image with each other, and The first light source (12) and the second light source (14) are arranged relative to the illumination plane (O 11 ) are tilted at the same angle (β).

2. The surface inspection system (10) of claim 1, wherein a viewing area of ​​the camera (16) covers the inspection area (5).

3. The surface inspection system (10) according to claim 1, wherein a plurality of the cameras (16) are arranged adjacent to each other, and viewing areas of the plurality of cameras (16) are combined to cover the inspection area (5).

4. The surface inspection system (10) according to any one of claims 1 to 3, wherein the illumination plane (O 11 ) and the middle plane (M) is in the range of 15° to 30°.

5. The surface inspection system (10) according to any one of claims 1 to 3, wherein the illumination plane (O 11 ) and the median plane (M) is 20°.

6. The surface inspection system (10) according to any one of claims 1 to 3, wherein the first light source (12) and the second light source (14) are aligned with the illumination plane ( 11 ) is in the range of 5° to 10°.

7. The surface inspection system (10) according to any one of claims 1 to 3, wherein the first light source (12) and the second light source (14) are arranged relative to the illumination plane ( 11 ) is inclined at an angle (β), and the angle (β) is 7.5°.

8. The surface inspection system (10) according to any one of claims 1-3, wherein the camera (16) is a line scan camera.

9. The surface inspection system (10) according to any one of claims 1 to 3, wherein the camera (16) is a 2D camera.

10. The surface inspection system (10) according to any of the preceding claims 1-3, wherein the camera (16) is a color camera.

11. The surface inspection system (10) according to any one of claims 1 to 3, wherein the camera (16) is a monochrome camera.

12. The surface inspection system (10) according to any one of claims 1 to 3, wherein the resolution of the camera (16) on the surface of the sheet material element (4) to be inspected is in the range of 0.05 to 0.6 mm.

13. The surface inspection system (10) according to any one of claims 1 to 3, wherein the resolution of the camera (16) on the surface of the sheet material element (4) to be inspected is 0.1 mm.

14. A method for inspecting the surface of a sheet element (4) using the surface inspection system (10) according to any one of claims 1 to 13, wherein a first light source (12) directs light at a first angle relative to the inspection area (5) to a same surface area of ​​the sheet element (4) to be inspected in the inspection area (5), and the camera (16) acquires a first line image (l) of the same area illuminated by the first light source (12). 12 ), then a second light source (14) directs light to the same surface area at a second angle different from the first angle relative to the inspection area (5), and the camera (16) acquires a second line image (l) of the same area illuminated by the second light source (14) 14 , wherein the image evaluation unit (18) acquires the first line image (l 12 ) and the second line image (l 14 ) compared with each other, and in, The first light source (12) and the second light source (14) are adjacently arranged on the illumination plane (O 11 ) on opposite sides and relative to the illumination plane (O 11 ) are tilted at the same angle (β).

15. The method according to claim 14, wherein the image evaluation unit (18) acquires the first line image (l 12 ) and the second line image (l 14 ) are subtracted from each other.

16. The method of claim 14, wherein the camera (16) is adapted to acquire more than 10,000 line images per second.

17. The method of claim 14, wherein the camera (16) is adapted to acquire more than 40,000 line images per second.

18. The method according to claim 14, wherein the sheet element (4) is moved relative to the surface inspection system (10) at a speed of 1 to 15 m / s.

19. Method according to any one of claims 14 to 18, wherein at least a portion of the inspected sheet element (4) has a reflective surface.

20. The method according to any one of claims 14 to 18, wherein the image obtained after subtraction (S n ) are combined to form a reconstructed image, and the image evaluation unit (18) analyzes the reconstructed image.

21. The method according to claim 20, wherein the image evaluation unit (18) analyzes the reconstructed image to identify changes in grayscale intensity.

Images