METHOD AND INSPECTION DEVICE FOR OPTICALLY INSPECTING A SURFACE

Abstract

A method for optically inspecting a surface (10) of an object (1) and an inspection device (9) are described. With the method a temporally periodic pattern (13) with different illumination patterns (130) is generated on the surface (10) by means of a illumination device (8) of the inspection device (9) during an image recording sequence (13), and in the image recording sequence a number of images of the pattern (13) on the surface (10) are recorded by means of an image recording device (7) of the inspection device (9), wherein generating one of the different illumination patterns (130) is synchronised, respectively, with the image recording of one of the images of the pattern (13), the phase of the pattern (13) is determined from the succession of the recorded known illumination patterns (130) in at least one image point and defects (4, 5) on the surface (10) are detected from deviations of the recorded illumination pattern (130) from the generated known illumination pattern (130). The illumination device (8) and the image recording device (7) are arranged in the reflection angle (α), wherein the object (1) is moved relative to the inspection device (9) and the duration of the image recording sequence is chosen such that a sequence reflection zone (17) can be regarded as constant (FIG. 4b).

Claims

1. A method for optically inspecting a surface (10) of an object (1) by means of an inspection device (9), in which by means of an illumination device (8) of the inspection device (9) a temporally periodic pattern (13) with different illumination patterns (130) is generated on the surface (10) during an image recording sequence and in the image recording sequence a number of images of the pattern (13) on the surface (10) are recorded by means of an image recording device (7) of the inspection device (9), wherein generating one of the different illumination patterns (130) is synchronised, respectively, with the image recording of one of the images of the pattern (13) such that each image from the image recording sequence is recorded, respectively, with a known illumination pattern (130) of the different illumination patterns (130); the phase of the pattern (13) is determined from the succession of recorded known illumination patterns (130) in at least one image point; defects (4, 5) on the surface (10) are detected from deviations of the illumination pattern (130) recorded in at least one image from the generated known illumination pattern (130); characterised in that the illumination device (8) and the image recording device (7) are arranged in the reflection angle (α), wherein during inspection of the surface (10) the object (1) is moved relative to the inspection device (9); the duration of the image recording sequence is chosen such that a sequence reflection zone (17), which is defined as the surface area covered in total by the reflection zones (17a, 17b) in the respective images from the image recording sequence, can be regarded as constant.

2. The method according to claim 1, characterised in that the size of the image point (12) is set during the performance of the method.

3. The method according to claim 2, characterised in that setting the size of the image point (12) is done by combining several pixels of a recording sensor (11) of the recording device (7).

4. The method according to claim 1, characterised in that the duration of the image recording sequence is set during the performance of the method.

5. The method according to claim 4, characterised in that when setting the duration of the image recording sequence at least one of the variables listed hereunder is adapted: exposure time of an image brightness of the pattern (13) generated on the surface (10) scanning frequency of the recording sensor (11) number of images per image recording sequence.

6. The method according to claim 1, characterised in that the illumination pattern (130) is generated by the illumination device (7) such that the area of the illumination pattern (130) 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, characterised in that the period length of the pattern (13) in the illulmination pattern (130) is chosen such that depending on a topology of the surface (10) in direction of the pattern course an intensity change can be regarded as sufficiently constant.

8. The method according to claim 1, characterised in that the periodic pattern (13) is generated along the movement direction of the object (10), transversely to the movement direction of the object (10) or alternately along and transversely to the movement direction of the object (10).

9. The method according to claim 1, characterised in that the recording device (7) is focussed such that the illumination pattern (130) recorded in the image is blurred.

10. The method according to claim 1, characterised in that during inspection of the surface (10) the three-dimensional topography of the surface (10) of the object (1) is determined by means of deflectometric processes.

11. A use of the method according to claim 1 for the inspection of web product or of treated surfaces (10).

12. An inspection device for optically inspecting a surface (10) of an object (1) with an illumination device (8) and a recording device (7), which are aligned to each other in such a way that a visual ray (15) emanating from the recording device (7) as a visual ray (19) reflected on the surface, is incident on the illumination device (8) then, when a surface normal (16) standing vertically on the surface (10) in the incident spot of the visual ray (15, 19) just halves the angle between the outgoing visual ray (15) and the reflected visual ray (19), wherein the illumination device (8) is designed to generate a temporally periodic pattern (13) with different illlumination pattern (130) during an image recording sequence 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 illumination pattern (130), wherein the inspection device (9) includes a computing unit for controlling the inspection device (9) and for avaluating the recorded images, characterised in that a processor of the computing unit is designed for performing the method according to claim 1.

13. The inspection device according to claim 12, characterised in that the illumination device (8) comprises individually controllable light elements arranged in a row or matrix and in that the recording device (7) comprises a sensor (11) for recording images mapped on the sensor (11) via a recording optics, wherein the sensor (11) comprises individual sensor pixels arranged in a row or a matrix.

14. The inspection device according to claim 12, characterised in that the recording device (7) and the illumination device (8) are arranged such that a flat viewing and illumination angle is provided between the respective visual ray (15, 19) and the surface (10) and/or in that a large illumination distance is provided between the surface (10) and the illumination device (8).

Description

[0061] In the drawing:

[0062] FIG. 1, in a schematic sectional view, shows an object with a surface to be inspected with a first typical defect;

[0063] FIG. 2, in a schematic sectional view, shows the object according to FIG. 1 with the surface to be inspected with a second typical defect;

[0064] FIG. 3a, shows a top view onto an inspection device according to an embodiment of the invention for the inspection of a planar surface;

[0065] FIG. 3b, shows a side view of the inspection device according to FIG. 3a;

[0066] The object 1 depicted in FIGS. 1 and 2, the surface 10 of which is to be inspected by the inspection device according to the invention, is an FCCL film, which is used as raw 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 the outer copper films 2 are laminated. The surface 10 of the object 1 is typically examined for surface defects.

[0067] This surface inspection is also to be used for detecting laminating defects, in particular so-called laminating folds 4 (FIG. 1) and inner folds 5 (FIG. 2). With laminating folds 4 the material has formed slight folds, which were pressed flat again during the laminating process. Inner folds 5 are created in that folds have formed in the inner plastic film 3, which were laminated in.

[0068] FIG. 3b shows a side view of the inspection device 9 with an illumination device 8 and a recording device 7. On the illumination device 8 a temporally periodic pattern 13 with different illumination patterns 130 is depicted, which illuminates the surface 10 of the object 1 (see also top view as per FIG. 3a). The illumination pattern 130 comprises a brightness distribution 14. This also causes the pattern 13 to be generated on the surface 10. The recording device 7 records the pattern 13 on the surface 10 in an image.

[0069] The recording device 7 also includes a recording sensor 11, which generates an image with many image points 12. Due to an optics of the recording device not depicted visual rays 15 emanating from the (each) image point 12 are reflected at the surface 10 and are incident as reflected visual rays 19 on the illumination device 8 on the pattern 13 generated there. The edge rays of these visual rays 15, 19 are plotted in FIG. 3b. The edge rays emanate from the edges of the image point 12 and delimit the reflection zone 17 on the surface 10. All visual rays 15 emanating from the image point 12 in the reflection angle a and incident on the surface lie in the reflection zone 17 on the surface 10 and are also reflected in the reflection angle a from the surface as reflected visual rays 19. They are incident on the illumination device 8 in the pattern area 17, because according to the inventive arrangement the recording device 7 and the illumination device 8 are arranged in the reflection angle a relative to the surface 10.

[0070] The reflection angle a is defined as the angle between the incident visual rays 15, 19 (emanating from the image point 12)/the exiting (reflected from the surface 10) and the associated surface normal 16. The surface normal 16 belonging to a visual ray 15, 19 extends vertically to the surface in the reflection point 170, in which the visual rays 15, 19 are incident on the surface 10.

[0071] FIG. 3a concretely shows a line of the recording sensor 11 of the recording device 7, which extends along the width of the surface 10 such as a web product moving in movement direction as object 1, such as an FCCL film. The recording device 7 may be constructed as a line camera with only one sensor line of the recording sensors 11, or as an area scan camera with several such sensor lines. An image point 12 may be formed from one or several sensor pixels. Via the optics not depicted an image point 12 of the recording device (camera) captures the reflection zone 17 on the surface 10. The visual rays 15 are deflected on the surface 10 and capture the pattern area 18, which is given by the area of the pattern 13/the respective illumination pattern 130 of the pattern 13 at the point in time of the image recording. In the example depicted in FIGS. 3a and 3b the illumination device is designed as an illumination line, which is aligned transversely to the movement direction 6 of the surface 10.

[0072] FIG. 3b shows the same arrangement in a side view, in which the reflection of the visual rays 15, 19 (plotted as edge rays as in all figures) is clearly recognisable with the reflection angle a relative to the surface normal 16. The plotted edge rays of the visual rays 15, 19 visualise the size/area of the reflection zone 17 on the surface 10 and of the pattern area 18 in the pattern 13.

[0073] FIGS. 3a and 3b show the state during an image recording, wherein it is assumed that the movement of the surface 10 moving in movement direction can be neglected during the short exposure time of the image recording. If this is not the case, the images recorded show a certain movement blur, which can be counteracted by shortening the exposure time (providing illumination is sufficiently bright).

[0074] As already described a number of images are recorded in chronological order with the method according to the invention during an image recording sequence. Because the surface moves during the image recording sequence in movement direction 6, the image point 12 does no longer see the same surface area in the respective reflection zone 17 of the successively recorded images. Rather the reflection zones 17 on the surface 10 are shifted relative to each other in the successively recorded images.

[0075] This is depicted in FIGS. 4a and 4b, in which the shift 61 of the surface 10 between the first and the last image recording in an image recording sequence is plotted. The reflection zone 17a is plotted as the reflection zone of the first image recording and the reflection zone 17b is plotted as the reflection zone of the last image recording from the image recording sequence, each shown as a hatching rotated by 90°. In the overlapping area the two hatchings are superimposed. The entire reflection zone 17 across all images of the recording sequence is correspondingly enlarged (relative to the surface 10 covered in total relative to reflection zones of individual recordings). The effect is basically similar also for the already discussed movement blur, the difference being that the entire reflection zone is integrated in one image. This makes the image look blurred, insofar as a movement blur is to be at all recognised.

[0076] Because the recording geometry does not change for a planar surface, the shift of the surface 10 does not have any effect on the pattern area 18; this remains unchanged during the recording sequence, wherein of course, as already described, the pattern illuminations are generated phase-shifted. This is, however, not shown in FIG. 4a for reasons of clarity.

[0077] FIG. 4b shows the same situation as FIG. 4a in a side view. The surface normals 16a during recording of the image a were at that time in the same position as the surface normals 16b during the recording of image b, which is shown here as a momentary recording of the arrangement. Because of the planar surface 10 the alignment of the surface normals 16a and 16b is the same, with the effect that the pattern area 18 does not change either.

[0078] FIGS. 3c and 4c show an arrangement of the inspection device 9, where the illumination device 8 comprises an illumination line aligned along the movement direction 6 of the surface 10. This can be achieved by a line illumination device (with correspondingly aligned line) or by a matrix illumination device, which is correspondingly controlled. Due to the planar surface a situation results also in this arrangement, which is comparable to that shown in FIGS. 3a, 3b and 4a, 4b. For a detailed description please refer to the above description.

[0079] FIGS. 3d and 4d show an arrangement of the inspection device 9 similar to the arrangement in FIGS. 3c and 4c, where not only the illumination line of the illumination device 8, but also the sensor line of the recording sensor 11 are aligned along the movement direction 6 of the surface 10. The recording device may be designed accordingly as a line camera (with only one sensor line) or as a matrix camera (with several sensor lines arranged side by side). Due to the planar surface a situation arises also in this arrangement which is comparable to the arrangement shown in FIGS. 3a, 3b, 3c and 4a, 4b, 4c. For a detailed description please refer to the above description.

[0080] The picture is different, when the surface is indeed not planar. This is depicted in FIGS. 5a, 5b, 5c and 5d/6a, 6b, 6c and 6d. The views and arrangements correspond to the views and arrangements discussed with reference to the views and arrangements relating to FIGS. 3a, 3b, 3c and 3d/4a, 4b, 4c and 4d. In view of the general description therefore reference should be made to the above. Due to the curvature of the surface 10, which impacts the alignments of the surface normals 16, 16′ and which influences the reflections of the visual rays 15, 19, different pattern areas 18a, 18b result as a consequence for the different images of an image recording sequence.

[0081] FIGS. 5a, 5b, 5c and 5d show the situation for one image respectively, for example the first image of the image sequences. FIG. 5a in essence corresponds to FIG. 3a, wherein the sides of the surface 10 depicted in a curved manner indicate the curvature of the surface 10 as extending transversely to the movement direction 6. Due to the curvature of the surface 10 the visual rays—in the top view—are then not reflected as a straight line, but deflected in the reflection point 170, 170′. Correspondingly the reflected visual rays 19 are incident on the pattern 13 in a pattern area 18, which lies in a different spot from that of the pattern area 18 according to FIG. 3a. FIG. 5b correspondingly shows that the surface normals 16 and 16′ are differently aligned in the reflection points 170, 170′ (and have therefore been marked with different reference symbols). The reflection angles α, α′ are therefore also different.

[0082] FIGS. 6a and 6b show the reflection zone 17a (for the visual rays 15, 19 reproduced in FIGS. 5a, 5b during recording) and the reflection zone 17b (for the visual rays 15, 19 reproduced in FIGS. 6a, 6b) together with the overlapping area 171. The pattern areas 18a and 18b and their overlapping area 181 are shown in a corresponding manner.

[0083] The image point 12 is illuminated in the recording sensor 11 by the area 18, 18a, 18b of the pattern limited by the edge visual rays 15 (prior to being mirrored at the surface 10)/19 (after being mirrored at the surface 10), wherein this area 18, 18a, 18b is mapped on the pattern 13 across the reflection zones 17, 17a, 17b of the surface 10 in the recording device 7. Each of the visual rays 15 is however deflected according to the surface normals 16, 16′, 16a, 16a′/16b, 16b′ present in this spot.

[0084] In FIGS. 5a, 5b, 5c and 5d the situation in the first image of the sequence is depicted. Again the camera pixel 12 in the image sensor 11 is illuminated by the area 18 of the pattern delimited by the edge rays 15 (prior to being mirrored at the surface)/19a (after being mirrored at the surface), wherein this area 18 is mapped on the pattern 13 across the area 17a of the surface 10 in the camera. Now, however, the visual ray 15 is deflected according to the surface normals 16a/16b present in this spot. The situation in the respectively last image of each image recording sequence is shown in FIGS. 6a, 6b, 6c and 6d. Now the area 18b of the pattern 13 is mapped across the area 17b across the shifted surface 10 in the image point 12. Now the surface normals 16b, 16b′ are relevant for the mirroring of the edge rays 15 emanating from the camera. Since these are different from those in the first image (FIGS. 5a, 5b, 5c and 5d), the area of the illumination pattern 130 in the illumination device 8, which is seen/mapped in the image point 12, is also shifted. In total, during the image sequence from the first to the last recording the image point 12 sweeps over the area 17 of the surface 10 in FIGS. 6a, 6b, 6c and 6d and thus over the entire area 18 of the pattern 13. The image point 12 sees the area, which is located both in the reflection zones 17a as well as 17b on the surface 10 and in the pattern areas 18a as well as 18b on the pattern 13. It, i.e. the image point 12, does not see the areas, which during the entire image sequence are only present in 17a or 17b/18a or 18b.

[0085] It should, however, be noted that the proportions in FIGS. 3a, 3b, 3c, 3d, 4a, 4b, 4c, 4d, 5a, 5b, 5c, 5d, 6a, 6b, 6c, 6d are not realistic. Nor do the cutting areas 171, 181 depicted with cross-hatching, respectively, correspond to realistic variables, but only serve illustrative purposes and to promote understanding. In fact, at least the pattern 13/the illumination pattern should be very much longer-wave compared to the depicted size of the image point 12, so that an image point 12 only covers a small fraction of a wavelength. If the proportions regarding size were realistic, the principle could no longer be recognised in the drawing.

[0086] As already explained, in an image recording sequence which is recorded for a multi-image phase shift process, the same spot of the surface 10, i.e. the same reflection zone 17, should really be mapped in all images in each image point 12. When several images are recorded one after the other, these are, relative to a moving surface 10, shifted from one another. What is decisive for the assessment as to whether that, which is recorded by an image point 12 during an image sequence, can still be regarded as “approximately the same spot” in terms of the invention, ultimately depends on to what extent the mapping of the periodic pattern 13 across the surface 10 in the recording device 7 changes during an image sequence. This in turn depends, on the one hand, on the pattern 13 (illumination pattern 130) itself and its distance from the surface 10, on the other hand on the reflection zone 17, which is mapped on the image point 12 during the entire image sequence, and how this area (reflection zone 17) changes. The area of the reflection zone depends on the optical pixel resolution (i.e. the area, which is mapped in the viewing plane on one pixel), the exposure time, the duration of the exposure sequence and the traversing speed (i.e. how far the surface 10 moves during a complete image sequence). Changes in the pattern area 18 depend on the surface topography (in particular on the change of the surface normal).

[0087] If the phase shift process is to be performed, the pattern 13 and the image point 12 (also in the case of a stationary surface 10) must be matched to each other such that in that part of the illumination pattern 130, which is covered by an image point 12 on the illumination pattern 130, the brightness can be regarded as almost constant/the medium brightness actually represents the brightness measured in the image point 12. Also the brightness is allowed to change to that extent that the brightness for the required minimum surface deflection (caused by a defect to be detected) changes sufficiently for the inspection device 9 to be able to perceive this. The former is the case, if the surface 10, which is covered by an image point 12 as reflection zone 19, can be regarded as almost planar. If this is not the case, a topographic measurement is no longer possible without further information; all that can still be detected is that a surface deviation exists. In addition the lateral resolution (i.e. the size of the area on the surface) must be adjusted such that the smallest surface deviations, which shall be identified during the inspection, are still resolved.

[0088] For the moving surface 10 it must further be taken into account that during the image recording sequence a larger area (entire reflection zone 17 on the surface 10 of FIGS. 4a, 4b, 4c, 4d e.g. 6a, 6b, 6c, 6d is covered by an image point. This impacts the lateral resolution. If the surface is additionally curved, a larger pattern area 18 on the pattern 13 is additionally covered by one image point. This impacts the depth resolution. If the surface 10 moves during the recording of images in the image recording sequence, the deciding factor is, how the respective visual ray 15, 19 of an image point 12 sweeps over the illumination pattern 130 (momentary recording of the pattern 13).

[0089] In the case of a planar surface 10 this effect does not occur anyway as per FIGS. 3 and 4. Thus errors do not occur because of mapping different recorded pattern areas in the images of a recording sequence. Albeit this only applies if there are no measuring errors in the undisturbed case. As soon as any fault occurs on the surface (or if this is curved anyway) this no longer applies. Therefore the case shown in FIGS. 5 and 6 also occurs in the case of measuring errors.

[0090] Due to the method according to the invention and the respective inspection device the system is laid out such that the above mentioned conditions are maintained also for exposure times/the entire recording time for a complete image recording sequence. To this end the images of an image recording sequence are recorded chronologically one of the other so quickly that the shifting of the surface 10 during the recording is so small that each image point 12 covers an area (reflection zone 17) on the surface 10, which can still be regarded as constant. Besides the period length of the pattern 31 is laid out such that the area, which is swept over by a visual ray 15, 19 of the recording device 7 mirrored or reflected at the surface during the recording of an image recording sequence, can still be regarded as constant/that the error arising therefrom is smaller than the required depth resolution.

[0091] The stronger the surface 10 is curved, the faster the images have to be recorded and the more long-wave the pattern 13 must become. However, both conditions must be maintained only for those areas on the surface 10, which are to be actually inspected. These are, in most cases, the constructively defect-free surface areas and those areas, in which flat, topographical long-wave defects exist. Most surfaces have moreover very small, mostly very steep topographical defects. With regard to these defects the conditions can no longer be maintained in most cases, wherein this applies mostly already for the static case. All that can be done here is detecting these defects (detecting of a defect), but measuring them (measuring the topography) is no longer possible.

[0092] Very high image recording frequencies are necessary for the method, in order for the required lateral resolution to be achieved for the entire image recording sequence. These in turn require very short exposure times, which in turn require very bright illumination.

[0093] For the phase-shift method used in a very advantageous manner in this context it is most advantageous, if the pattern 13 (i.e. each of the illumination patterns 130) is a sinusoidal brightness curve. This is typically achieved using e.g. screens or patterns projected on a surface. The sinus curve can be represented in a very good to perfect manner therewith. Unfortunately the brightness achievable at economically justified expense with these illuminations is often not sufficient, and the possible image frequency is limited so that they can only be used in slow processes.

[0094] With an LED line or an LED matrix, where individual LEDs or even individual LED modules, which consist of a number of single LEDs, can be separately controlled, both the required brightness and the required switching frequency can be realised, synchronised with the image recording of the cameras. Or a number of lines can be combined to form a matrix.

[0095] In the simplest form the individual LEDs/LED modules can only be switched on or off. This means that only a rectangular brightness curve can be realised, which is only a rough approximation of the actually desired brightness curve. This is already sufficient for performing the phase shift method, but the accuracy is limited. By taking various measures a better approximation to the desired curve can be achieved. The closer one comes to a sinusoidal curve, the better is the accuracy. The illumination line/illumination matrix can be modified such that intermediate brightnesses for individual LEDs can also be set. Depending on the size of the LEDs or LED modules a good approximation of the sinusoidal curve can thus be achieved. This is possible e.g. in that the individual LEDs/LED modules are only connected from time to time during the actual exposure time. However this method is expensive because extremely fast control electronics are then required. A solution preferred according to the invention provides for the pattern to be mapped blurred on the camera. This has already been described and is not repeated here.

[0096] It is pointed out that in terms of the above description the terms of camera and image recording device are used synonymously. All features and functions disclosed in relation to the camera apply correspondingly also for the image recording device and vice-versa.

LIST OF REFERENCE SYMBOLS

[0097] 1 object [0098] 2 copper film [0099] 3 plastic film [0100] 4 first defect [0101] 5 second defect [0102] 6 movement direction [0103] 61 shift [0104] 7 recording device [0105] 8 illumination device [0106] 9 inspection device [0107] 10 surface [0108] 11 recording sensor [0109] 12 image point [0110] 13 pattern [0111] 130 illumination pattern [0112] 14 brightness distribution [0113] 15 visual ray [0114] 16 surface normal [0115] 17 reflection zone [0116] 170 reflection point [0117] 171 cutting area of the reflection zones of individual images [0118] 18 pattern area [0119] 181 cutting area of the pattern areas in individual images [0120] 19 visual rays [0121] α reflection angle