WO2009083251A1

WO2009083251A1 - Method and apparatus for optically inspecting a surface of an object

Info

Publication number: WO2009083251A1
Application number: PCT/EP2008/011127
Authority: WO
Inventors: Klaus Knupfer; Bernd Spruck; Wolfgang Kimmig; Rolf Beck
Original assignee: Carl Zeiss Ag
Priority date: 2007-12-27
Filing date: 2008-12-29
Publication date: 2009-07-09
Also published as: DE102007063529A1

Abstract

In order to optically inspect a surface (17), a pattern is provided that forms a first spatial intensity curve (34a) with a first spatial period. The object having the surface (17) is placed relative to the pattern in such a way that the first spatial intensity curve (34a) falls onto the surface (17). A number of primary images is taken which show the surface (17) with the first spatial intensity curve (34a). Properties of the surface (17) are determined in accordance with the primary images. According to one embodiment of the invention, at least one additional spatial intensity curve (34b, 34c, 34d) with an additional spatial period is provided. The first and the additional spatial periods differ from one another. A number of additional primary images is taken which show the surface (17) with the additional spatial intensity curve (34b, 34c, 34d), and at least one parameter representing scattering characteristics (110, 112) of the surface (17) is determined in accordance with the first and additional primary images.

Description

Method and device for optically inspecting a surface on an object

The present invention relates to a method for optically inspecting a surface on an object, comprising the steps of:

Providing a pattern that forms a first spatial intensity course having a first spatial period,

Positioning the article with the surface relative to the pattern such that the first spatial intensity profile falls on the surface,

Taking a number of first primary images showing the surface with the first spatial intensity profile, and Determine properties of the surface as a function of the primary images.

The invention further relates to an apparatus for optically inspecting a surface on an article, having a pattern forming a first spatial intensity profile having a first spatial period, a receptacle for positioning the article with the surface relative to the pattern such that the first spatial Intensity profile falls on the surface, at least one image pickup unit for receiving a number of primary images, which show the surface with the first spatial intensity profile, and an evaluation unit for determining properties of the surface in response to the primary images.

Such a method and such a device are known for example from DE 198 21 059 Al.

In the industrial production of products, the quality of product surfaces plays an increasingly important role. These may be decorative surfaces, such as paint surfaces of motor vehicles, or technical surfaces, e.g. the surfaces of precision machined metallic pistons or bearings.

On the one hand, high quality can be achieved through high-quality and stable production processes. On the other hand, the relevant quality parameters should be checked as completely as possible in order to detect quality defects at an early stage. There are already a variety of concepts to inspect product surfaces. Frequently, however, the known methods and devices can only be used for a specific application because they require a high level of a priori knowledge about the surface to be inspected. This particularly concerns the question of whether it is a glossy and / or reflective surface or a matte surface. Matt, rough surfaces are inspected in particular by so-called fringe projection methods. In this case, a stripe pattern is projected from a known position and direction on the matte surface, and the resulting image is taken with an image pickup unit at a defined angle to the projection direction. The projected patterns are distorted due to height variations of the surface. Based on the known geometrical relationships between the pattern projector, the surface and the image acquisition unit, information about the local heights of the surface can be determined. However, such fringe projection methods are not suitable or only conditionally suitable for the inspection of shiny, reflecting surfaces, because the actual surface is not or only very poorly visible in such a case.

Also, for inspection of glossy and specular surfaces, various methods of striped patterns have been proposed. However, the striped patterns need not be projected from a known position. Rather, it is enough for the patterns to fall "somehow" onto the surface, using the glossy or reflective surface as part of an optical system that looks at the fringe patterns, and determining surface properties based on the system behavior of that optical system. These methods are often referred to as "deflectometric methods." A basic description of such methods can be found in a publication by Markus Knauer, "Surveying Reflective Surfaces - A Task of Optical 3D Sensing", published in the German magazine Photonik, issue 4 / 2004, pages 62-64 or in a publication by Sören Kammel, "Deflectometry for quality testing of specularly reflecting surfaces", published in the German magazine tm - Technisches Messen, issue 4/2003, pages 193-198 called Phase Measuring Deflectometry (PMD) is shifted with a sinusoidal intensity profile relative to the surface to be inspected. At intervals of 90 ° with respect to the period of the intensity profile, images are taken. These images are evaluated according to the so-called 4-bucket method, whereby one obtains phase information for each considered point of the object surface, which indicates the local surface tilt is representative of this object point. By integrating or differentiating the local surface slopes, the local height and local curvature of the surface can be determined so that dents, scratches, and other dimensional surface defects can be detected.

The aforementioned DE 198 21 059 Al proposes to use such a deflektometric method also for the inspection of a diffusely reflecting, that is non-reflecting surface. Although the diffuse scattering behavior of the surface leads to a blurring of the intensity distributions. However, this can be resolved by the phase reconstruction, if a sinusoidal intensity curve with a low spatial frequency is used.

US 6,239,436 Bl proposes a method and a device for inspecting a matte surface, wherein infrared radiation with a structured pattern falls on the surface and is recorded with an infrared camera. Also in this process, it is primarily about to discover bumps, in particular to examine unpainted body panels for motor vehicles.

DE 103 45 586 A1 discloses a method and an apparatus for determining the structure of a reflecting surface, wherein a plurality of areal patterns having mutually different dimensions are produced on the surface. The evaluation here corresponds more to a fringe projection method, since a height information of the surface is determined on the basis of the geometrical relationships between the pattern generator and the recorded image. For the investigation of weak or non-reflecting surfaces, DE 103 45 586 A1 also proposes the use of infrared radiation.

DE 199 44 354 B4 also discloses a method and apparatus for inspection of specular surfaces using a stripe pattern with a sinusoidal intensity profile. This document proposes, based on theoretical considerations, a formula by which the optimal period of the Intensity curve as a function of the wavelength of the light used and in dependence on the distance between the stripe pattern and the surface can be determined.

In a publication by Perard and Beyerer entitled "Three-dimensional measurement of free-form surfaces with a structured-lighting reflection technique", published in SPIE, Vol. 3204, pages 74-80, a method is described for determining the three-dimensional structure of a Thereafter, a mathematical model of the surface is deformed until the mathematically calculated distortions of a strip pattern reflecting on the surface coincide with the actually recorded image of a mirrored stripe pattern.

None of the previously proposed methods has been able to fully assert itself in industrial practice for the automatic inspection of surfaces. To date, many surfaces are inspected with the help of the human eye, even if some of the technical methods of the aforementioned type for preparing the images are used. One reason for this could be the many different surface and surface defects that can occur in reality and whose properties can vary considerably. None of the previously known methods seems to be able to reliably inspect different surfaces for different defects.

Against this background, it is an object of the present invention to provide a method and a device which enable an at least largely automated inspection of a surface. Advantageously, the method and the device for a variety of different surfaces should be used, such as for inspection of paint surfaces in motor vehicles, for the inspection of plastic parts or for inspection of finely machined metallic surfaces. The object is achieved according to one aspect of the invention by a method of the type mentioned, in which at least one further spatial intensity profile is provided with a further spatial period, wherein the first and the further spatial period are different from each other, wherein a number of further primary images is recorded, which show the surface with the further spatial intensity curve, and wherein, depending on the first and further primary images, at least one parameter is determined which is representative of a scattering characteristic of the surface.

According to a further aspect of the invention, this object is achieved by a device of the type mentioned, with at least one further spatial intensity curve with a further spatial period, wherein the first and the further spatial period are different from each other, and with a control unit, which is adapted is to take with the help of the at least one image pickup unit, a number of further primary images, wherein the further primary images show the surface with the further spatial intensity curve, and wherein the evaluation unit is adapted to determine depending on the first and the further primary images at least one parameter which is representative of a surface strike characteristic.

Particularly advantageously, the problem can be solved with a computer program with program code, which is stored on a data carrier and which is designed to carry out such a method when the program code is executed on a computer, in particular on a computer designed as a control and evaluation for a device of the aforementioned type.

The new method and device are based on the well-known idea of examining the surface of an object with the aid of a stripe pattern. For the first time, however, the aim is to measure the scattering characteristics of the surface using various striped patterns. As a result of this survey, the new method and device allow an automated statement as to whether it is a highly glossy, reflective surface, a dull, diffused surface scattering surface or is a mixed or intermediate form between these two extremes. In this case, in preferred embodiments of the invention, not only a qualitative value is determined, by means of which a reflecting surface can be distinguished from a non-reflecting surface. Preferably, the new method and the new device serve to determine the position, width and direction of the so-called scattering lobe of individual surface points or surface areas of the surface to be inspected with the aid of the various fringe patterns. The different stripe patterns differ with respect to the spatial period of the respective intensity profile. The surface to be inspected is picked up multiple times to obtain a number of first and further primary images, the primary images showing the surface with at least one of the intensity gradients. It is even possible and preferred in embodiments of the invention that the stripe patterns with the different intensity gradients are superimposed on patterns and form a common pattern, so that the first and further primary images are identical or at least can be recorded together. This preferred embodiment is described in detail in two earlier patent applications (German patent application with the file reference DE 10 2007 034 689.3 and international patent application with the file reference PCT / EP2008 / 005683). These two earlier patent applications are hereby incorporated by reference in their entirety,

With a strongly glossy, reflecting surface, the surface reflects incident light essentially according to the law of incidence angle = angle of reflection. An incident light beam is thus reflected in a defined direction or in a narrow space. The radiation lobe is accordingly narrow and inclined in the exit direction. In contrast, a diffusely reflecting surface, at least in the ideal case, has a largely spherical radiation lobe (Lambert radiator). Such a surface will appear the same light, no matter which direction you look at it.

As has been shown, it is possible to interpret the scattering characteristic of a surface as a low-pass filter over which an image pickup unit forms a striped pattern observed with a spatial intensity curve. If the fringe pattern has a very long period (ie, a low spatial frequency), this period can be resolved relatively well in the reflected image, even if the surface diffuses diffusely. In contrast, high spatial frequencies of a fringe pattern are "filtered out", ie the image recording unit sees only a washed-out gray image.

The new method and the new device take advantage of this property by measuring the low-pass behavior of the surface metrologically on the basis of stripe patterns with different periods (and thus different spatial frequencies). In preferred embodiments, the frequency response of the low-pass filter is measured using a plurality of stripe patterns with different periods. Depending on the measurement results, the scattering characteristic in the form of the scattering lobe can be determined on the basis of the measured frequency response. This gives a parameter that characterizes the scattering behavior of the surface very accurately. Depending on this, the surface can be classified, for example if a certain gloss level is a quality criterion of the surface to be inspected. In other embodiments, the characteristic may serve to select an optimal fringe pattern and / or measurement method for further automated inspection of the surface.

The new method and apparatus enable automated inspection of a surface without the need to know in advance a priori whether it is a highly glossy, specular, or a dull, diffusely diffusing surface. In addition, with the new method and the new device, mixing and intermediate forms between these two extreme examples can be determined metrologically. For example, it can be determined at which points a partially specular and partially diffusely scattering surface mirrors or does not reflect. Furthermore, local gloss levels of the surface can be determined.

Because of these properties, the new method and apparatus for automated inspection of a variety of different surfaces are suitable. chen. The new method and apparatus can be used particularly advantageously in technical surfaces, such as, for example, finely machined metal surfaces. In addition, the new method and the new device are also very suitable for inspection of paint surfaces in motor vehicles or the like. The above object is therefore completely solved.

In a preferred embodiment, the first and the further intensity profile have an at least substantially continuously changing light intensity. Most preferably, a sinusoidal intensity profile is used, although this is not the only possibility. For example, a sawtooth or triangular intensity profile could also be used.

In contrast to this preferred embodiment, some prior art methods use binary stripe patterns, i. Stripe pattern with evenly bright and evenly dark stripes. The intensity profile of such a stripe pattern jumps between light and dark. In principle, it seems possible to measure the scattering characteristic of the surface even with such an intensity profile. Preferably, however, an intensity progression which changes at least largely continuously is used, because such an intensity course contains phase information by means of which the position of the intensity profile relative to the surface can be determined more accurately. A sinusoidal intensity curve therefore allows a very accurate evaluation.

In a further embodiment, at least three primary images are recorded, which show the first and the further intensity profile, wherein the surface in each of the at least three primary images has a different position relative to the respective intensity profile. Preferably, in each case four primary images are recorded, wherein the intensity profile is shifted in each case by 90 °.

This embodiment allows a very simple and fast evaluation of the primary images according to the so-called 4-bucket method. As a result, can be also determine the local surface slopes on the basis of the primary images, so that in addition to the degree of gloss and a dimensional property of the surface is determined. This embodiment therefore allows a very efficient inspection of a surface.

In a further refinement, the at least three primary images are recorded with the same viewing direction relative to the surface points to be inspected. Preferably, the at least three primary images are recorded under the same optical conditions, ie also with the same focus setting, etc.

This embodiment further simplifies the new method and device. The individual primary images can be directly compared with each other and computationally linked to determine the frequency response and possibly other properties of the surface.

In a further refinement, the surface has a large number of surface points at which a local temporal intensity profile with a local amplitude value is produced by the respective other position, local scattering characteristics for the plurality of surface points being determined as a function of the local amplitude values.

This embodiment has the advantage that the scattering characteristic of the surface is determined in relation to the individual surface points. Therefore, glossy and less lustrous areas of a surface can be differentiated and automatically separated from each other. In other words, this design enables a spatially resolved, detailed "gloss analysis" of a surface It is easily understood that such information is of great advantage for automated inspection of a surface.

In a further embodiment, the parameter is determined as a function of the local scatter characteristics. In this embodiment, a global characteristic is determined as a function of the local scattering characteristics. In a very simple embodiment, the parameter represents a qualitative statement about the gloss level of the surface, such as "high gloss" or "matt". Such a global characteristic allows a simple classification and in particular a further inspection of the surface with the aid of specifically selected methods from the prior art. For example, a surface that has been classified as dull or diffusely scattered may be inspected by a known stripe projection method, while a glossy classified surface is further inspected using a deflektometric method.

In a further embodiment, local surface slopes for the plurality of surface points are also determined based on the primary images.

This embodiment uses the information from the individual primary images in a very advantageous and efficient manner. The pre-existing primary images are well suited for detecting scratches, splinters, small bumps, or other dimensional and localized surface defects.

In a further refinement, a defined spatial intensity profile is determined as a function of the parameter in order to determine the local surface inclinations.

In this embodiment, first the scattering characteristic of the surface is measured with the aid of a plurality of intensity profiles. Subsequently, an optimum intensity profile is selected, in particular to determine the local surface slopes by means of a deflektometric method. The previously determined scattering characteristic replaces a priori knowledge of the gloss properties of the surface. The optimal intensity curve enables a particularly high measuring accuracy and measuring speed for the further investigations of the surface. In a further refinement, the spatial intensity profiles are kept spatially stationary, with the object being displaced with the surface relative to the spatial intensity profiles in order to record the at least three primary images.

This embodiment allows a very fast and efficient inspection of large surfaces, such as painted surfaces of motor vehicles.

In a further embodiment, which also represents an inventive development in comparison with the prior art, a data record is determined as a function of the parameter, which represents a defect-free surface on the object.

The data set of this embodiment includes a number of quality parameters, which may be defined for example on the basis of a so-called feature cloud. Each quality parameter represents a defined property that the surface of an object to be inspected must or must meet in order to be classified as defect-free. For example, a quality parameter may represent the minimum and / or maximum gloss level that the surface is to have. For example, another quality parameter may be a maximum accepted number and / or size of scratches, voids, or dull surface areas on the inspected surface. The data record is individual for each type of test specimen and serves as a reference for all objects of a test specimen to be inspected. If an inspected item meets all the requirements that the record represents, it is classified as defect-free. If he does not meet the requirements or not completely, he is usually classified as defective. The advantage of this embodiment is that an already existing data set, which was created for a defined type of test object, can be transferred quite simply and purposefully to another type of test object as a function of the new parameter. This makes it easier to transfer experience gained during the inspection of one type of specimen to the inspection of items of another specimen type. For example, if you have a record for the inspection of chrome and thus created reflecting spoons and now wants to inspect chromed thermos for the first time, the existing data set for the spoon inspection can be taken quite easily and quickly and adapted to the individual characteristics of the thermoses, provided that the properties of the respective surfaces are largely the same. The latter can be easily and reliably determined with the help of the new parameter, preferably even automated. In a particularly preferred variant of this embodiment, the new device is a compact "hand-held" test device, which is designed essentially to determine the new characteristic and thus scattering characteristic of a surface on an object to be inspected for the first time, depending on one already In other words, the new parameter is advantageously used here as a criterion for the transferability of quality parameters from one type of test specimen to another type of test specimen in order to determine the suitability of an article for a particular type of surface inspection, in particular the suitability for a reflectometric surface inspection.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Show it:

1 is a simplified representation of an embodiment of the new device with an inspection tunnel for inspecting painted surfaces in motor vehicles,

2 shows the inspection tunnel of FIG. 1 with another strip pattern, FIG. 3 shows the inspection tunnel from FIGS. 1 and 2 in a cross section from the front,

4 is a schematic representation for explaining the method,

5 is a simplified flowchart for explaining an embodiment of the new method, and

Fig. 6 different scattering lobes in relation to different stripe patterns.

In Fig. 1 and 2, an embodiment of the new device is designated in its entirety by the reference numeral 10.

The apparatus 10 here includes a tunnel 12 having a front end and a rear end. The tunnel 12 has a longitudinal axis 14, along which a car 16 is moved with a paint surface 17 to be inspected in the direction of the arrow 18. The car 16 is arranged here on a transport vehicle 20, which is pulled through the tunnel 12, for example by means of an electric drive (not shown). Alternatively, the car 16 may be disposed on a conveyor belt, or the car 16 may be pulled or driven through the tunnel without a trolley 20. In any case, the tunnel floor, possibly together with the trolley or the conveyor belt, forms a receptacle for the object to be inspected.

As can be seen in Fig. 3, the tunnel 12 here has a substantially circular cross-section covering a circular angle of about 270 °. In principle, other tunnel cross sections are possible, for example in the form of a polygon or a rectangular tunnel cross section. A circular tunnel cross-section or other kink-free tunnel cross-section is preferred from today's perspective, because the patterns explained below can then be realized largely continuously and without joints, which simplifies the inspection of the paint surface. The tunnel 12 can also be realized with the aid of mirrors (not shown here), with which the degree of coverage can be increased in a simple manner. The new procedure and however, the new device is not limited to the use of a tunnel. In simple cases, stripe patterns of the type described below may also be provided over a flat screen or on a simple wall.

The tunnel 12 has an inner wall 24 on which here two strip patterns 26, 28 are arranged. The stripe pattern 26 consists of lighter stripes 30 and darker stripes 31, which run alternately next to one another and parallel to one another. The striped pattern 28 includes lighter stripes 32 and darker stripes 33, which are also arranged parallel to each other and side by side. In the illustrated embodiment, the darker stripes 31, 33 are spectrally different, as illustrated by different "dot densities" in Figure 1. For example, the darker stripes 31, 33 are realized in different colors, preferably blue and red is advantageously used to spectrally differentiate the different fringe patterns.

In preferred exemplary embodiments, the strips run spirally along the inner wall 24 of the tunnel. In one exemplary embodiment, the strip patterns 26, 28 are painted onto the inner wall 24 of the tunnel 12. In another embodiment, the inner wall 24 of the tunnel 12 is covered with a film on which the different stripes are printed. In a further exemplary embodiment, a plurality of light-emitting diodes are arranged behind a semitransparent screen on the inner wall 24. It is advantageous if the light-emitting diodes are individually controllable, and in one embodiment the light-emitting diodes can be switched over in color be either from the interior 24 of the tunnel or by a projection from the outside, wherein the outer wall of the tunnel in the latter case is a semi-transparent screen In another embodiment, the tunnel walls are made of a partially transparent material having frosted areas. The transparent material serves for light transmission via total reflection. The frosted areas shine in this case.

Each pattern 26, 28 forms a spatial intensity curve 34, which is sinusoidal in the illustrated embodiment. In principle, however, other brightness profiles are possible, such. Sawtooth or triangular course. What is common to all intensity curves is that they have an amplitude of 35 and a period of 36.

Reference numerals 38 and 40 designate two camera heads with the camera head 38 located at the front end of the tunnel 12 while the camera head 40 is located at the rear end. Each camera head 38, 40 here has four image recording units 42, 44, 46, 48, which are staggered with a defined distance from one another (see FIG. 4 and following explanations). The viewing directions 50 of the image recording units 42, 44, 46, 48 are parallel to each other, as shown schematically in FIGS. 1 and 2. In one embodiment, each camera head 38, 40 has a variable color filter 51, with the aid of which either the one or the other stripe pattern 26, 28 can be selected for image acquisition.

As can be seen in Fig. 3, the device 10 here has three camera heads 38a, 38b, 38c at the front end of the tunnel and three corresponding camera heads 40a, 40b, 40c at the rear end (not shown). The three camera heads 38a, 38b, 38c are distributed along the cross-sectional area of the tunnel 12 so that they can completely accommodate the car 16. Each image pickup unit 42, 44, 46, 48 is realized here as a line scan camera, ie the image pickup units 42, 44, 46, 48 each have an image sensor with a line-shaped arrangement of pixels. Within each camera head 38, the line sensors are staggered one behind the other in such a way that the viewing directions 50 shown in FIG. 1 result with the defined distances and the visual fans 54 shown schematically in FIG. To increase the light intensity, so-called TDI line sensors are preferably used. Alternatively, however, the image recording units 42 to 48 can also be realized as area cameras. In one exemplary embodiment, each image acquisition unit 42 to 48 is an area camera with a matrix-like arrangement of pixels (not shown), only individual rows or columns being read out from this matrix-type arrangement, so that the matrix-like arrangement is comparable to a staggered array of line sensors. In another embodiment, the matrix-like arrangements of the pixels are read out flat in order to capture in this way a larger section on the surface 17 of the car 16. Preferably, deviations of the feed movement from the ideal feed movement are computationally compensated on the basis of the camera images. For this purpose, it is advantageous if markings (not shown here) are arranged on the transport carriage 20, with the aid of which deviations from the ideal feed movement can be detected.

In a further, not shown embodiment, a plurality of image pickup units are arranged on holders which are fixedly coupled to the trolley 20. In this case, the image pickup units are moved together with the car 16 relative to the pattern 26, 28.

The reference numeral 60 denotes an evaluation and control unit, which is designed on the one hand to control the advancing movement 18 of the car 16 and the image recording. In preferred embodiments, the car 16 is continuously moved through the tunnel 12. In other embodiments, the feed takes place stepwise, wherein after each feed step, an image acquisition takes place with the image recording units 42 to 48.

4 shows the surface 17 at a total of four different positions Po, Pi, P2, P ₃ . In addition, the four image recording units 42 to 48 of one of the camera heads 38, 40 are shown. The reference numeral 62 denotes the relative distance from one image acquisition unit 42 to the next image acquisition unit 44, the distance being dimensioned parallel to the advance direction 18 of the surface 17. Reference numeral 64 denotes the distances over which the surface 17 of a Position P _{0 is moved} to the next position Pi, etc. Reference numeral 66 denotes a pattern image, ie, an image of the stripe pattern 26 or 28 which is reflected from the surface 17 or otherwise can be detected on the surface 17. Reference numeral 66 'shows the pattern image 66 on the surface 17' after being advanced by the distance 64.

As can be seen in FIG. 4, a surface point 68 is recorded with the image pickup unit 42 at the position Po of the surface 17. It is assumed that, at the time of image pickup, a dark stripe area is incident on the surface point 68, which is shown in FIG. 3 by the viewing direction of the image pickup unit 42 with respect to the pattern image 66.

Due to the selected distances 62 and distances 64, the same surface point 68 is recorded under the same optical conditions as previously with the image acquisition unit 44. However, at this time, another portion of the spatially stationary pattern 26 falls on the surface point 68, which is shown at reference numeral 66 '. The reason for the change of the pattern image is the relative movement of the surface 17 with respect to the pattern 26.

As can be understood from the illustration in FIG. 4, the same surface point 68 is subsequently also recorded with the further image recording units 46, 48. Due to the spatial displacement of the car 16 relative to the pattern 26, 28, a temporal intensity profile 70 arises at each surface point, which reflects the relative position of the surface point 68 with respect to the spatial intensity profile 34. With the four image pickup units 42, 44, 46, 48, four primary images representing instantaneous images of the temporal intensity profile 70 are obtained for each surface point 68.

As can be seen from FIGS. 1 and 2, the new device 10 has the possibility of producing different patterns 26, 28 with different spatial periods 36a, 36b and correspondingly different spatial frequencies. In one advantageous embodiment, in which the patterns 26, 28 are generated by means of (organic) light-emitting diodes, these LEDs are driven accordingly different. However, it is also conceivable that the tunnel 12 is so long that the patterns with the different periods 36a, 36b along the feed direction 18 are arranged one behind the other. In a further exemplary embodiment, which is not shown here in the figures, the intensity profiles 34a, 34b with the different periods 36a, 36b are arranged one above the other and / or one inside the other, wherein the different intensity profiles 34a, 34b can then advantageously be realized in different colors , Such an embodiment makes it possible to record the primary images with multiple patterns.

The various intensity profiles 34a, 34b are advantageously used according to the new method in order to measure the scattering characteristic of the surface 17 of the car 16. An embodiment of the new method will be explained below with reference to FIG. 5.

In step 72, the initial position x of the surface along the feed direction 18 is first determined. This can be done in a known manner with the aid of position sensors, which are arranged along the tunnel axis 15. Subsequently, in step 74, a count variable n is set to zero. In the next step 76, the count variable is incremented by one. According to step 78, a first intensity profile 34a having a first spatial period 36a is generated on the inner wall 24 of the tunnel. Subsequently, in step 80, primary images # 1a.n / # 2a.n / # 3a.n / # 4a.n are recorded with the four image capturing units 42 to 48. Image # la.n here denotes a primary image which was taken with the first image recording unit 42 in the iteration step n, wherein the first intensity profile 34a coincides with the first period 36a on the surface 17 to be inspected. Image # 2a.n accordingly denotes a primary image that was taken in the iteration step n with the second image recording unit 44, etc.

In accordance with step 82, a further intensity profile 34b is then generated with a further period 36b on the inner walls of the tunnel. According to step 84 another picture series # lb.n / # 2b.n / # 3b.n / # 4b.n is taken. Corresponding to the loop 86, further image series with further intensity profiles can be recorded. In preferred exemplary embodiments, at least four image series with respectively different intensity profiles are recorded. The more intensity curves with different periods / spatial frequencies are recorded, the more accurately the low-pass behavior of the surface 17 can be determined.

After step 88, the car 16 is then advanced with the surface 17 by the distance 64 (position Pi in Fig. 4). Subsequently, a query is made in step 90 as to whether sufficient iteration steps have been carried out, so that advantageously at least four primary images are available for each surface point 68 to be inspected. In the illustrated embodiment, it is checked whether the iteration counter n = 4 s, where s in the simplest case = 1. If the check in step 90 shows that not enough iteration steps have been completed, the method returns to step 76 according to loop 92, and further image series with the four image acquisition units are recorded.

After the required number of loop passes 90 has been reached, enough images are available for evaluation. According to step 94, firstly the local amplitudes a (x, y) of the temporal intensity profile which results from the displacement of the first intensity profile 34a relative to the surface 17 at the surface points 68 are determined. For the preferred case here, in which a sinusoidal intensity profile 34a is used and in which four primary images are recorded at intervals of 90 °, the local amplitudes a (x, y) can be calculated according to the following formula:

Here, Ii, I ₂ , I ₃ and I ₄ denote the local intensity values which result at the individual surface points 68 with the coordinates x, y in the four primary images of the image series.

According to step 96, the local amplitudes b (x, y) of the temporal intensity profile which results when using the additional spatial intensity profile or paths 34b are then determined.

According to step 98, local spatial frequency responses F (x, y) are determined on the basis of the different local amplitudes a, b. In other words, a characteristic is determined which, for each surface point to be inspected, indicates the local amplitude as a function of the spatial period of the respectively used intensity profile. According to step 100, a Fourier transformation of the local frequency responses F (x, y) is then performed. The Fourier transformation is used to obtain the local scattering characteristics of the surface at the individual surface points x, y.

In step 102, a parameter is then determined which is representative of a global scattering characteristic of the surface. The parameter may advantageously be an average of the local scattering characteristics. In other embodiments, the characteristic is a location-dependent, i. function dependent on the surface points of the surface, indicating the local scattering characteristics.

After step 104, an optimal intensity profile 34 is subsequently selected. "Optimal" means, for example, that an intensity profile with the smallest possible period is selected (maximum spatial frequency), in which the individual periods in the recorded primary images can still be distinguished from one another, in other words an intensity profile with the maximum possible spatial frequency is selected here , in which the diffuse scattering is still suppressed. Step 106 then determines further local surface properties. This includes in particular the local phase positions φ (x, y) of the temporal intensity profiles relative to the phase position of the selected, optimal intensity profile 34. The local phase positions φ (x, y) are representative of the local inclinations of the surface points. They can be determined according to the following formula:

φ (x, y) = atan2 [- (I ₂ -I ₄ J ₁ (I ₁ -I ₃ )]

With

arctan ^ for x> 0 arctan | - + π for x <0, y> 0 arctan | - -π for x <0, y <0 atan2 (y, x): = ^x

+ π / 2 for x = 0, y> 0

-π / 2 for x = 0, y <0

0 for x = 0, y = 0.

Furthermore, in a preferred embodiment, a local average Iav _g (x, y) of the intensity values is determined. The corresponding formula is:

This local mean value is representative of the gray value of the surface at the individual surface points and thus for the local reflectance.

Fig. 6 illustrates the concept of the new method and the new device. Like reference numerals designate the same elements as before.

FIG. 6 shows symbolically different intensity profiles 34a, 34b, 34c, 34d with different periods / spatial frequencies. The multiple intensity gradients can be color-coded so that they can be distinguished from each other. Alternatively, the various intensity traces are generated one after the other over the surface 17, or they are generated by means of a combined pattern, as described in the earlier International Application Serial No. PCT / EP2008 / 005683, which is incorporated herein by reference in its entirety.

Reference numeral 68 represents a surface point of the surface 17 having two distinct lobes 110, 112. The radiation lobe 110 shows the scattering characteristic of the surface point 68 when the surface point 68 is a diffuse scattering point (Lambert radiator). The radiation lobe 112, on the other hand, shows the case that the surface point 68 strongly reflects. Reference numeral 114 symbolically represents the angular range from which light rays falling on the surface point 68 are reflected to the image pickup unit 38. At reference numeral 116, the corresponding angular range for the narrow beam 112 is shown. As can be seen from the viewpoints 114, 116, a surface point 68 with Lambertian behavior reflects light from a much larger angle range to the image capture unit 38 than a surface point with a narrow radiation lobe 112. Therefore, an intensity trace 34b, 34c, 34d can be at higher spatial frequencies when viewed through the diffusely scattering surface point 68, they are no longer resolved into light and dark areas. In other words, the image acquisition unit 38 only sees a blurred gray image of the intensity curves with higher spatial frequencies. On the other hand, the light and dark stripes can be distinguished from each other when the intensity traces 34a to 34d are viewed through a specular surface point 68 having a narrow radiation lobe 112. The frequency response of the specular surface has a much higher transmission frequency than the frequency response of the diffusely diffusing surface. By now examining the surface with several intensity curves of different periods / spatial frequencies, the radiation lobe 110, 112 can be determined very easily and quickly by measurement. In preferred embodiments, the articles are classified into "defect-free" and "non-defect-free" by comparing the individual properties of the inspected surface with a reference. The reference is advantageously provided in the form of a parameter data set containing reference or comparison values for a plurality of surface properties representing a surface classified as defect-free. In preferred embodiments, a parameter data set created for an individual sample type, such as a chromed spoon, is used to classify similar but different types of samples when the particular characteristic indicates that the surface scatter properties are the same or at least the same for both types of samples are largely the same. It is understood that the existing parameter data record can be adapted to the individual properties of the new type of test object. For example, if a quality parameter represents the number of allowed surface defects per article, it may be necessary to vary that number depending on the size of the particular type of sample. On the other hand, if the surfaces of the two types of specimens are substantially the same, because they were made using the same manufacturing process, another quality parameter representing a minimum required gloss level can be maintained without adaptation. The decision on the transferability of quality parameters as a function of the parameter determined here is very advantageously possible with a compact hand-held device (not shown here), which has an integrated pattern generator for generating the different stripe patterns, an integrated display for displaying the different stripe patterns, an integrated camera for recording the primary images and an integrated evaluation and control unit, which is designed to automatically determine the parameter and display.

Claims

claims

A method of optically inspecting a surface (17) on an article (16), comprising the steps of:

Providing a pattern (26, 28) forming a first spatial intensity profile (34a) having a first spatial period (36a),

Positioning the article (16) with the surface (17) relative to the pattern (26, 28) such that the first spatial intensity profile (34a) falls on the surface (17),

Taking (80) a number of first primary images (66, 66 ') showing the surface (17) with the first spatial intensity profile (34a), and

Determining (94-102) properties of the surface (17) in dependence on the primary images (66, 66 '),

characterized in that at least one further spatial intensity profile (34b, 34c, 34d) is provided with at least one further spatial period (36b), the first and further spatial periods (36a, 36b) being different from each other, a number of others Primary images (66, 66 ') is recorded, showing the surface (17) with the further spatial intensity curve (34b, 34c, 34d), and wherein at least one characteristic determined in dependence on the first and further primary images (66, 66') (102) representative of a scattering characteristic (110, 112) of the surface (17).

2. The method according to claim 1, characterized in that the first and the further intensity profile (34a-34d) have a spatially at least substantially continuously changing light intensity.

3. The method of claim 1 or 2, characterized in that at least three primary images (66, 66 ') are recorded showing the first and the further intensity profile (34a-34d), wherein the surface (17) in each of the at least three Primary images (66, 66 ') has a different position relative to the respective intensity profile (34a-34d).

4. The method according to claim 3, characterized in that the at least three primary images (66, 66 ') are recorded with the same viewing direction (50) relative to the surface to be inspected (17).

5. The method according to claim 3 or 4, characterized in that the surface (17) has a plurality of surface points (68), at which by the respective other position, a local time intensity curve (70) is formed with a local amplitude value, wherein in dependence from the local amplitude values, local scattering characteristics (110, 112) for the plurality of surface points (68) are determined (94-100).

6. The method according to claim 5, characterized in that the characteristic in dependence on the local Streucharakteristiken (110, 112) is determined (102).

7. The method of claim 5 or 6, characterized in that based on the primary images (66) also determines local surface slopes for the plurality of surface points (106).

8. The method according to claim 7, characterized in that depending on the characteristic a defined spatial intensity curve (34) is determined (104) to determine the local surface slopes.

9. The method according to any one of claims 3 to 8, characterized in that the spatial intensity curves (34) are kept spatially stationary, wherein the object (16) with the surface (17) relative to the spatial intensity gradients (34) is moved to to record the at least three primary images (66).

10. The method according to any one of claims 1 to 9, characterized in that a data set is determined depending on the characteristic, which represents a defect-free surface (17) on the object (16).

11. A device for optically inspecting a surface (17) on an object (16), with

a pattern (26, 28) forming a first spatial intensity profile (34a) having a first spatial period (36a),

a receptacle (20) for positioning the article (16) with the surface (17) relative to the pattern (26, 28) such that the first spatial intensity profile (34a) falls on the surface (17),

at least one image pickup unit (38, 40) for picking up a number of first primary images (66, 66 ') showing the surface (17) with the first spatial intensity profile (34a), and

an evaluation unit (60) for determining properties of the surface (17) as a function of the primary images (66, 66 '), characterized by at least one further spatial intensity profile (34b, 34c, 34d) having a further spatial period (36b), the first and the further spatial period (36a, 36b) being different from one another, and a control unit (60) designed for this purpose is, with the help of the at least one image pickup unit (38, 40) to record a number of further primary images (66, 66 '), the further primary images (66, 66') the surface (17) with the further spatial intensity profile (34b, 34c , 34d), and wherein the evaluation unit (60) is designed to determine (94-102) at least one characteristic (94-102) dependent on the first and further primary images (66) that is suitable for a scattering characteristic (110, 112) of the surface ( 17) is representative.

A computer program with program code stored on a data carrier and adapted to carry out a method according to any one of claims 1 to 10 when the program code is executed on a computer.