WO2011098324A1

WO2011098324A1 - Method and device for surface examination by means of deflectometry

Info

Publication number: WO2011098324A1
Application number: PCT/EP2011/050584
Authority: WO
Inventors: Adam Bonemberg; Ralf Zink
Original assignee: Robert Bosch Gmbh
Priority date: 2010-02-09
Filing date: 2011-01-18
Publication date: 2011-08-18
Also published as: DE102010001715B4; CN102741649B; DE102010001715A1; CN102741649A

Abstract

The invention relates to a method for surface examination of a specimen according to the principle of deflectometry, in which at least one observation apparatus receives reflection images of a lighting apparatus which transmits electromagnetic radiation, wherein the reflection images of the lighting apparatus occur on the surface of the specimen and wherein an inclination of the surface of the specimen is determined from the shape of the reflection images. According to the invention, a lighting surface having one or more color modulations which can be specified in the color space is used as a lighting apparatus and at least one color line scan camera is used as an observation apparatus, wherein one or more chromatically coded image data are recorded by the color line scan camera simultaneously and pixel-accurate and information on the surface inclination is derived from the color information and information on the surface texture of a given pixel is derived from the intensity thereof. The invention further relates to a corresponding testing device for carrying out the method according to the invention, wherein the lighting apparatus is formed as a lighting surface having one or more color modulations which can be specified in the color space and the observation apparatus comprises at least one color line scan camera, with which one or more chromatically coded image data can be recorded simultaneously and pixel-accurately and, within an evaluation unit, information on the surface inclination can be derived from the color information and information on the surface texture of a given pixel can be derived from the intensity thereof. Compared to the previously conventional method for deflectometry, the method according to the invention and the testing device corresponding thereto provide particularly economical advantages. According to the invention, outer surfaces of rotation bodies can be recorded by a common color line scan camera in a short period of time and evaluated in respect of surface defects.

Description

description

title

METHOD AND DEVICE FOR SURFACE TESTING BY DEFLECTOMETRY

State of the art

The invention relates to a method for surface inspection of a specimen according to the principle of deflectometry, in which at least one observation device receives reflection images of a lighting device emitting electromagnetic radiation, in which the reflection images of the illumination device on the surface of the specimen arise, and in which from the shape of the Reflection images an inclination of the surface of the specimen is determined.

The invention further relates to a corresponding test device for carrying out the method according to the invention.

State of the art

For the automatic inspection of rotationally symmetrical products for damage and material defects, image processing systems with incident light or grazing light illumination, deflektometric systems with temporally and spatially modulated light as well as tactile systems are in use. Tactile measuring systems usually scan the surface of the test sample only on a narrow line and therefore require an extremely large amount of time for a complete test of a surface. Many products are still visually inspected manually due to technical limitations of the methods mentioned as well as economic considerations.

For example, image processing systems with reflected light and grazing light illumination can not distinguish between the feature representation in a gray value image. ometric defects, such as scratches or surface dents and discoloration of the surface differ. Since brightness fluctuations of the surface occur in many test objects, but these are usually not function-relevant, such systems have an increased pseudo error rate.

The optical measurement of reflective free-form surfaces is carried out by methods that are referred to as deflectometry. In these methods, an observation device is provided with one or more cameras, which are directed to the reflective surface of the measurement object and observe there a reflection image of an extended illumination device, which is usually a structured screen, a structured illuminated screen or television or Monitor representing structures acts. The reflective surface itself is not visible. By evaluating the structures of the reflection image, one can draw conclusions about the local inclination of the reflecting surface and thus on its shape. An example of a deflektometric measuring method is described in EP 1 882 8996 A1.

Available deflectometric systems can separately detect geometric features and the texture of a surface. However, they modulate in time and therefore require several images of the test object. In the case of rotationally symmetrical bodies, several images with different illumination modulations are difficult and therefore very difficult to implement, with the condition that each pixel represents the same object point in all images. If the object to be measured rotates, the camera must be triggered very quickly and spatially very precisely.

It is therefore an object of the invention, based on conventional deflektometric surface testing methods, to provide a surface inspection method which is simplified compared with the prior art.

It is a further object of the invention to provide a device suitable for carrying out the method. Disclosure of the invention

The object of the method is solved by the features of claims 1 to 7.

The method provides that a lighting surface with one or more predeterminable in the color space color modulations and as observation device at least one color line camera is used, wherein the color line camera one or more chromatically coded image data simultaneously and pixel precision are detected and the color information information to

Surface slope and from the intensity of which information about the surface texture of each pixel are derived.

The object relating to the device is achieved by using a corresponding test device for carrying out the method according to the invention, wherein it has at least one observation device and at least one illumination device with which reflection images of the illumination device in the visible spectral range can be detected, which arise on the surface of the test object , And in which in an evaluation of the shape of the reflection images, an inclination of the surface of the specimen is derivable. In this case, the illumination device is designed as an illumination surface with one or more predeterminable in the color space color modulations and the observation device has at least one color line camera, with the one or more chromatically coded image data simultaneously and pixel-accurately detectable and within the evaluation of the color information information about the surface slope and the intensity thereof Information about the surface texture of each pixel are derivable.

With the method according to the invention and the apparatus for carrying out the method, the expense involved can be significantly reduced compared to the methods of deflectometry that have hitherto been usual, which offers particular economic advantages. Elaborate engineering designs to ensure that each pixel represents the same object point in each modulation shot, as required in previously known methods for deflectometry, can be eliminated with the proposed method. A preferred variant of the method provides that electromagnetic radiation in the visible spectral range is emitted by the illumination device and the reflection image is detected therein.

In a further preferred variant of the method it is provided that a color value of the chromatically coded image data is assigned a slope value of the surface of the test object by means of a calibration. Thus, deviations in the topography during the inspection of the surface of the specimen can be quickly detected.

In a preferred variant of the method, it is provided that the color modulations of the illumination surfaces are clearly specified and adapted to the respective measurement task. It can thus be achieved that, depending on the inclination of the surface and corresponding to the reflection angle, each pixel of the chromatically coded image data can be unambiguously assigned corresponding surface regions of the test object.

A particularly effective color modulation in one direction can be achieved if the color modulation of the illumination source uses a cosine modulation of the individual RGB color channels whose phase is shifted by 120 ° in each case. This modulation allows a direct calculation of the slope and texture from the chromatically coded image data by means of a simple HSI color space transform.

If an overlay of a horizontal and a vertical modulation is used for the color modulation of the illumination source, it is possible to achieve, compared to a modulation in only one spatial direction, tilt values from the image data for the surface of the test object both in the x-direction and in the y-direction can be.

To increase the spatial resolution of the tilt data, it is advantageous if a superimposition of higher and lower frequency modulations is used for the color modulation of the illumination source.

In principle, it should be noted that different color modulations in the x and / or y direction of the illumination surface are used for the color modulation of the illumination source. che with color modulation, in which different spatial frequencies are superimposed, can be combined as desired.

A particularly preferred use of the method, as previously described, provides surface inspection of rotationally symmetric specimens, with the specimens rotated and scanned by the color line camera. Mantle surfaces of such bodies of revolution can be inspected with a standard color line camera in a short time and evaluated with respect to surface defects.

The invention will be explained in more detail below with reference to an embodiment shown in FIGS. Show it:

FIG. 1 shows a schematic illustration of a test device for surface testing;

FIG. 2 shows different embodiments of a lighting surface with different color modulations,

FIG. 3 shows further embodiments of the illumination surface with different color modulations,

FIG. 4 shows an example of a surface defect, present as chromatically coded image data and as calculated topology image data with calculated inclination data,

FIG. 5 shows the surface defect according to FIG. 4 as calculated topology image data in comparison to reference image data measured with other methods and FIG

FIG. 6 shows another surface defect, shown in different image data.

Figure 1 shows schematically the structure of a test apparatus with which the inventive method can be performed.

Shown is a rotationally symmetrical DUT 1 1, which is suspended rotating about its longitudinal axis. Illuminated the specimen 1 1 with a lighting surface 12, which has a color modulation 20, so that the surface to be tested of the specimen can be illuminated chromatically modulated. A color line camera 13 records the differently colored specular reflections for determining the surface inclination and the texture of the surface of the specimen 11. Schematically this is represented by the differently colored beam paths (red 21, blue 23 and green 25).

The method described above expressly relates to a color modulation in the visible spectral range of the light. In principle, the method can also be extended to other spectral ranges, for example to the near infrared range (NIR) or to the near UV range. To be considered here are appropriate light sources for lighting and corresponding recording systems that must have a sufficiently high sensitivity in these areas. Furthermore, in this context, in particular, the reflection or the transmission properties of the specimen 1 1 are to be considered in the spectral range used.

In order to be able to determine the reflection angle of each pixel, the illumination surface 12 imaged on the object is unambiguously chromatically modulated. Depending on the inclination of the surface is, according to the law of reflection of the beam optics, another point of the illumination surface 12 on the surface of the specimen 1 1 imaged on the camera chip of the color line camera 13.

Calibration allows each color value of the lighting to be assigned a slope value of the surface. In order to completely detect the lateral surface of a rotationally symmetrical object, the test object 11 is rotated and scanned with the color line camera 13. The image data 30 acquired in this way (see FIGS. 4 to 6) contain in their color information (color angle in the HSI color space system) information on the surface slope and in its intensity information on the surface texture of each pixel.

Depending on the sought-after feature on the specimen 1 1 different color modulations 20 of the illumination surface 12 are targeted. FIGS. 2 and 3 show corresponding examples of such color modulations 20 of the illumination surface 12.

FIG. 2 shows in the left image a cosine modulation of the individual color channels (red 21, blue 23 and green 25) whose phase is shifted by 120 ° in each case. This results in a color gradient surface which, seen from top to bottom, results in the colors red 21, gray-blue 22, blue 23, cyan 24, green 25 and yellow 26. Another example represents that is a middle image in FIG. 2. A gray value of coded color angles is shown. The right picture shows an intensity coding.

Since the modulations for the illumination surface 12 shown in FIG. 2 are chromatically modulated in only one direction, only one direction of inclination of the surface of the device under test 1 can be detected with this type of color modulation 20. In order to detect two mutually independent inclination directions on the specimen 11, and thus the entire information of the surface in the 3D space, a superposition of the horizontal color modulation 20 with a vertical color modulation 20 is necessary.

FIG. 3 shows a corresponding two-dimensional color modulation 20 of the illumination surface 12 in the left image. This results in a color gradient surface which, viewed in the clockwise direction, gives the colors green 25 (bottom left), cyan 24, blue 23, magenta 22, red 21 and yellow 26.

To increase the resolution of the tilt data, a variation of the modulation slopes or modulation frequencies of the individual color channels is helpful. For this purpose, in the right-hand image of FIG. 3, low-frequency green and red modulations (transitions of green 25, yellow 26 and red 21) are combined with high-frequency blue modulation (blue 23).

In FIGS. 4, 5 and 6, as a result of a feasibility study, various surface defects 50 are shown by way of example in different image data 30.

FIG. 4 shows as a surface defect 50 a geometric defect in the form of a dent. The left-hand illustration in FIG. 4 shows chromatically coded image data 31 and corresponds to the raw data acquired with a CMOS color camera in row camera operation. The right-hand figure shows calculated topology image data 33 from which calculated inclination data 36 result in the y-direction of the surface of the specimen 11. Bright pixels correspond to a rising surface 37 (positive slope). Dark pixels correspond to a descending surface 39 (negative slope). A flat surface 38 (slope = 0) is represented by a particular average gray value in the topology image data 33. FIG. 5 shows the topology of the surface defect 50 from FIG. 4. The left image shows a height map, calculated from the inclination data of the deflectometric method (calculated topology image data 33). Bright pixels correspond to a high-lying surface 34. Dark pixels correspond to a low-lying surface 35. The right-hand image shows corresponding reference image data 40 recorded with a high-quality white-light interferometer. The dent (surface defect 50) in the surface of the specimen 11 is clearly visible in both exposures. Structures such as the bars 41 colored in the white-light interferometer (WLT image, right image) image have been deliberately suppressed in the calculated topology image data 33 by the type of illumination modulation (vertical color modulation only 20, cf. FIGS. 2 and 3) ,

FIG. 6 shows image data 30 of a feasibility study for the automatic testing of a component of a common rail injector.

These illustrate the separation of texture and topology data, which is possible by the system presented. The left image shows the raw data as chromatically coded image data 31 of the surface of the test piece 1 1, which was recorded with the test apparatus 10 shown in FIG. The middle image represents the calculated texture of the surface (intensity image data 32). Clearly visible are irregularities of the surface and two dark marks 51, which were applied to the left and right of the surface defect 50. In the right image, the topology is shown in the form of calculated topology image data 33. It can be clearly seen that the surface defect 50 (here as a scratch) only has a significant depth in a small area. The irregularities of the surface as well as the marks 51 have a negligible height and thus appear only schematically in the topology image data 33.

Claims

claims

1 . Method for surface testing of a test specimen (1 1) according to the principle of deflectometry, wherein at least one observation device receives reflection images of a lighting device emitting electromagnetic radiation, at which the reflection images of the illumination device on the surface of the specimen (1 1) arise, and in which an inclination of the surface of the test object is determined from the shape of the reflection images, characterized in that a lighting surface (12) having one or more color modulations (20) which can be predetermined in the color space and at least one color line camera (13) is used as the illumination device, wherein the color line camera (13) one or more chromatically coded image data (31) are detected simultaneously and pixel-accurate and derived from the color information information on the surface slope and from the intensity information on the surface texture of each pixel w earth.

2. The method according to claim 1, characterized in that emitted by the illumination device electromagnetic radiation in the visible spectral range.

3. The method according to claim 1 or 2, characterized in that by means of a calibration each color value of the chromatically coded image data (31) is assigned a slope value of the surface of the specimen (1 1).

4. The method according to any one of claims 1 to 3, characterized in that the color modulations (20) of the illumination surfaces (12) are clearly defined and adapted to the respective measurement task.

5. The method according to any one of claims 1 to 4, characterized in that for the color modulation (20) of the illumination source (12) is a cosine modulation of individual RGB color channels is used whose phase is shifted in each case by 120 °.

6. The method according to any one of claims 1 to 5, characterized in that for the color modulation (20) of the illumination source (12) a superposition of a horizontal and a vertical modulation is used.

7. The method according to any one of claims 1 to 6, characterized in that for the color modulation (20) of the illumination source (12), a superposition of higher and lower frequency modulations is used.

8. Use of the method according to any one of claims 1 to 7 for the surface inspection of rotationally symmetrical test specimens (1 1), wherein the specimens (1 1) are rotated and scanned by the color line camera (13).

9. test device (10) for surface inspection of a test specimen (1 1) according to the principle of deflectometry, which has at least one observation device and at least one illumination device, with the reflection images of the illumination device can be detected, which arise on the surface of the specimen (1 1) and in which an inclination of the surface of the test object can be derived in an evaluation unit from the form of the reflection images, characterized in that the illumination device is designed as illumination surface (12) with one or more color modulations (20) predefinable in the color space and the observation device is at least one color line camera (13), with the one or more chromatically coded image data (31) simultaneously and pixel-accurately detectable and within the evaluation unit from the color information information about the surface slope and from the intensity information on the surface texture of each pixel are conductive.

10. The device according to claim 9, characterized in that with the illumination device light in the visible spectral range can be emitted.