KR20040012743A - Method and device for surface evaluation - Google Patents

Abstract

The invention records the visual properties of a surface comprising a sample area 6 on which a sample with a surface to be tested is located, a light source 2 and an imaging device 3 recording the interaction (reflection or transmission) of the light with the surface. As a device to

The imaging device, the light source and the sample region can be recorded as a function of the continuous range of angles between at least one surface characteristic in an image between the illumination direction (7, 9) and the viewing direction (4, 5), the image The device is a CCD camera, characterized in that the device and method are suitable for evaluating and imaging visual properties that depend on optical geometry such as flop properties and gloss.

Description

METHOD AND DEVICE FOR SURFACE EVALUATION

The present invention utilizes a sample area, a light source and an imaging device in which a sample having a surface to be tested is located, for example, the color, gloss, texture, etc. of a colored coating film, A method and apparatus for recording the visual characteristics of a surface. The visual appearance of the surface may depend on the optical geometry, which is defined by the position of the object of interest with respect to the observer and the light source. The dependence can occur, for example, in a coating film comprising an effect pigment. The dependence of visual properties on optical geometry is commonly referred to as goniochromatism or flop behaviour. In optical geometry, the viewing direction is the line between the observer and the viewing part of the object, and the illumination direction is the line between the light source and the viewing part. The reflection direction is the direction of the line reflecting the illumination direction by the reflector plane perpendicular to the specimen surface. The flop angle, also called the reflection angle, is the angle between the reflection direction and the viewing direction. The gloss of the effect or non-effect film also depends on the optical geometry.

Effect pigments are used in coatings to obtain optical effects such as metallic or pearlescent appearance. In general, coating film containing metallic pigment pigment brightness depends on the optical geometry, while the color of the coating material comprising pearlescent pigments is changed to optical geometry. This deteriorates the properties of the visual appearance of the coating film. A further deterioration in this aspect is that because of the presence of pigment fragments, the observation from a short distance shows that it is partially strongly dispersed in the coating film. While solid colors can be characterized by the spectral distribution of reflectance values, coatings containing effect pigments must take angles and spaces according to calculations.

To date, the dependence of the brightness and color of metallic and pearlescent coatings on optical geometry has been measured using spectrophotometers on a limited number of different measured geometries. However, this results in insufficient results obtained with only a very limited number of optical geometries.

US 5,550,632 discloses a method and arrangement for evaluating paint films using a digital optical camera. Only one optical geometry was used for recording, because the camera focuses on only one flop angle. Since the appearance of the coating layer containing the effect pigments depends on the flop angle, the method cannot be used for evaluation of the effect coating material in one record.

It is an object of the present invention to allow the system to have a surface evaluation in a single record of a continuous range of flop angles.

The object of the present invention is obtained by a device for recording a flop angle depending on the surface properties, which device is used to record the interaction of light by the surface, the light source and the sample area on which the sample with the surface to be tested is located. A device, wherein the range of the imaging device covers all of the continuous range of the viewing direction, and the imaging device, the light source and the sample area are arranged such that at least one surface characteristic is recorded as a function of the flop angle in one image. The recording can be used depending on the capacity of the imaging device to measure the amount of recorded signal for measurement or data processing or for visual inspection and comparison. Specific examples of use are gloss measurement and color matching.

The device or arrangement according to the invention is particularly suitable for evaluating flop properties or goniochromatism of surfaces coated with coatings comprising effect pigments.

In order to obtain a useful phase of flop quality, the range of the flop angle is preferably at least 40 degrees, more preferably at least 50 degrees, as defined above.

In some forms of surfaces, such as coating films containing metallic pigments, the color is dark when the flop angle is large. In order to use the full measurement range in this case, the light distribution is preferably varied in the viewing scope of the imaging device in accordance with an increase or decrease function. The light distribution can change the light emission of the light source as a function of the illumination angle. Alternatively, the light source can be diversified using suitable filters.

In a preferred arrangement, the light source can be a line source such as TL strip light, a horizontal slit in a light diffusor, an array of point sources such as LEDs or glass fibers. . Alternatively, the light source may be a point source.

Suitable imaging devices in the arrangement according to the invention are CCD cameras or charge coupled device cameras. Suitable CCD optical cameras are, for example, Ricoh® RDC 5000, Olympus® C-2000Z, Minolta® Dimage® RD 3000, and Nikon® Coolpix® 950.

Digital video cameras are another group of suitable devices that can be used to practice the present invention. Using video cameras, the flop angle can be varied not only as a function of position, but also alternatively or alternatively as a function of time. Using a video camera also makes it possible to record time-dependent changes in the visual appearance of the surface to be tested for a period of time, such as the appearance of the coating film during curing.

Every image recorded using a digital camera consists of a large number of pixels. Every pixel has a red value R, a green value G and a blue value B. Ideally, the calibrated R, G, and B values for pure black surfaces are all zero, and ideally pure white surfaces will each be equal to a predefined maximum value of each of these three values. The maximum value is equal to 2 ⁿ −1, where n is the number of pieces that define the pixel. If an 8-piece pixel depth is used, the maximum value is 255.

When testing metallic coatings, the brightness may partially exceed that of white. This can be taken for example in an amount that selects a maximum white value lower than 2 ⁿ −1.

For accurate color measurement, it is desirable to calibrate the measurement periodically. When using a CCD camera, the calibration can be performed, for example, by first separating and recording the black and white samples. The R, G and B values of the black value are subtracted from the R, G and B values of the suspected measured and white samples. The R, G, and B values of the measured sample are then divided by the corresponding values of the white calibrated sample and multiplied by the maximum white value. This was calculated using Equation 1 below for the corrected value R _cal of the R value for all the pixels in the image.

(In Equation 1, R _black is the R value of the pixel in the black sample, while R _white is the R value of the pixel in the white sample.) The corrected values of the B and G values were correspondingly calculated. The correction calculated the variability in the light intensity and the deviation of the light sensitivity of the pixel as an optical geometry function.

Optionally, R, G and B values can be corrected as time-dependent changes in light intensity. This can be done, for example, by applying a parallel white strip to the sample. For the purpose of correction, the sample and the white strip are considered by dividing the hardness side of the sample into a plurality of virtual images. For all sample parts, determine the average R, G and B values R _av , G _av and B _av . Similarly, for all sample parts, determine the average R, G and B values R _white-av , G _white-av and B _white-av . The fixed R value R _cor for each sample portion is then calculated using Equation 2 below:

G _cor and B _cor were also calculated correspondingly.

Most common systems for complementary color data are Commission International de l'Eclairage (CIE), ie CIELab (L ^* , a ^* , b ^* ), CIEXYZ (X, Y, Z) and CIELuv (L ^* , u ^* , v ^* ). The system takes the visual sensitivity as a value. The R, G and B values measured by the CCD camera can be converted into L ^* , a ^* and b ^* values of the CIELab system.

The mathematical model selected will be a model known to those skilled in the art. Examples are disclosed in HR Kang, Color Technology for Electronic Imaging Devices , SPIE Optical Engineering Press, 1997, chapters 3 and 11 and US 5,850,472. The model may be straight or non-linear. One example of a non-linear model is a 3 ^rd order polynomial with 20 parameters or a 2 ^nd order polynomial with 10 parameters. It is more preferable to use a straight model with four model parameters.

One example of a linear model with four parameters is the following model, in the case of the R, G and B data, the measured color signal of the corrected color is converted to colorimetric data, in the case of the CIELab data:

L _i ^* = c ₀ + c ₁ R _i + c ₂ G _i + c ₃ B _i

a _i ^* = d ₀ + d ₁ R _i + d ₂ G _i + d ₃ B _i

b _i ^* = e ₀ + e ₁ R _i + e ₂ G _i + e ₃ B _i

(R _i , G _i , B _i are the measured signals, and L _i ^* , a _i ^* and b _i ^* are the colorimetric data of the correction color i.)

Linear regression is used to calculate twelve model parameters c ₀ -c ₃ , d ₀ -d ₃ and e ₀ -e ₃ from the measured RGB data and known CIELab data of the corrected color. The model parameter is used to convert the measured RGB data of the selected color into CIELab data.

One example of a non-linear 3 ^rd order polynomial with 20 parameters is as follows:

L _i ^* = c ₀ + c ₁ R _i + c ₂ G _i + c ₃ B _i + c ₄ R _i ² + c ₅ G _i ² + c ₆ B _i ² + c ₇ R _i G _i + c ₈ R _i B _i + c ₉ G _i B _i + c ₁₀ R _i ³ + c ₁₁ G _i ³ +

c ₁₂ B _i ³ + c ₁₃ R _i ² G _i + c ₁₄ R _i ² B _i + c ₁₅ G _i ² R _i + c ₁₆ G _i ² B _i + c ₁₇ B _i ² R _i + c ₁₈ B _i ² G _i + c ₁₉ R _i G _i B _i

a _i ^* = d ₀ + d ₁ R _i + d ₂ G _i + d ₃ B _i + d ₄ R _i ² + d ₅ G _i ² + d ₆ B _i ² + d ₇ R _i G _i -d ₈ R _i B _i + d ₉ G _i B _i + d ₁₀ R _i ³ + d ₁₁ G _i ³ +

d ₁₂ B _i ³ + d ₁₃ R _i ² G _i + d ₁₄ R _i ² B _i + d ₁₅ G _i ² R _i + d ₁₆ G _i ² B _i + d ₁₇ B _i ² R _i + d ₁₈ B _i ² G _i + d ₁₉ R _i G _i B _i

b _i ^* = e ₀ + e ₁ R _i + e ₂ G _i + e ₃ B _i + e ₄ R _i ² + e ₅ G _i ² + e ₆ B _i ² + e ₇ R _i G _i -e ₈ R _i B _i + e ₉ G _i B _i + e ₁₀ R _i ³ + e ₁₁ G _i ³ +

e ₁₂ B _i ³ + e ₁₃ R _i ² G _i + e ₁₄ R _i ² B _i + e ₁₅ G _i ² R _i + e ₁₆ G _i ² B _i + e ₁₇ B _i ² R _i + e ₁₈ B _i ² G _i + e ₁₉ R _i G _i B _i

The regression line is used to calculate 60 model parameters c ₀ -c ₃ , d ₀ -d ₃ and e ₀ -e ₃ from the measured RGB data and known CIELab data of the corrected color. The model parameter is used to convert the measured RGB data of the selected color into CIELab data.

Notwithstanding the above, it is possible to add a large weight to the correction color near the selected color when calculating the model parameters. In the case of the above linear model example with four parameters, this means that during the regression line each correction color is given a weight factor based on the distance in the RGB color space between the selected color and the questionable complementary color. In the regression method, the sum of squares of the following equation (3) is minimal:

(In Equation 3, w _i is a weighting coefficient, y _i is L ^* _i , a _i ^* or b _i ^* based on the spectroscopic measurement, _i is the calculated value of L ^* _i , a _i ^* or b _i ^* based on the CIELab conversion of RGB)

_i is the same as c ₀ + c ₁ R + c ₂ G + c ₃ B (see above), w _i is ((R _i -R) ² + (G _i -G) ² + (B _i -B) ² ) equal to ^−2, and the sum is equal to the following Equation 4:

(In Equation 4, n is the number of correction colors, R, G, B is the measured signal of the selected color)

Alternatively, a correction color around the selected color can be used for security.

If desired, gray balancing can be performed with signals measured for black, white and gray according to R = G = B = f (L ^* ) or with corresponding values for L ^* in other colorimetric systems. Can be. The gray balancing is described in HR Kang, Color Technology for Electronic Imaging Devices , SPIE Optical Engineering Press, 1997, chapter 11.

Using image processing software such as Optimas (trademark, Media Cybernetics) or program image ProPlus (ProPlus, trade name, Media Cybernetics), which are computer programs, individual particles can be used to vary the luminance of the background of the particles. Can be identified by recognition. For example, the particles can be metallic pigments or clusters of metallic pigments. After recognizing the particles, not only the R, G and B values of the particles, but also the number of particles and image parameters such as particle size, particle shape, short axis length and long axis length can be measured by the image processing software. Alternatively, the data can be averaged over the strip portion, or larger portions if desired.

Data measured based on the image can be used, for example, to search for a coating composition, to provide a matching surface coating. This allows the measured data to be compared with the data in the color manufacturing database.

In order to enhance the range of the imaging device, the light source optionally comprises a set of mirrors. It has been found that by using a reflector in a suitable arrangement, the range can be improved to about 90 degrees or more.

The light source may be a permanent light source, but a flashlight is preferred to minimize the use of energy. If a permanent light source is used, the camera should be set for a suitable exposure time. Suitable light sources are for example tungsten halogen lamps or xenon lamps.

In a particularly preferred embodiment, the light source comprises a light diffusing housing that emits light through the slit. The longitudinal surface of the slit is arranged substantially parallel to the surface of the sample, while the cross section is arranged substantially perpendicular to the surface of the sample. In this arrangement, a light sensor can be used to regulate the emission of light. In a preferred embodiment of the diffuser, the slits are bounded by walls that are substantially horizontal at the inner surface of the diffuser. As such, the brightness at the sample surface is a function of the flop angle. At locations at smaller angular distances to the diffuser, the luminous intensity is smaller than at larger angles.

In a further preferred embodiment of the device according to the invention, the spectral distribution of the light in one image varies as a function of the position of the sample, for example by using another light source or by using a filter set, a diffraction grating or a prism. This improves the accuracy of the color by increasing the number of independent measurement data. This can be done, for example, by changing the light at the light source or by changing the spectral distribution of the light just before it is introduced into the imaging device. Preferably, the spectral distribution of the illumination is changed perpendicular to the deviation of the optical geometry.

In order to eliminate certain effects of ambient light, the device according to the invention preferably comprises a protective structure.

The present invention includes a surface evaluation method as described above, in which the action of normally recorded light is the light reflection of the sample. However, if the sample is transparent or translucent, the action of the recorded light may be transmission of light. In this case, the sample is located between the imaging device and the light source.

Flat samples can be used. However, if required, a curved sample may be suitable for testing flop behaviour.

The invention is further described and illustrated by the following figures. In the drawings below:

1 is a schematic diagram of a recording arrangement according to the present invention;

2 shows the recording by the arrangement of FIG. 1;

3 shows a schematic selective arrangement according to the invention;

4 shows an illustration of the flop angle as a function of position when recorded by the arrangement of FIG. 3;

5 shows a third optional arrangement according to the invention;

6 shows a fourth optional embodiment of the present invention;

7 shows the change in the filtered wavelength range for the sample to be imaged in the device according to FIG. 6;

8 shows samples for parallel control samples in gloss measurement.

1 shows an arrangement 1 according to the invention, which comprises a light source 2, a CCD camera 3 as a recording device, which is arranged in the first outer surface viewing direction 4 closest to the light source. It has the viewing angle (alpha) which is a range to 2 outer surface observation direction (5). The coated sample 6 is located under the camera 3. The light source 2 is a line source parallel to the sample surface. The light source 2 is located outside the direct range of the CCD camera 3. The line between the light source 2 and the point where the observation direction 4 meets the sample 6 is defined by the first light source direction 7, which is defined by the first reflection direction 8. 6) is reflected. Similarly, the line between the light source 2 and the point where the observation direction 5 meets the specimen 6 is defined as the second illumination direction 9, which is defined as the second reflection direction 10. Is reflected by the sample (6). In the figure the outer flop angle Θ ₁ is the angle between the first viewing direction 4 and the first reflection direction 8, while the outer flop angle Θ ₂ is the second viewing direction 5 and the second Angle between reflection directions 10. The range of angles between Θ ₁ and Θ ₂ is less than about 90 degrees.

FIG. 2 shows the recorded image to be recorded by the arrangement of FIG. 1. This is an image of a sample coated with metallic paint. The figure shows how the brightness changes with the flop angle. The image in FIG. 2 also shows a change in roughness with respect to the length of the sample when tested by the naked eye.

3 shows an optional arrangement according to the invention, wherein the flop angle range is increased by using a reflector 11. The reflecting mirror 11 is arranged to reflect the right outer side end of the part of the specimen in the viewing angle range of the camera farthest from the light source 2. In the image observed by the camera 3, the range closest to the light source 2 is replaced by extending the range of the other side. From right to left, the record shows the range of Θ ₃ to Θ ₅ , and continuously the range of Θ ₄ to Θ ₂ . The range of Θ ₁ to Θ ₄ is no longer visible in the recording.

4 shows the floc angle as a function of position in a similar arrangement to the arrangement of FIG. 3, with position 0 being the lower right of the camera. The reflected portion of the sample overlaps the portion of about 20 mm to 25 mm.

Optionally, the sample is recorded in two (or more) separate parallel strips, one having an extended range with a reflector and the other not. As such, a fully extended range of Θ ₁ to Θ ₂ and a range of Θ ₅ to Θ ₃ can be recorded. If Θ is ₅ equal to the Θ _2, Θ range of ₁ to Θ ₃ may cover.

FIG. 5 shows an arrangement similar to that of FIG. 1, where the camera 3 is on the upper right of the specimen 6 and the light source 12 uses a standard flashlight 13 (trade name Metz 45CT-1 may be used). Include. The flashing light 13 includes a transparent surface 14. On the transparent surface 14, a flashlight is connected to the flat top surface 15 of the diffuser 16 with a semi-cylindrical portion 17. The flat surface 15 is open, which is connected to the transparent surface 14 of the flashing light 13. The inner surface of the diffuser 16 was covered with a white coating. Although not coincident with the transparent side 14 of the flashing light 13, the flat side 15 is closed by the horizontal wall 18, the inside of which is covered with a white coating. The edge between the outer surface of the horizontal wall 18 and the semi-cylindrical portion 17 provides a vertical slit 19 that extends with respect to the width of the diffuser 16.

When the strobe light 13 shines, light is diffused by the reflective coating on the inner surface of the diffuser 16. Part of the light is reflected by the reflective layer of the horizontal wall 18 through the slit 19 to the part of the specimen 6 to be imaged. Referring to FIG. 5, in the external viewing direction of the viewing angle range of the camera closest to the diffuser 16, the specimen 6 of the reflective horizontal wall 17 is more than in the external viewing direction of the camera away from the diffuser 16. Reflected by quite a few parts. As such, light density is a function of angular distance from the light source. The function is varied by the placement of the slits relative to the sample surface.

The glass fiber cable 20 connects the space containing the flashing light 13 to the light detector 21 which controls the time intervals in which the flashing light 13 shines. Some of the light diffused from the diffuser 16 goes through the fiberglass cable 20 to the light detector 21. The amount of light passing through the glass fiber cable 20 is measured by the light detector 21. When the predetermined amount of light passes through the glass fiber cable 20, the detector 21 stops the flashing light 13. As such, every glance allows to provide an exact same amount of light.

6 shows an additional optional embodiment of the device according to the invention, which comprises an imaging device 3 and a light source 2. The specimen 6 is located below the imaging device 3. A filter set and diffraction grating or prism 24 are positioned between the light source 2 and the sample 6. The spectral distribution of the illumination changes in one image as a function of the position of the sample. 7 shows the results of the preferred embodiment, wherein the spectral distribution of the illumination is changed perpendicular to the deformation of the optical geometry.

8 shows that the gloss properties of the sample are characterized as a function of optical geometry. This particular sample shows in one image the difference between the high gloss sample 25 and the low gloss sample 26.

Claims (13)

A device for recording the visual properties of a surface, comprising: a sample area in which a sample having a surface to be tested is located, an optical source, and an image device for recording light interacting with the surface And said imaging device, light source and sample area are arranged such that at least one surface characteristic can be recorded in one image as a function of each angular range between the viewing direction and the illumination direction. The method of claim 1, Wherein the camera, the light source and the sample region are located in a triangular array. The method according to claim 1 or 2, The flop angle is at least 40 degrees, preferably at least 50 degrees. The method according to any one of claims 1 to 3, Light distribution by the light source varies over a range of imaging devices. The method according to any one of claims 1 to 4, And the light source is a linear light source. The method according to any one of claims 1 to 5, And said image device is a CCD camera. The method according to any one of claims 1 to 6, Wherein the device comprises a set of reflectors that broaden the scope of the imaging device. The method according to any one of claims 1 to 7, The light source is a flashlight comprising a light emission control unit. The method according to any one of claims 1 to 8, Wherein the device comprises a prism or a diffraction grating and a filter set for varying the spectral distribution of light or the spectral distribution of illumination entering the recording device as a function of position on the sample. In a method of recording the visual characteristics of a surface using an imaging device that records the sample area, the light source and the light interacting with the surface on which the sample with the surface to be tested is located, The imaging device, the light source and the sample region are arranged in such a way that at least one surface characteristic in one image is recorded as a function of a continuous angular range between the illumination direction and the viewing direction. The method of claim 10, Said visual property being the gloss of a sample. The method of claim 10 or 11, The interaction of the recorded light is the light reflection of the sample. The method of claim 12, The interaction of the recorded light is the light transmission of the translucent sample.