JP3933581B2 - Method and apparatus for surface evaluation - Google Patents

Description

  The present invention records the visual properties of the surface of a colored coating film, such as color, gloss, feel, etc., using an imaging device, a light source and a sample area for placing a sample having a surface to be tested. Relates to a method and an apparatus.

  The visual appearance of the surface can depend on the optical geometry, which is defined as the placement of the observed object relative to the observer and the light source. This dependence can occur, for example, in a film of a coating that includes a color pigment. The dependence of visual properties on optical contours is commonly referred to as goniochromatism or flop behavior. In the optical contour, the observation direction is a line between the observer and the observation point on the observed object, and the illumination direction is a line between the light source and the observation point. The reflection direction is the direction of a line reflecting the illumination direction by a mirror surface perpendicular to the sample surface. The flop angle (also called the reflection angle) is the angle between the observation direction and the reflection direction. The gloss of a color or non-color film depends on the optical contour.

  Color pigments are used in coatings to obtain optical effects such as a metallic or pearlescent appearance. Typically, in films of coatings containing metal pigments, the lightness depends on the optical profile, whereas in coatings containing pearlescent pigments, the hue changes with the optical profile. This complicates the characterization of the visual appearance of such coating films. More complicated in this regard is the presence of strong local scattering points in the coating film when viewed from a short distance due to the presence of pigment flakes. Whereas solid colors are characterized by a spectral distribution of reflectance values, coatings containing color pigments need to take into account angular and spatial dependence.

  To date, the dependence of metal and pearlescent coatings on brightness and color optical profile has been investigated using a spectrophotometer with a limited number of different measurement profiles. However, this results in imperfect images that give data with only a very limited number of optical profiles.

  US Pat. No. 5,550,632 discloses a method and arrangement for evaluating a paint film using a digital optical camera. Since only one optical profile is used for each record and the camera is focused, only one flap angle is used. Since the appearance of coating layers containing color pigments depends on the flap angle, this method cannot be used to evaluate such color coatings in a single recording.

  An object of the present invention is a system that allows the evaluation of a surface with a single recording over a continuous range of flop angles.

  The object of the present invention is an apparatus for recording surface flop angle-dependent properties, including an imaging device for recording light interaction by the surface, a light source and a sample area for placing a sample having a surface to be examined. The imaging device field of view encompasses a continuous range of viewing directions, and the imaging device, light source and sample region have at least one surface property recorded in one image as a function of the flop angle. This is achieved by a device characterized by being arranged in such a way. The recording can be used for visual inspection and comparison, or for measurement or data processing, depending on the ability of the imaging device to quantify the recorded signal. Specific examples of applications are gloss measurement and color matching.

  The device or arrangement according to the invention is particularly suitable for evaluating the flop behavior or goniochromatism of a surface coated with a coating containing color pigments.

  In order to obtain a useful image of the flop behavior, the range of flop angles as defined above should preferably be greater than 40 degrees, more preferably greater than 50 degrees.

  For some types of surfaces, such as coating films containing metal pigments, darker colors with larger flop angles. In order to use the entire measurement range in such a case, the light distribution preferably varies over the visible range of the imaging device, preferably according to an increasing or decreasing function. This function depends on the type of substance. The light distribution can be changed by changing the light output of the light source as a function of the angle of illumination. Alternatively, the light distribution can be changed using a suitable filter.

  In a preferred arrangement, the light source can be a linear light source, such as a TL strip light, a planar slit in a light scatterer, an array of point light sources, such as an LED or glass fiber. Alternatively, the light source can be a point light source.

  A suitable imaging device in an arrangement according to the invention is a CCD camera or an electronic coupling device camera. Suitable CCD optical cameras are, for example, Ricoh ™ RDC 5000, Olympus ™ C-2000Z, Minolta ™ Dimage ™ RD 3000 and Nikon ™ Coolpix ™ 950.

  Digital video cameras form another group of suitable devices that can be used to practice the present invention. With a video camera, the flop angle can be changed not only as a function of position, but additionally, as a function of time. Using a video camera also makes it possible to monitor time-dependent changes in the visual appearance of the surface being examined over time, for example the appearance of the coating film during curing.

With a digital camera, every recorded image consists of a 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 a pure black surface should all be zero, whereas for an ideal pure white surface, each of these three values is pre- Must be equal to the maximum specified. The maximum value is equal to 2 ⁿ -1, where n is the number of bits that define the pixel. If 8-bit pixel depth is used, this maximum is 255.

When a metal coating is tested, the lightness can locally exceed the lightness of white. This should be taken into account, for example, by selecting the largest white value below 2 ⁿ -1.

For accurate color measurements, it is preferable to calibrate the measurements periodically. When a CCD camera is used, calibration can be performed, for example, by first recording a black sample and a white sample separately. The R value, G value and B value of the black value are subtracted from the R value, G value and B value of the white sample and the R value, G value and B value of the measurement sample in question. Next, the R, G, and B values of the measurement sample are divided by the corresponding values of the white calibration sample and multiplied by the maximum white value. This means that for every pixel in the image, the R value calibration value R _cal is calculated using the following formula:
[Equation 1]
R _cal = 255 * (R−R _black ) / (R _white− R _black )
In this equation, 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. Calibration values for B and G values are calculated accordingly. This correction accounts for deviations in the light sensitivity of the pixels and changes in light source intensity as a function of optical profile.

Optionally, the R, G and B values can be corrected for time dependent changes in light intensity. This can be done, for example, by applying a white strip parallel to the sample. For calculation purposes, the sample and the white piece are considered to be separated along the sample's longitudinal axis by a number of virtual parts. For every sample part, the average R, G and B values R _av , G _av and B _av are determined. Similarly, R _white-av , G _white-av, and B _{white-av, which} are average R values, G values, and B values, are determined for every white piece. R _cor , the corrected R value for each sample part, is then calculated using the following formula:
[Equation 2]
R _cor = 255 * (R _av / R _{white- av} )
G _cor and B _cor are calculated accordingly.

  The most common systems for colorimetric data are the Commission International de IEclairage (CIE), eg CIELab (L *, a *, b *), CIEXYZ (X, Y, Z) and CIELuv. Submitted by (L *, u *, v *). These systems take into account the sensitivity of the human eye. The R, G, and B values measured by the CCD camera can be converted into CIELab L *, a *, and b * values.

  The selected mathematical model can be any model known to those skilled in the art. HR Kang, Color Technology for Electronic Imaging Devices, SPIE Optical Engineering Press, 1997, Chapters 3 and 11, and US Pat. No. 5,850,472 Examples are given in the description. This model can be non-linear or linear. One example of a nonlinear model is a second order polynomial with 10 parameters or a third order polynomial with 20 parameters. Preferably, a linear model is used. More preferably, the linear model used has four model parameters.

An example of a linear model with four parameters is the following model, where the measured color signal of the calibration color (in this case R, G and B data) is converted to colorimetric data, which in this case In the CIELab data:
[Equation 3]
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
(Where R _i , G _i and B _i are the measured signals and L _i *, a _i * and b _i * are the colorimetric data of the calibration color i)
It is.

To calculate the 12 model parameters c ₀ -c ₃ , d ₀ -d ₃ and e ₀ -e ₃ from the measured RGB data of the calibration color and the known CIELab data (CIE1964 standard colorimetric observer), a linear regression is performed. used. These model parameters are used to convert the measured RGB data of the selected color into CIELab data.

である。 An example of a nonlinear cubic polynomial with 20 parameters is

It is.

Linear regression is used to calculate the 60 model parameters c ₀ -c ₁₉ , d ₀ -d ₁₉ and e ₀ -e ₁₉ from the measured RGB data of the calibration color and the known CIELab data. These model parameters are used to convert the measured RGB data of the selected color into CIELab data.

（ここで、W_iは重み因子であり、y_iはスペクトル測定に基づくL_i*、a_i*またはb_i*であり、y^~ _iはRGBに基づいてCIELab換算へのL_i*、a_i*またはb_i*についての計算値である）。 Notwithstanding the above, it is possible to give a greater weight to the calibration color in the vicinity of the selected color when calculating the model parameters. In the case of the above example of a linear model with four parameters, this means that during the linear regression each calibration color is based on the distance in the RGB color space between the calibration color in question and the selected color. Means that a weight factor can be given. In the linear regression procedure, the following sum of squares is minimized:

(Where, W _i is a weight factor, y _i is L _i * based on spectral measurements are a _i *, or b _i *, L to y ^~ _i is CIELab conversion based on RGB _i *, a It is a calculated value for _i * or b _i *).

（ここで、n:は較正色の数であり、R、G、B:は選択された色の測定された信号である）。 y ^~ _i is equal to 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 If they are equal, the sum is:

(Where n: is the number of calibration colors and R, G, B: are the measured signals of the selected color).

  Alternatively, it is possible to use a calibration color regardless of the color selected for the interpolation method.

  If desired, gray balancing is performed with signals measured for black, white and gray according to the formula R = G = B = f (L *) or equivalent values for L * in different colorimetric systems. be able to. Such gray balance maintenance is described in HR Kang, Color Technology for Electronic Imaging Devices, SPIE Optical Engineering Press, 1997, Chapter 11. Has been.

  Using image processing software such as the computer program Optimas ™, which is commercially available from Media Cybernetics, or Image ProPlus ™, available from the same company, the difference in brightness relative to the background of the particles Individual particles can be identified by recognition. These particles can be, for example, metal pigments or clusters of metal pigments. After particle identification, the number of particles and image parameters such as particle size, particle shape, minimum and longest axis lengths and particle R, G and B values can be determined by image processing software . The data can optionally be averaged piece by piece or, if necessary, larger pieces.

  Data determined based on the image can be used, for example, to look for a coating formulation that provides a matching surface coating. To this end, the measured data can be compared with data from a color formulation database.

  In order to increase the field of view of the imaging device, the light source can optionally include a set of mirrors. It has been found that the field of view can be increased to about 90 degrees or more when the mirror is used in the proper arrangement.

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

  In a particularly preferred embodiment, the light source includes a light scattering housing that emits light through a slit. The longitudinal side of the slit is arranged substantially parallel to the sample surface, while the short side is substantially perpendicular to the sample surface. In such an arrangement, a light sensor can be used to control the light output. In a preferred embodiment of such a scatterer, the slit is bordered by a substantially horizontal wall inside the scatterer. Thus, the light intensity at the sample surface is a function of the flop angle. In an arrangement with a smaller angular distance to the scatterer, the light intensity is smaller than for a larger angle.

  In a further preferred embodiment of the device according to the invention, the spectral distribution of the light is a function of the position on the sample, for example by using different light sources or by using a set of filters, diffraction gratings or prisms. Change in one image. This increases the number of independent measurement data, thus improving the color accuracy. This can be done, for example, by changing the light from the light source or by changing the spectral distribution of the light just before the light enters the imaging device. Preferably, the spectral distribution of the illumination changes perpendicular to the change in optical profile.

  In order to eliminate the effects of ambient light, the device of the present invention preferably includes a housing.

  The present invention includes a method for surface evaluation as described above, where generally the recorded light interaction is light reflection of the sample. However, if the sample is transparent or translucent, the recorded light interaction can be light transmissive. In that case, the sample is placed between the imaging device and the light source.

  A flat sample can be used. However, if necessary, bent samples may also be suitable for testing flop behavior.

The invention is further described and illustrated by the drawings.
FIG. 1 shows an arrangement 1 of the present invention, a CCD camera as a recording apparatus having a light source 2 and a viewing angle α extending from a first outer end observation direction 4 closest to the light source to a second outer end observation direction 5. Including 3. The coated sample 6 is located under the camera 3. The light source 2 is a line light source parallel to the sample surface. The light source 2 is located outside the direct field of view of the CCD camera 3. The line between the light source 2 and the point where the viewing direction 4 meets the sample 6 defines a first illumination direction 7, which is reflected by the sample 6 in the direction defined as the first reflection direction 8. . Similarly, the line between the light source 2 and the point where the observation direction 5 meets the sample 6 defines a second illumination direction 9, which is defined by the sample 6 in the direction defined as the second reflection direction 10. Reflected. In the drawing, the outer flap angle θ ₁ is the angle between the first observation direction 4 and the first reflection direction 8, whereas the outer flop angle θ ₂ is the second observation direction 5 and the first reflection direction 8. It is an angle between two reflection directions 10. The angular range between θ ₁ and θ ₂ can extend up to about 90 degrees.

  FIG. 2 shows a recorded image recorded by the arrangement of FIG. It is an image of a sample coated with a metal paint. The figure shows how the brightness varies with the flap angle. The image in FIG. 2 also shows the change in roughness over the length of the sample, as also experienced by the human eye.

FIG. 3 shows an alternative arrangement of the present invention, where the field of flap angle is increased by the use of the mirror 11. The mirror 11 is arranged in such a way as to reflect the sample part right next to the outer edge of the viewing angle range of the camera farthest from the light source 2. In the image observed by the camera 3, the field of view closest to the light source 2 is replaced by an extension of the field of view on the other side. From right to left, the recording shows a field of view from θ ₃ to θ ₅ and then a field of view from θ ₄ to θ ₂ . The field of view from θ ₁ to θ ₄ is no longer visible on the record.

FIG. 4 shows the flop angle as a function of position in an arrangement similar to that of FIG. 3, position 0 being the lower right of the camera. The reflected part of the sample overlaps with the part of about 20 mm to 25 mm.
Optionally, the sample can be recorded in two (or even more) separate parallel strips, one with an increased field of view with a mirror and the other without a mirror. In this way, a fully extended field of view extending from θ ₁ to θ ₂ and a field of view extending from θ ₅ to θ ₃ can be recorded. If θ ₅ is equal to θ ₂ , the closed range from θ ₁ to θ ₃ is covered.

  FIG. 5 shows an arrangement similar to that of FIG. 1 with the camera 3 in the upper right of the sample 6 and the light source 12 includes a standard flashlight 13 available under the trademark Metz 45CT-1. The flashlight 13 includes a transparent surface 14. With a transparent surface 14, the flashlight is connected to a flat top surface 15 of a scatterer 16 that includes a semi-cylindrical portion 17. The flat surface 15 is open and is connected to the transparent surface 14 of the flashlight 13. The inner surface of the scatterer 16 is covered with a white coating. Where it does not coincide with the transparent surface 14 of the flashlight 13, the flat surface 15 is closed by a horizontal wall 18, which has a white coating on its inside. The end between the outer end of the horizontal wall 18 and the semi-cylindrical portion 17 is provided with a vertical slit 19 that extends across the width of the scatterer 16.

  When the flashlight 13 emits light, the light is scattered by the reflective coating inside the scatterer 16. Part of the light is reflected by the reflective layer of the horizontal wall 18 through the slit 19 on the part of the sample 6 to be imaged. As can be seen in FIG. 5, in the viewing direction outside the viewing angle range of the camera closest to the scatterer 16, the sample 6 is one of the significantly smaller reflective horizontal walls 17 than in the viewing direction outside the camera far from the scatterer 16. Illuminated by the department. Thus, light density is a function of angular distance from the light source. This function can vary with the orientation of the slit relative to the sample surface.

  The glass fiber cable 20 connects the space in which the flashlight 13 is confined to an optical sensor 21 that controls the time interval during which the flashlight 13 emits light. Part of the light scattered in the scatterer 16 escapes to the light source 21 by the glass fiber cable 20. The amount of light passing through the glass fiber cable 20 is measured by the optical sensor 21. When a predetermined amount of light passes through the glass fiber cable 20, the sensor 21 stops the flashlight 13. In this way, all flashes are guaranteed to give exactly the same amount of light.

  FIG. 6 shows a further alternative embodiment of the device according to the invention comprising an imaging device 3 and a light source 2. The sample 6 is located under the imaging device 3. A set of filters and a diffraction grating or prism 24 is placed between the light source 2 and the sample 6. The spectral distribution of illumination changes in one image as a function of position on the sample. FIG. 7 shows the result of a preferred embodiment where the spectral distribution of illumination changes perpendicular to the change in optical profile.

  FIG. 8 shows that the gloss behavior of the sample can be characterized as a function of optical profile. This particular example shows the difference between the high gloss sample 25 and the low gloss sample 26 in one image.

FIG. 1 shows a schematic overview of the recording arrangement of the present invention. FIG. 2 shows a recording according to the arrangement of FIG. FIG. 3 schematically illustrates an alternative arrangement of the present invention. FIG. 4 shows a plot of the flop angle as a function of position recorded by the arrangement of FIG. FIG. 5 shows a third alternative arrangement of the present invention. FIG. 6 shows a fourth alternative arrangement of the present invention. FIG. 7 shows the change of the wavelength range through the filter for the sample imaged in the apparatus according to FIG. FIG. 8 shows a sample with a parallel reference sample for gloss measurement.

Claims (11)

For recording visual properties of a surface, including an imaging device for recording light interaction with the surface over a continuous region in the viewing direction , a light source and a sample region for placing a sample having a surface to be tested An apparatus, wherein the imaging device, the light source and the sample area are arranged in a triangular arrangement for recording at least one surface characteristic as a function of a continuous range of flop angles in one image, the flop angles being observed An angle between a direction and a specular direction, wherein the imaging device is oriented substantially perpendicular to the sample area . Flop angle claim 1 Symbol mounting device is greater than 40 degrees. Light intensity, device according to any one of claims 1-2, which varies over the field of view of the imaging apparatus. The apparatus of any one of claims 1 to 3 the light source is a linear light source. The apparatus of any one of claims 1-4 imaging device is a CCD camera. Device, device according to any one of claims 1 to 5 comprising a set of mirrors to increase the field of view of the imaging apparatus. Light source device according to any one of claims 1 to 6 is a flashlight having a light output control device. Device, for changing the spectral distribution of light entering the spectral distribution or recording device of the illumination as a function of position on the sample, any one of claim 1 to 7 including a set of filters and a diffraction grating or a prism The device described. An imaging device for recording light reflection by a surface, a method for recording visual characteristics of a surface using a light source and a sample area for placing a sample having a surface to be tested, comprising: an imaging device; A method characterized in that the light source and the sample region are arranged such that in one image at least one surface property is recorded as a function of a continuous range of angles between the reflection direction and the observation direction. 10. The method of claim 9 , wherein the visual property is sample gloss. An imaging device for recording light transmission of a translucent sample, a method for recording visual characteristics of a sample using a light source and a sample area for placing a sample to be tested, comprising: an imaging device; A method characterized in that the light source and the sample area are arranged such that in one image at least one visual characteristic is recorded as a function of a continuous range of angles between the illumination direction and the viewing direction.

Images