JP7261725B2 - Color difference measurement method - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Law Date September 17, 2019 Name of meeting The 54th All-Toyota Research Presentation Venue Toyota Central R&D Labs., Inc. (41-1 Yokomichi, Nagakute City, Aichi Prefecture) [Published Materials] Date: November 1, 2019 Name of meeting: 2019 Japan Painting Techniques Association 2nd Lecture Venue: Nippon Paint Holdings Co., Ltd. Tokyo Office Center Building A Hall (4-1 Minami-Shinagawa, Shinagawa-ku, Tokyo 15)

The present invention relates to a method for measuring multi-site color differences.

A multi-angle colorimetric device having a plurality of light-receiving angles is known as a colorimetric device for quantitatively measuring colors. However, the color difference measured using a multi-angle colorimetric device often does not match the color difference perceived by the naked eye. In this case, it is very difficult to match the color difference visually sensed. In other words, even though there should be a clear color difference in the color difference using a multi-angle colorimetry device, it is not perceived visually, and vice versa. rice field.

Therefore, visual evaluation is indispensable in factories that produce color products, and colorimetry equipment is used only as a supplementary role. It is the actual situation that it is evaluated. However, it is not easy to visually evaluate the color difference accurately, and only some skilled workers can do it, so the work efficiency is very poor. In addition, since the sensitivity to color difference tends to vary from operator to operator, the reliability of the color difference evaluation results is not sufficient.

Visual evaluation can be omitted if the color difference evaluation by the multi-angle colorimetric device can be matched with visual evaluation. For example, in Patent Document 1 below, the reflected A color difference measurement method for calculating a continuous approximation curve representing the relationship between the light acceptance angle and each stimulus value X, Y, Z is shown. This makes it possible to calculate the color difference not only at the angle at which the reflected light is actually received, but also at any angle. Color differences between multiple sites can be covered as measurements. The continuous curve of the stimulus values XYZ thus obtained is converted to the coordinates of a uniform color space (for example, L ^* a ^* b ^* color space or L ^* u ^* v ^* color space) to calculate the color difference. It is possible to obtain an evaluation result close to the feeling of evaluation.

JP 2017-90060 A

However, even if a continuous curve of the stimulus values X, Y, and Z is used as described above, there are cases where it does not match the sense of visual evaluation.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a color difference measuring method that is closer to the color difference sensitivity of visual observation.

In the conventional visual color difference evaluation, the evaluator visually and intuitively evaluates the color difference between the reference color and the color of each part. , Δ when a slight color difference with respect to the reference color is observed but can be shipped, and × when collection is impossible due to a considerable color difference with respect to the reference color. Conventionally, by comparing the results of such visual color difference evaluation with the results of color difference measurement using a colorimetric instrument and analyzing the correlation between them, the results of the color difference measurement using a colorimetric instrument can be determined visually. I was checking to see if it matched the evaluation. However, since the visual evaluation of the color difference varies depending on the evaluator, it is difficult to compare it with the color difference measurement result using a colorimetric instrument, and it is difficult to determine whether the color difference measurement result has a high correlation with the visual evaluation. It was difficult to judge accurately.

As described above, the visual evaluation of the color difference with respect to the reference color tends to vary depending on the evaluator. There was almost no variation in the ranking among evaluators. Therefore, the present inventors compared the ranking of the colors of each part in order of proximity to the reference color with the results of color difference measurement using a colorimetric instrument, and analyzed the correlation therebetween.

As a result, the color difference calculation result using the continuous curve in the Munsell color coordinates is as follows compared to the conventional case of using the continuous curve in the L ^* a ^* b ^* color space or the L ^* u ^* v ^* color space. There was a high correlation with visual evaluation (ranking each target site close to the reference color). The Munsell color system is a color system in which hue, lightness, and saturation are differentiated step by step based on human senses. was never used for In particular, since the Munsell color system is a color system for solid colors that do not contain glitter materials, it has not been used to evaluate colors that contain glitter materials (metallic colors, pearl colors, etc.). However, the present inventors obtained a continuous curve in the Munsell color coordinates based on the spectral reflectance of each part, and calculated the color difference from the continuous curve in the Munsell color coordinates of each part. It was found that even when measuring the color difference, a result close to the color difference evaluation by visual observation can be obtained.

Based on the above findings, the present invention provides a method for measuring the color difference of a plurality of parts, comprising the steps of measuring the spectral reflectance of each part, and based on the spectral reflectance, a continuous curve in Munsell color coordinates and calculating the color difference of the plurality of parts based on the continuous curve in the Munsell color coordinates of each part.

The present invention also provides a method for measuring color differences at a plurality of sites, comprising the steps of measuring the spectral reflectance of each site, and obtaining a continuous curve in an xy chromaticity diagram based on the spectral reflectances. a step of identifying a Munsell grid to which each point on the continuous curve in the xy chromaticity diagram belongs; and a coefficient set for each Munsell grid to determine each point on the continuous curve in the xy chromaticity diagram to coordinates in the L ^* a ^* b ^* color space or L ^* u ^* v ^* color space; and based on the coordinates in the L ^* a ^* b ^* color space or L ^* u ^* v ^* color space, said and calculating color differences at a plurality of sites.

In this way, when each point on the continuous curve in the xy chromaticity diagram is transformed into coordinates in the L ^* a ^* b ^* color space or the L ^* u ^* v ^* color space, the coefficients used for the transformation are determined by the coefficients to which each point belongs. By setting for each Munsell grid, a color difference measurement result closer to visual evaluation can be obtained.

As described above, according to the present invention, it is possible to obtain color difference measurement results that are closer to visual evaluation.

1 is a perspective view conceptually showing a multi-angle spectrophotometer; FIG. FIG. 2 is a side view of the multi-angle spectrophotometer of FIG. 1, showing a case where incident light is emitted from an irradiation unit 1a with an incident angle of 45°. FIG. 2 is a side view of the multi-angle spectrophotometer of FIG. 1, showing a case where incident light is emitted from an irradiation unit 1b with an incident angle of 15°. 2 is a plan view of the multi-angle spectrophotometer of FIG. 1; FIG. 1 is a flow diagram of a color difference measuring method according to one embodiment of the present invention; FIG. FIG. 6 is a diagram illustrating step (2) of FIG. 5 in detail; FIG. 6 is a flow diagram illustrating in detail step (3) of FIG. 5; It is a figure which shows the state which superimposed the Munsell grid on the approximated curve of the xy chromaticity diagram. FIG. 9 is an enlarged view of the Munsell grid containing the point α of FIG. 8; FIG. 10 is a flow diagram of step (3) of a color difference measurement method according to another embodiment; 5 is a graph showing a comparison between color difference values (new color difference values) measured by the method according to the present invention and color difference values measured by a conventional method;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.

The color difference measuring method according to the present embodiment includes a multi-angle spectrophotometer as a multi-angle colorimetry device, and a calculation unit (computer, tablet, etc.) that performs various calculations using the amount of received light measured by the multi-angle spectrophotometer. This is done using a color difference measuring device. The multi-angle spectrophotometer and the computing unit are connected by wire or wirelessly so as to be communicable.

The multi-angle spectrophotometer, as shown in FIGS. 2b and 2c. 1 to 3, the plane containing the incident light and specularly reflected light is defined as the principal plane _P0 , which is indicated by an arc.

The irradiating units 1a and 1b are provided at positions where the site O to be measured is irradiated with incident light L _in from different angles. Specifically, the irradiating unit 1a irradiates the incident light L _{in (45)} from a direction of 45° to the perpendicular V passing through the measurement target site O (see FIG. 2), and the irradiating unit 1b Incident light L _{in (15)} is applied from a direction of 15° (see FIG. 3).

The light receiving portion 2a receives reflected light on the principal plane P0. In this embodiment, a plurality of light receiving portions 2a are provided on the principal plane _P0 , and in the illustrated example, a plurality of light receiving portions 2a are provided on both the incident light side and the specularly reflected light side of the vertical line V, respectively. The light receiving portions 2b and 2c receive reflected light in a direction intersecting the principal plane _P0 . As shown in FIG. 4, the light-receiving unit 2b receives the reflected light in the direction of 90° in plan view with respect to the specularly reflected light _Lref0 , and the light-receiving unit 2c receives the specularly reflected light Lref0 ₍₄₅₎ and Lref0 _{( 15)} , the reflected light L _ref is received in a direction less than 90° in plan view (45° direction in the illustrated example). In the illustrated example, the light receiving portion 2b and the light receiving portion 2c are provided at symmetrical positions with respect to the main plane _P0 .

In this embodiment, the color difference between parts A and B is measured through steps (1) to (5) shown in FIG. Each step will be described in detail below.

In step (1), the spectral reflectance of each portion A and B is measured. In this embodiment, the multi-angle spectrophotometer described above is used to measure the spectral reflectance of a plurality of geometrical optics. Specifically, in each geometrical optics system, the reflectance is measured for each of a number of wavelengths at predetermined intervals set within the visible light wavelength range (for example, wavelengths at intervals of 10 nm in the range of 400 to 700 nm). Specifically, first, incident light L _{in (45)} with an incident angle of 45° is irradiated from the irradiation unit 1a to the measurement target site O, and reflected light L _{ref (45)} in multiple directions reflected at the measurement target site O is The light is received by the light receiving units 2a, 2b, and 2c, and the respective spectral reflectances are obtained (see FIGS. 2 and 4). Next, an incident light L _{in (15)} having an incident angle of 15° is irradiated from the irradiation unit 1b to the measurement target site O, and the reflected light L _{ref (15)} in multiple directions reflected at the measurement target site O is received by the light receiving unit. Light is received by 2a, 2b, and 2c, and respective spectral distributions are obtained (see FIGS. 3 and 4). By performing these operations for each of the portions A and B, data of the reflectance Y with respect to the wavelength λ is obtained for each geometrical optical system for each of the portions A and B (see the upper diagram in FIG. 6). Note that the geometrical optics refers to a light ray composed of one incident light and one reflected light.

In step (2), an approximate curve of spectral reflectance is calculated based on the spectral reflectance data acquired in step (1). In this embodiment, the data measured by the multi-angle spectrophotometer (data of the reflectance Y with respect to the wavelength λ for each geometrical optical system at each part A and B) is sent to the calculation unit, and the calculation unit calculates the spectral reflectance. Calculate an approximate curve for the angle of acceptance.

Specifically, as shown in Fig. 6, the spectral reflectance data for wavelength (see the upper diagram) is rearranged into the spectral reflectance data for the light receiving angle (see the lower diagram), and an approximate curve is plotted from each graph. calculate. Specifically, since the attenuation factor of the amount of light received with an increase in the light receiving angle approximates an exponential function, the plot for each wavelength in each system is approximated by an exponential curve. If it is not possible to trace all points with an exponential curve, or if it is not possible to trace with an exponential like a solid paint color, a Bezier curve is also used for tracing. In this way, by defining an approximate curve using the spectral reflectance, which is a simple physical phenomenon before conversion to the tristimulus values X, Y, and Z, the coefficient of each reflectance with the light receiving angle as a variable is obtained. , the reflectance for each light receiving angle can be calculated by not only approximating but also by completely tracing the measurement points. If there is no problem with the accuracy of the approximation, the approximation curve may be calculated in a later step (for example, after calculating the tristimulus values X, Y, and Z).

In step (3), the reflectance data (approximate curve) of each of the portions A and B calculated in step (2) are converted into color coordinates. Specifically, as shown in FIG. 7, the reflectance Y data (approximate curve) to the light receiving angle .theta. , Z (step (3a)), and the approximated curves of the tristimulus values X, Y, Z are converted into approximated curves in the xy chromaticity diagram (step (3b)). A specific method for calculating the tristimulus values X, Y, Z and x, y from the spectral reflectance conforms to the method described in JIS Z8781-3:2016.

Next, as shown in FIG. 8, the Munsell grid (see dotted line) is superimposed on the xy chromaticity diagram (step (3c)). Then, the Munsell grid to which each point on the approximated curve of the xy chromaticity diagram belongs is specified. For example, the point α shown in FIG. 8 belongs to the Munsell grid with saturation: C1 to C2 and hue: 2.5G to 5G.

Next, each point on the approximated curve in the xy chromaticity diagram is converted into coordinates in the Munsell color system (step (3d)). Specifically, for example, as shown in FIG. 9, the distance from the straight line of saturation C1, C2 and the straight line of hue 2.5G, 5G that divides the Munsell grid to which the point α on the approximate curve belongs to the point α is and locate the point α in that grid. At this time, since the grid has a distorted shape, it is necessary to devise ways to specify the position of the point α. For example, with regard to the coordinates in the direction of saturation, a straight line g of the slope is obtained by performing a weighted average of the slopes of the straight lines of hues 2.5G and 5G in the xy chromaticity diagram with respect to the point α. is calculated based on the distances L1 and L2 from the point of intersection with the straight line to the point α. Similarly, regarding the coordinates in the hue direction, a straight line c of the slope is obtained by performing a weighted average of the slopes of the straight lines of the saturations C1 and C2 in the xy chromaticity diagram with respect to the point α. is calculated based on the distances L3 and L4 from the point of intersection with the straight line to the point α. Specifically, the Munsell color coordinates (saturation C, hue H) of the point α in the illustrated example are calculated by the following formula, and the saturation C=1.27 and the hue H=4.0G.
Saturation C=(2−1)·{L1/(L1+L2)}
Hue H = (5G-2.5G) · {L3 / (L3 + L4)}

By performing the above calculations for all points on the continuous curve in the xy chromaticity diagram, the curve is converted to a continuous curve in Munsell color coordinates. The Munsell color coordinates are a color system consisting of discontinuous points with different hues, lightness, and saturation in stages. By calculating detailed coordinates (coordinates within each grid) in , a continuous curve in Munsell color coordinates can be obtained.

Returning to FIG. 5, in step (4), from the distance between the coordinates of each point on the approximated curve in the Munsell color coordinates of the parts A and B, the color difference ΔE for each light receiving angle θ in the Munsell color coordinates between the parts A and B is calculated. Calculate _ms .

In step (5), the color difference between parts AB is evaluated based on the color difference ΔE _ms for each light receiving angle θ calculated in step (4). For example, the color difference can be evaluated based on the maximum value and integrated value of ΔE _ms for each system. If the color difference between parts A and B is out of a predetermined range, it is determined that the color difference between them is abnormal. In this case, the coating is applied after adjusting the formulation of the coating material and the coating conditions, and the color difference is measured again according to the above procedure. By repeating the above process until the color difference falls within a predetermined range, it is possible to set the optimal paint composition and coating conditions.

The invention is not limited to the above embodiments. For example, when converting an approximated curve in an xy chromaticity diagram into L ^* a ^* b ^* coordinates or L ^* u ^* v ^* coordinates, coefficients for conversion may be set for each Munsell grid to which each point belongs. . Specifically, as shown in FIG. 10, after obtaining an approximated curve in the xy chromaticity diagram in step (3b), a Munsell grid is superimposed on the xy chromaticity diagram in step (3c). At this time, coefficients for conversion to L ^* a ^* b ^* coordinates or L ^* u ^* v ^* coordinates are set for each grid to which each point on the approximation curve belongs (step (3d1)). Then, using this coefficient, each point on the approximated curve in the xy chromaticity diagram is converted to L ^* a ^* b ^* coordinates or L ^* u ^* v ^* coordinates. After that, the color difference ΔE ab or ΔE _uv is calculated from the distance between the coordinates in the L ^* a ^* b ^* color system or the L ^* u ^* v ^* color system of each of the parts A and B, and the color difference ΔE _ab or _{ΔE uv is} calculated as Evaluate the color difference between parts AB based on.

The above-described color difference measurement method can be applied not only to the color difference measurement of solid colors, but also to the measurement of the color difference of colors including luster materials such as metallic colors and pearl colors.

The site to which the above-described color difference measurement method can be applied is not particularly limited. Specifically, for example, it is possible to measure the color difference between parts made of different materials that are painted in the same color (for example, a steel body and a resin bumper) and the color difference between multiple parts on the same part.

A test was conducted to confirm whether or not the color difference measurement result obtained by the above color difference measurement method correlates with the visual color difference evaluation. First, four kinds of colored plates with slightly different colors were prepared for green (solid color), silver (metallic color), and white pearl (mica color), and visually evaluated when viewed from the front and diagonally. were ranked in order of closeness to the reference color. In addition, the spectral reflectance of each color plate and the reference color is measured with a multi-angle spectrophotometer, and based on the stimulus value XYZ calculated from the spectral reflectance, the conventional method (coordinates in L ^* a ^* b ^* color space distance between coordinates) and the method of the present invention (distance between coordinates in Munsell color coordinates). As a result, as shown in FIG. 11, the color difference values measured by the conventional method sometimes did not correlate with the results of ranking the color plates by visual evaluation (see the portion enclosed by the dotted line). On the other hand, the color difference values measured by the method of the present invention generally correlated with the results of ranking the color plates by visual evaluation.

1a, 1b irradiating parts 2a, 2b, 2c light receiving part L _in incident light L _ref reflected light L _{ref 0} specular reflected light O measurement target site P ₀ principal plane

Claims (1)

A method for measuring color differences at multiple sites, comprising:
A step of measuring the spectral reflectance of each part, a step of calculating an approximate curve of the measured spectral reflectance data, and a step of obtaining a continuous curve in Munsell color coordinates based on the approximate curve of the spectral reflectance. and calculating the color difference of the plurality of parts based on the continuous curve in the Munsell color coordinates of each part.

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