JP3353596B2 - How to measure color difference - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the number of color difference evaluations in which the difference in color quality is quantified.

[0002]

2. Description of the Related Art Conventionally, there is a so-called flop value (DuPont) as a numerical value (index) for evaluating the metallic degree of a coating.

[0003]

As shown in FIG. 7, this flop value (indicated by the symbol F) irradiates light to the surface of the sample (paint) at an angle of 45 °, as shown in FIG. Of the reflected light, lightness indices L1 ^* , L2 ^* , L3 ^* were measured in three directions (60 °, 90 ° in the same direction as the light incident direction, and 25 ° in the opposite direction) in the incident plane and were obtained. Lightness index L
Calculated by substituting 1 ^{* to} L3 ^* into the following equation: F = 2.69 (L1 ^* -L3 ^* ) ^1.11 ÷ L2 ^{* 0.86} When the flop value F is large, the degree of metallicity is high, and when the numerical value is small, the color is solid color. In other words, whether the flop value F is large or small is distinguished between metallic and solid.

[0004] However, in recent metallic paints, not only lightness but also color quality (an attribute of color sensation when hue and saturation are considered at the same time, also referred to as chromatics) often vary depending on angles. However, it is becoming difficult to determine the perceptual difference (color difference) of color only by brightness.

The present invention has been made in view of the above problem in the evaluation of the degree of metallicity of a coating, and a numerical value representing objectively and quantitatively a color appearance based on a change in color quality is a color difference evaluation number. It is an object of the present invention to provide a new method for measuring the number of evaluations of the color difference.

[0006]

The perceived color includes lightness, hue,
There are three attributes of saturation, tristimulus values are defined in order to express correlation with human visual sensation, and the concept of a uniform color space is used. Of the three-dimensional coordinates that make up the uniform color space,
The coordinates corresponding to lightness are a lightness index and two coordinates representing a position in the iso-brightness plane, and those corresponding to chromaticness (color quality: an attribute of a color sensation consisting of hue and saturation) are called a chromaticness index. ing. In the present invention, an L ^* a ^* b ^* color system is used as a uniform color space.

However, as described above, in recent metallic coating, depending on the angle of illumination and the direction of observation, not only does it appear brightly shining or dark, but also colors match under certain lighting and observation conditions. However, there is a characteristic that the color quality (chromaticness) varies depending on the angle without always appearing consistently under other conditions. In order to objectively and quantitatively express such changing color appearance, it is necessary to know not only the three attributes but also the spatial distribution of the reflection intensity of light and the accompanying changes in the three attributes. Therefore, when specifying this, the directionality and intensity of diffuse reflection light are more important than the regular reflection light of light from the object surface (anisotropic reflection characteristics). Accordingly, the present invention provides a method of measuring diffuse reflection light reflected in a plurality of angular directions on a light incident surface and quantifying a perceptual difference in color quality.

First, the invention according to claim 1 is a method for measuring an evaluation number of a color difference based on a change in a color quality. The process of measuring the reflected light in a predetermined angle direction at a plurality of measurement points corresponding to the angle direction to obtain color data for each measurement point, and the L ^* a ^* from the obtained color data of each measurement point ^.
The process of obtaining the b ^* color system chromaticness index a ^* , b ^* , and the obtained chromaticness index a ^* b of each measurement point
^{From *} , the process of calculating the angle difference in the a ^* b ^* coordinate plane between all two measurement points and determining the maximum value, and using the determined maximum angle difference, the color by the predetermined defined formula Calculating a color difference evaluation number indicating a difference in quality.

According to a second aspect of the present invention, there is provided a method for measuring the evaluation number of a color difference based on a change in color quality, wherein the method comprises the steps of: To the sample at an angle of 45 ° with respect to the surface of the sample in the same direction as the light incident direction with respect to the surface of the sample within the light incident surface.
Light reflected at angles of 0 °, 70 °, 90 °, and 70 ° and 30 ° in the direction opposite to the incident direction of light is measured at five measurement points corresponding to the angle directions, and the measurement points Of obtaining color data for each measurement point, obtaining L ^* a ^* b ^* color system chromaticity indices a ^* and b ^* from the obtained color data of each measurement point, and obtaining the calculated chromaty index of each measurement point. From the Kness index a ^* b ^* , the angle difference in the a ^* b ^* coordinate plane between all two measurement points is calculated, and the maximum value Hsmax is calculated.
And a step of calculating a color difference evaluation number Fh representing a difference in color quality defined by the following equation using the obtained maximum angle difference Hsmax, and Fh = (Hsmax × 10) / 180. It is a feature.

In the above-described method, a difference in color quality is mainly regarded as a change in hue, and the change is extracted in the form of an angle difference in an a ^* b ^* coordinate plane, and the evaluation number is calculated accordingly. However, when the saturation is small even if the angle difference is large, the perceptual difference in color becomes small. Therefore, in the case where a color having a small saturation (below a predetermined value) is included, it is desirable to take into account the change in the color saturation and take the difference in the overall color quality and digitize it. Specifically, when the saturation is small, the angle difference is corrected according to the value of the saturation.

The invention according to claim 3 introduces such a concept of correction, and is a method for measuring the evaluation number of color difference based on a change in color quality, wherein the sample is irradiated with light at a predetermined angle. Measuring the reflected light in a plurality of predetermined angular directions within a light incident surface at a plurality of measurement points corresponding to the angle directions, and acquiring color data for each measurement point; and from the color ^data, L ^* a * ^{b *} color system of the chromaticness index ^a *, and the process for obtaining the ^{b *,} the chromaticness index ^a * ^{b *} of the measurement points determined, among all second measurement point of a ^The saturation of each measurement point is calculated from the process of calculating the angle difference in the ^* b ^* coordinate plane and the chromaticness index a ^* b ^* of each measurement point, and there are measurement points whose saturation is equal to or less than a predetermined value. The angle between two measurement points, including the measurement point, Is corrected according to the saturation, the step of selecting the maximum value from the angle differences between all the two measurement points after the correction, and the step of correcting the predetermined value using the selected maximum angle difference. Calculating a color difference evaluation number indicating a difference in color quality by using the above equation.

According to a fourth aspect of the present invention, there is provided a method for measuring the evaluation number of a color difference based on a change in color quality, wherein the method comprises the steps of: To the sample at an angle of 45 ° with respect to the surface of the sample in the same direction as the light incident direction with respect to the surface of the sample within the light incident surface.
Light reflected at angles of 0 °, 70 °, 90 °, and 70 ° and 30 ° in the direction opposite to the incident direction of light is measured at five measurement points corresponding to the angle directions, and the measurement points Of obtaining color data for each measurement point, obtaining L ^* a ^* b ^* color system chromaticity indices a ^* and b ^* from the obtained color data of each measurement point, and obtaining the calculated chromaty index of each measurement point. Calculating the angular difference Hs in the a ^* b ^* coordinate plane between all two measurement points from the Kness index a ^* b ^* , and the chromaticness index a ^* b ^* of each measurement point. Calculate the saturation C, and when there is a measurement point at which the saturation C is equal to or less than a predetermined value, multiply the angle difference Hs between two measurement points including the measurement point by a correction coefficient k defined by the following equation. And k = 0.88C / (0.4 + 0.8C) All after correction Selecting the maximum value Hsmax from the angle difference Hs between the two measurement points, and using the selected maximum angle difference Hsmax, calculating a color difference evaluation number Fh representing a difference in color quality defined by the following equation: And Fh = (Hsmax × 10) / 180.

According to a fifth aspect of the present invention, in the fourth aspect of the present invention, preferably, the predetermined value is 5.

[0014] The invention according to claim 6 is the invention according to claim 3,
In the invention described in the item 4 or 5, preferably, when the angle difference Hs is corrected by the saturation, the angle difference Hs is corrected using a smaller value of the saturation C.

[0015]

According to each of the first to sixth aspects of the present invention, a difference in color quality is mainly regarded as a change in hue, which is then a ^* b ^*.
By extracting in the form of angle differences in the coordinate plane, a method was provided for calculating the evaluation number of color differences representing the difference in color quality.For example, the metallic degree of paint, which was conventionally expressed only in lightness, was provided. It is also possible to digitize the change in color quality. Therefore, the difficulty of the reproducibility of the paint in the colorimetry of the paint can be quantified, and the workability at the time of toning or starting a new color is improved.

According to the third or fourth aspect of the present invention, when the image includes a color with a small saturation, the angle difference is corrected in accordance with the value of the saturation, and the change in the saturation is also reduced. Considering the difference in overall color quality in consideration of the numerical value, the difference in color quality can be expressed more accurately.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of an apparatus configuration for realizing the color difference measuring method according to the present invention.
This system irradiates a sample with light, measures reflected light at a plurality of predetermined angles from the sample, and outputs respective color data. A multi-angle type color measuring unit 1 and a color signal from the color measuring unit 1 A signal processing operation device 2 for processing and calculating an evaluation number of color difference (color quality), and a result of the signal processing operation device 2 is output to a data output device 3. The signal processing operation device 2 includes a color signal interface 5 for inputting a color signal from the color measurement unit 1, an operation processing unit 5 for performing predetermined operation processing, and a memory 6 for storing programs and various data. ing.

The color measuring unit 1 is a television camera (color CC)
D camera) (for example, Japanese Patent Application No. Hei 4-32).
8466) or a device using a photodiode (see, for example, Japanese Patent Application No. 4-100476). The optical system of the color measurement unit 1, that is, the angle of the incident light and the angle of the reflected light to be measured are appropriately set in advance as described later (multi-angle type). For example, taking a case where a television camera is used as an example below, the color measuring unit 1
From the (television camera), RGB image signals (color data) in each measurement direction are sent to the signal processing operation device 2.

The signal processing arithmetic unit 2 receives the RGB image signals sent from the color measuring unit 1 through the color signal interface 4 and coordinates values (color values) in a predetermined color system.
Is calculated. More specifically, data for color system conversion is stored in the memory 6 in advance, and first, XY
Calculate Z color value (tristimulus value in XYZ color system),
The calculated values are further converted to L ^* a ^* b ^* color values. Methods (calculation formulas) for these conversion processes are well known. In the present invention, as described in detail below, the obtained L ^* a ^* b ^*
Using the chromaticness indices a ^* and b ^* of the color specification values, a color difference evaluation number (hereinafter referred to as a hue flop value Fh) representing a difference in color quality is calculated.

The data output device 3 is, for example, a monitor. The hue flop value Fh calculated by the signal processing operation device 2 is output to a monitor or the like and displayed.

A preferred method for measuring the hue flop value Fh according to the present invention is as follows. FIG. 2 is a flowchart of the measurement processing of the preferable hue flop value Fh. Step S1 First, the measurement system of the present embodiment will be described. As shown in FIG. 3, light from a light source (not shown)
The sample is illuminated at an angle of °, and the reflected light is measured at five measurement points to within the light incident surface. Measurement point ~
The angle direction is, in order, 30 ° (measurement point), 70 ° (measurement point), 90 ° (measurement point) in the same direction as the light incident direction with respect to the surface of the sample, and opposite to the light incident direction. Direction is 70 ° (measurement point) and 30 ° (measurement point). By measuring diffuse reflection light reflected in a plurality of angular directions in this manner, a perceptual difference in color quality (chromaticness) can be quantified. This measuring system is incorporated in the color measuring section 1. Therefore, the color measurement unit 1
Outputs color data (RGB image signals) for each measurement point measured at each measurement point to.

Step S2 Based on the color data at each measurement point obtained in step S1, the L ^* a ^* b ^* color specification value at each measurement point is calculated. This calculation is performed by the arithmetic processing unit 5 using the data for color system conversion stored in the memory 6 (all the processing from step S2 to step S8 is executed by the arithmetic processing unit 5). . More specifically, as described above, first, XYZ is calculated from a RGB image signal input through the color signal interface 4 using a well-known predetermined formula.
The colorimetric value is calculated, and the L ^* a ^* b ^* colorimetric value is calculated from the calculated value according to a known formula. Here, the L ^* a ^* b ^* color values of the respective measurement points are represented by (Li ^* , ai ^* , bi).
^* ) (Where i = 1 to 5). Note that L ^* a
The three-dimensional coordinates (Li ^* , ai) in the ^* b ^* color system
^* , Bi ^* ), the two-dimensional coordinates (ai ^* , bi ^* ) in the a ^* b ^* coordinate plane are the chromaticness index representing the color quality.

In this case, all coordinates including the lightness index L ^* are calculated. However, since the hue flop value Fh is calculated based only on the chromaticness indexes a ^* and b ^* , step S2 is performed. Only the chromaticness indices a ^* and b ^* may be obtained.

Step S3 The chromaticness indices (ai ^* , bi ^* ) of the respective measurement points calculated in step S2 are plotted on the a ^* b ^* coordinate plane (see FIG. 4), and all plotted arbitrary 2
A ^* b ^* between points (ai ^* , bi ^* ) (aj ^* , bj ^* )
Calculate the angle difference Hsij in the coordinate plane. This angle difference Hsij
Is given by the following equation. Hsij = | tan ^-1 (bi ^* / ai ^* )-tan ^-1 (b
j ^* / aj ^* ) | However, when Hsij> 180 °, Hsij = 360 °
−Hsij In the calculation of tan ⁻¹ in the above equation, a value of 0 to 360 ° is output by looking at the signs of the coordinates a ^* and b ^* . The combination (i, j) of two points is i, j = 1 to
5, and i <j, that is, (1,2), (1,
3), (1,4), (1,5), (2,3), (2,
4), (2,5), (3,4), (3,5), (4,
5) There are 10 ways. Thus, 2 in the a ^* b ^* coordinate plane
By obtaining the angle difference Hs between the points, when the difference in color quality is mainly regarded as a change in hue, the change in hue can be extracted (expressed) in the form of a numerical value.

Step S4 In this step, the saturation Ci of each point is calculated from the chromaticness index (ai ^* , bi ^* ) of each measurement point. The saturation Ci is given by the following equation. Ci = {(ai ^* ) ² + (bi ^* ) ² } ^1/2 where i = 1 to 5, that is, the saturation C is represented by the coordinates (a ^* , b ^* ) on the a ^* b ^* coordinate plane. Expressed as the distance from the origin. In general, the smaller the saturation C, that is, the closer to the origin, the duller, so that even if the hues are different, the perceptual difference is smaller.

Step S5 Then, in this step, it is determined whether or not there is one having a small saturation C (not more than a predetermined value). Specifically, for example, it is determined whether or not the saturation C may be 5 or less (C ≦ 5). As a result, if there is a case where the saturation C becomes 5 or less, the angle difference Hs is corrected according to the saturation value in order to capture the difference in overall color quality in consideration of the change in saturation. I do. In this case, even if the angle difference Hs is large, the perceptual difference in color becomes small. On the other hand, if there is no case where the saturation C becomes 5 or less, such correction is not performed.

Step 6 In this step, the angle difference Hs including the measurement point at which the saturation C is determined to be 5 or less in Step S5 is corrected. For example, taking the case of FIG. 5 as an example, three measurement points,
, Since the saturation C is 5 or less, a combination between two points including these measurement points, specifically,
(1,2), (1,3), (1,5), (2,3),
(2,4), (2,5), (3,4), (3,5),
Correction is made to each angle difference Hs according to (4, 5).

The correction is performed by multiplying the angle difference Hs by a correction coefficient k of a value corresponding to the saturation C. That is, Hs = Hs × k. At this time, in calculating the correction coefficient k, the smaller saturation Cmin (i, j) (where min (i, j) means that the smaller one of Ci and Cj is selected) is used. To make corrections. The relational expression between the correction coefficient k and the saturation C is, for example, as shown in FIG. According to this equation, the saturation C
Becomes smaller, the value of the correction coefficient K rapidly approaches 0, so that the corrected angle difference Hs also becomes smaller, and a numerical value suitable for the actual appearance is obtained. Therefore, in summary, the angle difference Hsij between two points to be corrected is
Is given by the following equation: Hsij = Hsij × ｛0.88 Cmin (i, j) / (0.4+
0.8Cmin (i, j))｝.

Step S7 In this step, the maximum value is selected from among the ten angle differences Hsij obtained in step S3 (including the corrected angle difference Hsij corrected in step S6 if corrected). select. The maximum angle difference selected here is represented as Hsmax. For example, in FIG. 4, if no correction is required, the angle difference Hs15
Is the maximum angle difference Hsmax.

Step S8 Finally, in this step, the hue flop value Fh according to the present invention is calculated using the maximum angle difference Hsmax selected in step S7.
Is calculated. The hue flop value Fh is defined by the following equation. Fh = (Hsmax × 10) / 180 The hue flop value Fh is a color difference evaluation number indicating a difference in color quality. As a result, the degree of metallicity of paint, which was conventionally expressed only in lightness (Dupont flop value)
Is quantified as a change in color quality.

Step S9 The hue flop value Fh calculated in step S8 is output to a data output device (such as a monitor) 3 and displayed.

The above method is merely an example of the present invention, and is not limited to this. For example, the angle of the incident light and the measurement angle of the reflected light are not limited to those shown in FIG. 3 and can be set appropriately. In that case, in order to improve the accuracy of the evaluation formula itself, it is preferable to provide an appropriate number of measurement points in the same direction as the light incident direction and in the opposite direction. The expression of the hue flop value may be appropriately modified according to the set angle relationship.

In the above example, the correction is performed when the saturation C is equal to or less than a predetermined value, but this is not essential. Also, in the case of correction, the value of the predetermined value to be compared with the saturation C is not limited to 5, but can be appropriately changed according to conditions. Further, the equation for calculating the correction coefficient k is not limited to the one shown in FIG. 6, and can be appropriately changed according to conditions.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of an apparatus configuration for realizing a color difference measuring method according to the present invention.

FIG. 2 is a flowchart illustrating an example of a hue flop value measurement process according to the present invention.

FIG. 3 is a diagram illustrating an example of a measurement direction.

FIG. 4 is a diagram illustrating an example of an angle difference relationship in an a ^* b ^* coordinate plane.

FIG. 5 is a diagram illustrating an example of a case where correction is required.

FIG. 6 is a diagram illustrating a relationship between a correction coefficient and saturation.

FIG. 7 is a diagram provided for explanation of a conventional principle of measuring a flop value.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01J 3/00-3/52

Claims (6)

(57) [Claims] 1. A method for measuring an evaluation number of a color difference based on a change in color quality, comprising irradiating a sample at a predetermined angle with light and reflecting reflected light in a plurality of predetermined angles in a light incident surface. The process of acquiring color data for each measurement point by measuring at a plurality of measurement points corresponding to the angular direction, and the chromaticness index of the L ^* a ^* b ^* color system from the acquired color data of each measurement point. a ^*, and the process for obtaining the ^{b *,} the chromaticness index ^a * ^{b *} of the measurement points determined,
The process of calculating the angle difference in the a ^* b ^* coordinate plane between all two measurement points and obtaining the maximum value, and the difference in color quality by a predetermined defined equation using the obtained maximum angle difference. Calculating a color difference evaluation number representing the following: and a method for measuring a color difference. 2. A method for measuring an evaluation number of a color difference based on a change in color quality, comprising irradiating light to a sample at an angle of 45 ° with respect to the surface of the sample, and applying light to the surface of the sample within a light incident surface. On the other hand, light reflected at angles of 30 °, 70 °, and 90 ° in the same direction as the incident direction of light, and 70 ° and 30 ° in the direction opposite to the incident direction of light, respectively, corresponds to five angles corresponding to the angular directions. A process of measuring color data at each measurement point, and a process of obtaining L ^* a ^* b ^* color system chromaticness indices a ^* and b ^* from the obtained color data of each measurement point. From the obtained chromaticness index a ^* b ^{* of} each measurement point,
The process of calculating the angle difference in the a ^* b ^* coordinate plane between all two measurement points and calculating the maximum value Hsmax, and the difference in color quality defined by the following equation using the calculated maximum angle difference Hsmax Calculating a color difference evaluation number Fh representing the following: and Fh = (Hsmax × 10) / 180. 3. A method for measuring an evaluation number of a color difference based on a change in color quality, comprising irradiating a sample with light at a predetermined angle, and reflecting reflected light in a plurality of predetermined angles in a light incident surface. The process of acquiring color data for each measurement point by measuring at a plurality of measurement points corresponding to the angular direction, and the chromaticness index of the L ^* a ^* b ^* color system from the acquired color data of each measurement point. a ^*, and the process for obtaining the ^{b *,} the chromaticness index ^a * ^{b *} of the measurement points determined,
From the process of calculating the angle difference in the a ^* b ^* coordinate plane between all two measurement points and the chromaticness index a ^* b ^* of each measurement point, the saturation of each measurement point is calculated. Correcting the angle difference between the two measurement points including the measurement point in accordance with the saturation if there is a measurement point equal to or less than the predetermined value; and the angle difference between all the two measurement points after the correction. And selecting a maximum value from among them; and, using a selected maximum angle difference, calculating a color difference evaluation number representing a difference in color quality by a predetermined defined equation. Color difference measurement method. 4. A method for measuring an evaluation number of a color difference based on a change in color quality, wherein the sample is irradiated with light at an angle of 45 ° with respect to the surface of the sample, and the light is applied to the surface of the sample within a light incident surface. On the other hand, light reflected at angles of 30 °, 70 °, and 90 ° in the same direction as the incident direction of light, and 70 ° and 30 ° in the direction opposite to the incident direction of light, respectively, corresponds to five angles corresponding to the angular directions. A process of measuring color data at each measurement point, and a process of obtaining L ^* a ^* b ^* color system chromaticness indices a ^* and b ^* from the obtained color data of each measurement point. From the obtained chromaticness index a ^* b ^{* of} each measurement point,
From the process of calculating the angle difference Hs in the a ^* b ^* coordinate plane between all the two measurement points and the chromaticness index a ^* b ^* of each of the measurement points, the saturation C of each measurement point is calculated. When there is a measurement point whose degree C is equal to or less than a predetermined value, correcting the angle difference Hs between two measurement points including the measurement point by a correction coefficient k defined by the following equation; 0.88C / (0.4 + 0.8C) A process of selecting the maximum value Hsmax from the angle differences Hs between all the two measurement points after correction, and using the selected maximum angle difference Hsmax, Calculating a color difference evaluation number Fh representing a difference in color quality defined by: Fh = (Hsmax × 10) / 180. 5. The method according to claim 4, wherein the predetermined value is 5. 6. The color difference measurement according to claim 3, wherein, when the angle difference Hs is corrected by the saturation, the correction is performed using the smaller value of the saturation C. Method.