JPH10253455A - Chromaticity measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure chromaticity with high accuracy in a wide luminance range from low luminance to high luminance by using power values below 1 of RGB output signal values, and using a three-line, nine-row charomaticity transformation matrix when the chromaticity is calculated from the RGB output signal values of a color television camera. SOLUTION: A phtometric object 1 is photographed by a color television camera 3. The R output signal 41, G output signal 42, and B output signal 43 of the camera 3 are digitized by A/D converters 51-53 and stored in image memories 61-63. A computer 7 reads out the contents of the image memories 61-63 and calculates the chromaticity of the photometric object 1. A colorimeter 8 is used in obtaining the coefficients for converting the output R, G, B of the camera 3 into tristimulus values X, Y, Z, and the computer 7 knows an accurate tristimulus value 9 via the colorimeter 8, i.e., an exponent approximating the relation between the output R of the camera 3 and the tristimulus value X is obtained. Exponents are likewise obtained from the relation between G and X and the relation between B and X. Exponents are likewise obtained from the relation between the outputs R, G, B of the camera 3 and the tristimulus values Y, Z.

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 a color using a color television camera.

[0002]

2. Description of the Related Art The relationship between a color television camera and chromaticity is as follows.
For example, it is described in the Color Science Handbook (published by the University of Tokyo Press, 1985, pages 934 to 937). That is, the output signals (R, G, B) of the camera can be converted into tristimulus values (X, Y, Z) by a matrix of 3 rows and 3 columns.

[0003]

(Equation 1)

Further, it is known that the xy chromaticity of CIE1931 can be calculated from the tristimulus values by the following equation. x = X / (X + Y + Z), y = Y /
(X + Y + Z)

[0005]

However, when colorimetry is performed using a color television camera, there is a phenomenon that when the brightness of an object changes, the measurement accuracy deteriorates. This is because the conversion coefficient slightly changes depending on the brightness of the object, and this change is due to the nonlinearity of the input / output characteristics of the color television camera. That is, in the television camera, even if the gamma is 1, the input light amount and the output voltage are not exactly proportional to the entire brightness range. In particular, when the input light amount is small, the influence of the non-linearity of the sensor and the video circuit becomes large. For this reason, even if the conversion coefficient between the output (R, G, B) of the color television camera and the chromaticity is obtained with a normal light quantity, an error occurs in the colorimetric value at a place where the light quantity is small. . An object of the present invention is to provide a method capable of performing accurate colorimetry using a color television camera even when the brightness of an object changes.

[0006]

For this purpose, the output of the color television camera may be approximated by a non-linear function. This non-linear characteristic can be realized by combining several power functions. The power value is selected based on the nonlinearity data for each color. Next, the conversion coefficients of the R, G, and B values and the X, Y, and Z values are obtained by using a colorimeter. Used. Thereby, although the number of conversion parameters increases, conversion with less error can be realized.

[0007]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an apparatus configuration for implementing the present invention. Generally, colorimetric objects are classified into those that emit light by themselves (light source color), such as color display devices, and those that reflect light from a light source without emitting light (object color). FIG. 1 shows the case of the light source color, and the colorimetric object 1 is a color display. Various colors are projected on the color display by the signal generator 2, and the colors are measured. The color television camera 3 images the colorimetric object 1. The R output signal 41, the G output signal 42, and the B output signal 43 of the color television camera 3 are supplied to A / D converters 51 to 5 respectively.
3 and is stored in the image memories 61 to 63. The computer 7 reads the contents of the image memories 61 to 63, and calculates the chromaticity of the colorimetric object 1. The colorimeter 8 converts the output R, G, B of the color television camera into tristimulus values X, Y,
The computer 7 uses the tristimulus value 9 of the measurement object 1 to calculate the coefficient to be converted into Z (calibration).
You can know by.

FIG. 2 shows a screen 10 used for calibration. At the time of calibration, a color display area 11 is set at the center of the screen.

The chromaticity conversion method using power approximation according to the present invention will be described below. FIG. 3 shows a relationship between a camera output (for example, G) and a tristimulus value (for example, Y). The calibration for obtaining the conversion coefficient is performed at the calibration point 12 in the middle / high luminance region. For this reason, the light conversion characteristic becomes the linear approximation conversion characteristic 14 and does not become the actual conversion characteristic 13, and an accurate tristimulus value cannot be obtained in a low luminance range. Therefore, in the present invention, the actual conversion characteristic 13 is approximated by using the power value of the camera output. The conversion formula is shown below.

[0011]

(Equation 2)

Here, power exponents α1 to 3, β1 to 3,
A method for determining γ1 to γ3 will be described. In FIG. 1, the color television camera 3 and the colorimeter 8 capture an image of the color display area 11 of FIG. Here, the luminance value of white is changed, and the camera outputs R, G, B and the tristimulus values (X,
Y, Z) 9 is measured. This is shown in FIG. The camera output on the horizontal axis is the output voltage of the color television camera 3. The vertical axis is the tristimulus value 9 of the output of the colorimeter 8. Power exponent α that best approximates the relationship between camera output R and tristimulus value X
Find 1 Similarly, β1 is obtained from the relationship between G and X. Γ1 is obtained from the relationship between B and X. Similarly, from the relationship between the camera outputs R, G, B and the tristimulus value Y, α2,
β2 and γ2 can be obtained. Similarly, from the relationship between the camera outputs R, G, B and the tristimulus value Z, α3, β
3, γ3 can be obtained. Next, as calibration
Nine colors are displayed in the color display area 11, and R,
From the G and B values and the X, Y and Z values of the colorimeter 8, a conversion coefficient a ij is obtained. From the above, power exponents α1 to γ3
And the chromaticity conversion coefficients a _{11 to} a ₃₉ could be obtained. The exponent index was about 0.85-0.95 in the experiment. Conventionally, when the measurement target 1 is 20 cd / m ² or more, x
The y chromaticity measurement error was about 0.005, and increased to about 0.01 at about 10 cd / m ² . However, with the method according to the present invention, a measurement accuracy of about 0.002 could be obtained from high luminance to low luminance of about 5 cd / m ² .

The embodiments of the present invention have been described for the case of light source colors. In the case of the object color, as shown in FIG.
The brightness of 5 is performed using the lighting device 16.

[0014]

According to the present invention, colors can be measured with high accuracy from low luminance to high luminance using a color television camera.

[Brief description of the drawings]

FIG. 1 is a block diagram of a color measurement device using a color television camera according to the present invention.

FIG. 2 is an explanatory diagram of a calibration screen.

FIG. 3 is a characteristic diagram showing a non-linear relationship between luminance and tristimulus values.

FIG. 4 is an explanatory diagram of how to calculate a power exponent.

FIG. 5 is an explanatory diagram of a calibration method for an object color.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Color measurement object, 2 ... Signal generator, 3 ... Color television camera, 41 ... R output signal, 42 ... G output signal, 43 ... B output signal, 51-53 ... A / D converter, 61-63 ... Image memory, 7: Computer, 8: Colorimeter, 9: Tristimulus value XYZ.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Nobuo Kawai 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Co., Ltd.

Claims (3)

[Claims] 1. A method for measuring color using a color television camera, comprising the steps of: storing the color television camera, an A / D converter for digitizing RGB output signals, an image memory for each color, and the contents of the image memory. It has a computer to analyze and calculate, and when calculating chromaticity from the RGB output signal value of the color TV camera, by using a power value less than 1 of the RGB output signal value, a wide range from low luminance to high luminance can be obtained. A chromaticity measurement method that enables highly accurate chromaticity measurement in a luminance range. 2. A color measuring method using a color television camera, wherein the color television camera, an A / D converter for digitizing RGB output signals, each color image memory, and the contents of the image memory are analyzed. A computer that calculates the chromaticity from the RGB output signal value of the color TV camera,
A chromaticity conversion matrix of 3 rows and 9 columns using a power value of less than 3 to enable high-precision chromaticity measurement in a wide luminance range from low luminance to high luminance. Measurement method. 3. A color television camera, an A / D converter for digitizing the RGB output signal, an image memory of each color, and an image memory for measuring the chromaticity of white of a television or a color display device. It has a computer that analyzes and calculates the contents of the memory,
When the chromaticity is calculated from the B output signal value, a power value less than 1 of the RGB output signal value is used, and by using a chromaticity conversion matrix of 3 rows and 9 columns, a wide luminance from low luminance to high luminance is obtained. A chromaticity measurement method characterized by enabling highly accurate chromaticity measurement within a range.