The three-color theory of color vision assumes a coordinate for each perception of color, and gives the red, green, and blue amounts of color, because the amount of light reaching the retina depends on the effective light amount. These three coordinate values ​​[tristimulus value x ( λ), y (λ), and z (λ) are the reflectance values ​​weighted by the tristimulus value of illumination. The y (λ) stimulus value of illumination is usually normalized to 100 when calculating.
The y (λ) value in the tristimulus value is a measure of the visual brightness response (based on the average value obtained by approximately 200 observers). The x (λ) value and z (λ) value in the tristimulus value express the red amount and Blue volume.
Because TV technology is mainly related to the additive color mixing of light, TV engineers will explain that the tristimulus value is simulated with red, green, and blue phosphors on the fluorescent screen. Just as any color (within the color gamut of a fluorescent screen) can be generated by correctly selecting the combined intensity of red, green, and blue phosphors, any color can be generated with an appropriate combination of three stimulus values. Unfortunately, the tristimulus value is imaginable, because the tristimulus value must be more saturated than the spectral color.
The visual sensations of the tristimulus values ​​x (λ), y (λ) and z (λ) are not evenly spaced, and the 5 unit color difference generated in the bright area is much smaller than the 5 unit color difference generated in the dark area. We can recall what we learned in the past. The tristimulus value is proportional to the reflectance value, so the reflectance value is not evenly distributed to the visual perception, even for neutral colors. Because of this, much research has been done on the conversion method of tristimulus values.
Because the reflectance values ​​are not equidistant from the eye, in order to obtain a relatively uniform brightness scale, the y (λ) stimulus value can be transformed. The first uniform brightness scale is the Munsell value scale, which uses the square root of the tristimulus value y (λ) as the brightness. It was redefined in 1943, and brightness is no longer treated as a function of Y.
The cube root brightness scale was proposed by Rad and Pini, and later redefined by Glosser et al. The expression is:
L * ï¼ 116 / y (λ) / y (λ) 0ï¼16
Where: L *-brightness;
y (λ) 0——The tristimulus value of illumination (usually 100).
The above formula can make the brightness into a visually uniform color space coordinate, and the other two stimulus values ​​x (λ) and z (λ) must also become visually uniform coordinates.
Adams considered that the reflectivity does not form a uniform visual scale, he suggested that the x (λ), z (λ) stimulus values ​​should also be transformed like the y (λ) value in order to obtain the Munsell value, and record the converted value Vx, Vy and Vz. In order to convert with Hering's opposite color model, Adams suggested subtracting Vy from Vx to get red-green coordinates, and subtracting Vy from Vz to get yellow-blue coordinates, and black-white coordinates are defined by Vy, which is brightness, This is the Adams color space. After undergoing a series of corrections (mainly using the relative scale of the coordinate axis and the cube root correction to approximate the Munsell value), CIE adopted the 1976 (L * a * b *) color space, abbreviated as CIELAB, the first A coordinate L * is about 10 times the Munsell value, so that L * = 0 is ideal black, L * = 100 is absolutely white, a * coordinate is red-green coordinate, b * is yellow-blue coordinate.
Adams suggested correcting the chroma retention through numerical transformation to make the chroma (subtracting the luminance component from the color of an object) more uniformly arranged. Then the brightness is used to multiply the uniform chromaticity in order to remove the luminance coordinates and the scaled chromaticity from the chromaticity space. Adams calls it the Color Valence Space. Later, the Adams color space was redefined by Hunter and Westinki. Hunter's color space is based on Adams' uniform chromaticity space, and the square root formula is used for brightness; while Wessinki's color space is based on a uniform chromaticity scale and cubic root brightness formula. The latter has been improved and was adopted by CIE in 1976, and is called CIE1976 (L *, u *, v *), abbreviated as CIELUV, where L * parameter is the same as CIELAB color space, visual brightness, coordinate u * Represents redness (compared to greenness), coordinate v * represents yellowness (compared to blueness), these coordinates are relatively uniform.
CIELAB and CIELUV coordinates can also be converted into intuitive coordinates, because the coordinate L * has intuitive meaning, no longer need to be converted, and a *, b * or u *, v * can be converted into polar coordinates.
The saturation value can be calculated according to the CIELUV coordinates, and the saturation can be obtained by dividing the chromaticity by the brightness, but it cannot be calculated from the CIELAB coordinates.
In a uniform color space, not only absolute colors but also chromatic aberrations can be compared. Therefore, not only can the color measurement technique be used to measure the color of the ink, but also the color difference of the printed color can be evaluated. In either direction of the CIELAB and ELUV color space, the color difference of one unit is approximately equal to the smallest difference that a standard observer can perceive. The color difference between two objects is the Euclidean distance of its coordinates and is represented by the symbol ΔE *.
When Adam studied the color tolerance range, he found that 6 color difference units were acceptable. Because the ΔE * values ​​come from a distance scale, their sizes are comparable. For example, when one printed sample is compared with a proof sample, the color difference is 4 and the other printed sample has a color difference of 2. It can be considered that the latter is nearly twice as close to the exact match as the former. In this way, the closeness of matching can be evaluated.
The chromaticity detection method can evaluate the color of an object with a visually uniform and accurate scale, so it has a wide range of uses in the printing industry. The chromaticity detection method can determine the absolute color of a printed surface or provide a sample with a certain tolerance, and can also evaluate different processes through color difference comparison. [next]
In the CIE color system, any color can be identified based on its tristimulus value, but the same color is expressed in different parameters in different color systems, but these parameters can be converted from tristimulus values ​​by simple mathematical conversion get.
There are currently at least 20 color systems. Most color metering systems are aimed at obtaining the direct relationship between the measured parameter and visual color, but various color metering systems have not completely achieved this goal. Each color-metering system has its own unique strengths, so different color-metering systems have different applications. For the printing industry, the following five color-measuring systems are of interest (see Figure 2-1, Figure 2-2, Figure 2-3).
(Figure 2-1)
(Figure 2-2)
(Figure 2-3)
1. Munsell (1929) system. This is one of the oldest color expression systems and has undergone many improvements. For equal chromatic aberration, the Munsell system gives an equal difference in visual perception, but the system is limited to the case of C light source illumination. Even using a computer to convert tristimulus values ​​into Insell values ​​is very complicated. The Munsell system can be used as a standard method for comparing other color systems.
2. CIE (1931) RGB system. When using different standard light sources A, B, C, and D65, the system can classify colors well, but it is not a spatial system with uniform visual perception, which is different from the Munsell system. The system is designed to define colored light that complies with the law of additive color, not to identify colorants that follow the law of subtractive color on a reflective surface. It can be used in soft proofing and color desktop publishing systems.
3. Hunter (L, a, b) (1947) system. It is a conversion form of the CIE (1931) color representation system. It is an excellent visual color space system. It distinguishes saturated colors better than light colors. It also improves the measurement of certain color differences except printing. Outside the industry, the application areas of the system are extensive.
4. CIELAB (CIE 1976 L *, a *, b *) system. The system is similar to the Hunter system but with some improvements. The International Lighting Commission hopes that it will replace the Hunter system in some applications and become a standard color system. If the standard deviations in all directions are found for a color position, the results will show that the connection of the endpoints of the line segment where the standard deviation is found is closer to the standard chromaticity diagram than other color systems Round. So this system is widely used in the printing industry. [next]
5. CIELUV (CIE 1976L *, u *, v *) system. This system is similar to the CIEIAB system, but in the psychological chromaticity diagram, the line connecting the component color and its mixed color is closer to a straight line. In the standard chromaticity diagram, the color difference of visual perception is more uniform than other systems. In the printing industry, these characteristics are important.
Psychological chromaticity Figures 2-4, 2-5 and 2-6 are based on the measurement values ​​of the same set of color blocks after conversion. The data used are listed in Table 2-1. It can be seen from the figure that the connection between the Hunter Lab system and the CIELAB system is very similar regardless of the direction and curvature, while the CIELUV system is very different from the previous two cases, and the connection appears straighter.
(Figure 2-4)
(Figure 2-5)
(Figure 2-6)
Table 2-1 Color category x (λ) y (λ) z (λ) L
L *
L * L **
L **
L Δa
Δu *
ΔV * Δb
Δa *
Δb * Cyan 19.2 24.9 72.3 56.8 46.7-18.3-53.2 60.6 46.7-59.5-75.8 62.5 46.7-23.8-46.1 magenta 73.4 20.7 26.3 42.3 41.4 67.5 -4.7 55.9 41.4 107.9-20.9 57.9 41.4 67.4 -5.0 yellow 67.9 74.6 20.8 93.3 91.4 -20.5 51.1 94.4 91.4 25.9 106.5 94.5 91.4 -10.5 89.1 red 33.6 19.4 8.1 51.0 39.6 59.0 18.0 54.5 39.6 129.3 26.0 56.5 39.6 60.6 31.8 green 8.5 18.5 7.6 49.8 38.4-39.7 17.4 53.3 38.4-63.3 52.0 55.4 38.4-62.9 31.5 blue purple 7.1 4.9 22.5 29.0 13.7 18.7 -46.9 28.5 13.7 -3.6 -61.7 31.7 13.7 25.9 -44.2 three-color 3.9 4.2 4.0 27.2 11.8 -1.2 0.6 26.2 11.8 -0.6 5.1 29.5 11.8 -2.1 2.5 four-color 2.7 2.7 2.9 23.6 7.3 0.6 -1.4 20.3 7.3 1.2 0.6 24.0 7.3 0.9-0.7 black 6.7 6.7 7.7 22.7 18.0 0.9-1.7 33.4 18.0 1.1 -0.6 36.3 18.0 1.2 -1.5 paper 84.77 86.61 98.85 99.4 99.2 0.9 1.1
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