The problem of color difference evaluation has always been an important issue in the field of color science and actual industrial production. Objectively measuring and evaluating the difference between the size of the color difference and the visual difference of the human eye has long been considered a key technology that must be solved in the industry. Chromatic aberration refers to the difference in color perception between two colors by numerical means. If both color samples are color-coded by L*, a*, b*, the total color difference ΔEab between the two samples and the individual color difference can be calculated by the following formula: Total color difference ΔEab=[(ΔL*)2+(Δa* ) 2+(Δb*) 2] 1/2. Since 1976, the International Commission on Illumination (CIE) has continuously recommended new color difference formulas in order to achieve the goal of consistent color difference values ​​with visual results, thus controlling and guiding the color quality in industrial production processes. For the color tolerance North Group, many scientists have done research, the ability to distinguish the human eye is different for different wavelengths. Through a large number of experiments, it has been found that the sensitivity of human vision to color differences at different wavelengths is different.
In this paper, through the measurement of L*, a* and b* values, the color difference calculation results and visual evaluation are compared and analyzed. The latitude and visual consistency of the currently widely used CIE1976L*a*b* color space are compared. Aspects were analyzed.
First, the experiment
1. Experimental equipment, instruments, materials and main parameters
IGT printing suitability instrument C1; electronic balance; X-Rite 530 spectrophotometer; X-RiteColorMaster color matching software; 157gAPP double-sided coated paper; Wansi ink, Hanghua thinner; standard dedicated chromatography; car wash water, cotton cloth; The entire experiment was carried out in an environment with a temperature of about 25 degrees and a humidity of 50% to 60% RH.
Prepress process
According to the theoretical principles of the Chinese color system, considering the influencing factors in the colorimetric colorimetric experiment, the observation conditions used in this experiment are as follows: D65 light source rubber product, 10 degree field of view, reflection angle 45°; observation background environment: proceed When the surface color is colorimetric, it is necessary to avoid the influence of the background color on the test color, and the background color of the experiment is required to be dull, and the reflectance is Y10 ≈ 30% neutral ash. A total of 20 color-sensing observers were selected for the paperback wireless binding line installation volume survey, including 10 males and 10 females. Age 18-25 years old, have some experience and training in color discrimination.
2. Experimental process
Before the color matching, the basic color sample should be made, and the basic data file should be established. Before the color matching, the following preparations should be made: select the ink font to be used, and dilute the agent, and adjust the ink weight ratio to 6.25%, 12.5%, 25%, 50%. Institution/Organization, 75%, 100% ink, proofed at six ink weight ratios. Repeat the above steps, respectively, the experiment of the inks of various colors in four different colors is completed, and the data paper package required for the experiment is measured by X-rite 530 type density meter, and stored in the color matching software to establish a base library.
After the ink data base library is established, the required color is measured on the chromatogram using an X-rite 530 densitometer. According to the hue circle theory, only orange, dark green, lake blue, and yellow green areas are selected for this experiment. There are seven main color blocks in the dark blue area, purple area and blue-green area. Each color patch is published according to the three primary color ink ratios provided by the color matching software, and then samples are prepared based on these ratios, and finally the experimental data is measured by an X-rite 530 densitometer, and the excellent difference is calculated. In this experiment, the color difference between the sample color and the proof color is about 2.0. If the difference is large, the color matching software is used to change the gravure of the data, so that the color difference between the proofed color and the sample color is close to 2.0.
CTP
Second, the experimental results and discussion
Experimental data analysis: After the color matching sample, the average data obtained by measuring three times with an X-rite 3200 densitometer, the color difference is calculated, as shown in Table 1:
Table 1 L*, a*, b* values ​​and ΔEab values ​​of the sample after color matching
It can be seen from Table 1 that the color difference of the 7 color blocks used in the experiment is controlled at about 2.0, and the sample after standard chromatography and color matching under the above observation conditions is observed, that is, 7 pairs of colors, and the statistical observation results are as follows. Table 2 shows:
Table 2 observer perception level
In the experiment, the measurement object is a series of color gradient samples, and its color coverage is more comprehensive. The Chinese Printing Blue Book has a certain representative color. From the experimental results in Table 2, it can be seen that the colors 1, 3, 5, and 7 have visually obvious chromatic aberrations, because the color discrimination ability is highest in the vicinity of the blue region and the orange region, and the color has a slight difference to be felt. However, the colors 2, 4, and 6 are not easily visually perceived to have chromatic aberration, because the color discrimination in the vicinity of the purple area and the green area is very low, and the color is changed a lot to change the color. Therefore, the color tolerance of different colors is different, such as the latitude of the blue part is smaller, the tolerance of the green part is larger, that is to say, the wine packaging, when the same color difference is calculated under the color difference calculation formula, the human eye can see A greater amount of various blues can be seen, and only a few kinds of green can be seen.
It can be seen that the calculation method of the color difference ΔEab is not an ideal calculation method. The uniformity of the color space on CIELab is still not ideal for visual enterprises. The chromatic aberrations in different color gamuts and even different directions are very different. It can correctly reflect the visual effect of color, so in some demanding fields, CIELab color difference formula can not meet the requirements of practical applications.
Government policy and regulation
Third, the conclusion
The visual inhomogeneity of CIELab's color space makes people often make a wrong impression process when observing different colors. It seems that the distance between two colors in space is a measure of their color difference. If this kind of false impression is produced, it will affect the color matching and the accuracy of color copying, and may also add unnecessary work to the color copying technology. In the color space, the human eye discerns the color insensitive area. Although the distance between the coordinate point of the original color and the coordinate point of the copied color is larger and square, the effect of copying may still be better, and in the area where the color is visually sensitive, even two The distance between the colors is relatively close, and the quality of the copy may still be poor. Therefore, the "China Printing Industry Annual Report" cannot be simply defined according to the spatial distance of the two colors in the calculation of the color difference formula. It is necessary to distinguish the color according to the human eye. The ability and color difference formula are further modified and improved.
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