Experimental Analysis of ΔEab Computation of Color Difference and Visual Observation Consistency
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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 in the value of the color difference and the visual difference of the human eye has long been regarded as 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 calculations with visual results, thus controlling and guiding the color quality in industrial production processes. For color latitude, many scientists have done research. For different wavelengths, the human eye has different discriminating power. 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 CIE1976 L*a*b* color space are compared. Analyzed.
First, the experiment
1. Experimental equipment, instruments, materials and main parameters
IGT printing suitability instrument C1; electronic balance; X-Rite 530 spectrophotometer; X-Rite Color Master color matching software; 157gAPP double-sided coated paper; Wansi ink, Hanghua thinner; standard dedicated chromatography; car wash water, cotton Several; the whole experiment was carried out in an environment with a temperature of about 25 degrees and a humidity of 50%-60% RH.
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, 10 degree field of view, reflection angle 45°; observation background environment: surface color When 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 matt, and the reflectance is Y10≈30% neutral gray. Twenty normal observers with color vision were selected, 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 to be used, and dilute the agent, and adjust the ink weight ratio to 6.25%, 12.5%, 25%, 50%, respectively. 75%, 100% ink, were sampled at six ink weight ratios. Repeat the above steps, respectively, the experiments of the inks of various colors in four different colors were completed, and the data required for the experiment was measured with an X-rite 530 densitometer, 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 block is based on the three primary color ink ratios provided by the color matching software, and then samples are prepared based on these ratios. 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 data, so that the color difference between the sampled color and the sample color is close to 2.0.
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 seven 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 shown in Table 2. Shown as follows:
Table 2 Observer perception level
In the experiment, the measurement object is a series of color gradient samples, the color coverage is more comprehensive, and the selected color has a certain representativeness. 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. 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 zone and the green zone is very low, and the color has to be changed a lot to feel the difference in color. Therefore, the color tolerance of different colors is different, for example, the latitude of the blue part is smaller, and the tolerance of the green part is larger, that is, when the same color difference is calculated under the color difference calculation formula, the human eye can see more. The amount of various blues, 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 visually. The chromatic aberrations in different color gamuts and even different directions are very different. Correctly reflect the visual effect of color, so in some demanding areas, CIELab color difference formula can not meet the requirements of practical applications.
Third, the conclusion
The visual inhomogeneity of CIELab's color space often causes people to make false impressions 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 large, the effect of copying may still be better, and in the area where the color is visually sensitive, even two The distance of the color is relatively close, and the quality of the copy may still be poor. Therefore, the calculation of the color difference formula cannot be simply defined according to the spatial distance of the two colors. According to the ability of the human eye to distinguish the color, the color difference formula is further modified. And perfect.