Printing Quality Control--Objective Evaluation (4)

3. Color reproduction faithfulness measurement evaluation. The evaluation of the color reproduction reproduction of a print screen is a complicated issue. Copying is based on manuscripts, and its color reproduction should be faithful to the manuscript. The color manuscript printing that satisfies the color standard or the main body of the image is generally required to be as faithful as possible to the original color. If only the specific color is used to evaluate the faithfulness of the printed matter to the color reproduction of the original, the instrument can measure the color metrics of the same color point on the original and the printed matter, calculate the color difference, and measure the degree of difference between the two colors by an absolute value. Comprehensive evaluation of print color reproduction provides a reference.

There are two types of commonly used measuring instruments. The first type is spectrophotometer or colorimeter. During the measurement, the original color points of the original and the printing screen are respectively measured by their respective tristimulus values, their respective chromaticity coordinates are converted, and their respective color point positions are marked on the CIE chromaticity diagram, and the distance between the two color points is That means the degree of difference between the two colors.

In order to obtain a visual color difference between the two colors, it is best to use the CIE 1976 (L*a*b*) uniform color space for the three-dimensional spatial coloration. L represents the metric brightness value of the color, and a and b represent the metric value. According to the tristimulus values ​​measured for the two color samples, the respective chromaticity coordinates and the brightness coordinates are converted. From the difference of the chromaticity and the brightness coordinates of the two color samples, the difference ΔL in illuminance and the difference in chromaticity can be obtained. Δa, Δb. Use the formula again:

ΔE=(ΔL2+Δa2+Δb2>1/2

Calculating the total color difference ΔE of the two color samples in the CIE1976 (L*a*b*) uniform color space can accurately represent the visual color difference of the two colors in three-dimensional space.

Although the method of measuring and calculating the color difference is very accurate, both the instrument and the measurement calculation are complicated.

The second category is colorimetric colorimeters. Measuring the color difference data of different colors is a direct, simple and practical color measurement method.

The colorimetric colorimeter can directly measure the tristimulus values ​​X, Y, Z and its chromaticity coordinates of the color sample. Through the microcomputer processing, the color difference value ΔE of the two color samples can be quantitatively given, and two color samples can be accurately expressed. The color difference in the three-dimensional space and the visual color difference.

The color difference meter can also intuitively indicate the difference data of the two colors in hue, saturation and brightness. The same color difference is expressed in absolute quantity, which is more accurate and specific than the value measured by the density meter. The color correction data can be indicated from a specific angle, effectively replacing the human eye function, and making up for the inability of the human eye to consistently remember the defects of specific color-accurate data. Through the input of color tristimulus values, the color difference meter can also give several alternative color schemes.

Comparative evaluation of the relative reproduction of color reproduction: Due to defects in the color characteristics of the ink and the limited reproduction color density of printed light, the ink color gamut diagram has shown that not only a large part of the high-purity original color cannot be reproduced on the print. It is its reproducible part of the color, except that color and color can faithfully reproduce and reproduce, its color saturation and brightness are difficult to faithfully reproduce and reproduce at the same time. Therefore, it is difficult to realistically evaluate the exact balance of printed color reproduction and the degree of relative reproduction, by measuring the color metric difference and the absolute color difference between the printed color and the original color. Because absolute color difference is inevitable.

The color printing screen cannot completely faithfully restore the color of the original, and it is feasible that the color of the original is nearly reproduced. Even if the color and the brightness of the reproduced color are slightly off (the hue should not deviate), the color effect can be achieved visual psychological satisfaction, it is a good color print. For this reason, establishing a relative standard for color reproduction and reproduction can more realistically evaluate the relative loyalty degree of the printed matter to the color reproduction of the original.

The establishment of relative standards should be based on the description of the scope of the color gamut of the paper and ink, and the standard printing density as a precondition. Based on this, the accuracy of the comprehensive balance of color separation and color reproduction and printing reproduction should be measured.

Compared with the standard, the principle of three-dimensional color of Munsell color can be imitated, and the single cone model can be used for color marking, as shown in Figure 6-21. The bottom surface of the cone can be divided into circles as shown in Figure 6-21, indicating the hue of colors, and the six halves represent three primary colors Y, M, C and secondary colors R, G, and B, respectively. The circle radius indicates the color purity, the color purity on the circumference is 100%, the center of the circle is black (K), the color purity is 0%, and the gray scale is 100%. The vertical center axis of the cone indicates the percentage of “degree of collapse” of the color, the vertex W is white, the degree of blush is 0, and the bottom center circle K is 100%.

The color measurement calculation, plotting and comparison measurement steps are as follows:

Select the same color level points on the original and print screens to be compared and evaluated, single primary color, secondary color, or mixed polychromatic color. A color transmission densitometer was used to measure the three-color optical density of the original color point, and the highest density D and the lowest density D0 of the original were measured. For example, a mixed blue-gray point has a green density of 1.2, a red density of 2.0, a blue density of 0.3, a maximum density of the original document D of 3.0, and a minimum density D0 of 0.25. Substituting the corresponding density value into the color shift formula to obtain the hue deviation as:

Color shift=M-L/H-L×100%=1.2-0.3/2.0-0.3×100%≈253%

Substituting the gray formula, find:

Gray level=L/H×100%=0.3/2.0×100%=15%

The degree of fall of this color point in the original tone is:

Dropping degree=H-D0D-D0×100%=2.0-0.25/3.0-0.25×100%≈64%

At this time, the single-cone model in Figure 6-21 can be used to determine the hue of its hue on the circumference of the single-cone model and determine its hue on the circumference of the point S0 by its gray value (complementary to the color purity) on the radius KS. To determine the point N, the color point should be on the hypotenuse WN that connects the vertex W of the brightness axis with the N point. Then on the brightness axis WK, from the top to the bottom, the point F is determined according to the percentage of the percentage of the polish, FE ∥ KS, and the hemline WN at point E. The point E is the color of the original color point in the cone model. Relative position in space.