Printing Quality Control--Objective Evaluation (II)

There are three different concepts in the reproduction of colors: First, the color reproduction in the physical sense requires that the reproduced colors be exactly the same as the spectral distribution of the original colors on each color point. However, the printed matter is for visual viewing, and it is difficult to achieve and it is not necessary to achieve the same-spectrum color reproduction in the physical sense. The second is the reproduction of the sense of color, so that the printed reproduction image is the same as or close to the color point of the original, ie, the effect of different spectrum and the same color. This is the measure standard of objectively evaluating the color reproduction. The third is psychological reproduction of color. That is, the color of the print reproduction may be somewhat different from the color of the original in terms of chromaticity, but it may meet the visual psychological satisfaction in the color effect, and subjective evaluation factors are added here.

Due to the lack of color performance characteristics of printing materials (paper, ink, etc.), imperfections of separation and reproduction of color separation equipment, and the lack of color representation of the printing reproduction method itself, the actual printing science and technology cannot faithfully reproduce originals. Or all the colors of the original scene, even if they are reproducible parts, can not achieve the degree of faithful restoration, can only be relatively close. This brings certain difficulties to the objective evaluation of print reproduction quality. It can only be compared from the results of colorimetric measurements on the degree of closeness of the print to the original or original scene. Between similarity and dissimilarity, people's psychological psychological requirements of color vision are reconstructed to make a comprehensive and comprehensive evaluation of print color reproduction.

If the objective color scale of the original color is to be set in terms of the degree of closeness of the reproduction of the original color, it should include: measurement and verification of the color reproduction range of the printing ink, measurement and verification of the gray balance reproduction, and absolute color reproduction accuracy of the original color. The metric test, as well as the calculation of the relative degree of reproduction measurements.

1. Grey balance reproduction measurement evaluation. Gray print reproduction is a prerequisite for determining whether color reproduction can be accurately reproduced. Whether the neutral grey level of the original or original scene is reproduced in the neutral grey on the print screen, or whether the ratio of the tri-color gray balance dots on which the plate is based has reached the gray balance reproduction, and the three original-color version of the plate-making network The ratio and density of the printing ink and the control of dot gain data are decisive. Gray balance reproduction is the main objective technical standard to measure the overall color tone and evaluation color of the print screen.

To measure the gray balance printing reproduction, the color density meter can be used to measure the three-color neutral gray ladder attached to the printed matter, or the three-color optical density in the measurement screen should be a neutral gray level, and the three-color optical density is equal as an objective technical evaluation standard. If observation is used instead of judging, it is necessary to compare the gray level in the print screen with a standard neutral gray block under the condition that the light source and ambient color temperature are very standard.

If the overall tone of the manuscript is correct, the neutral gray level is for faithful reproduction and print reproduction. The best measurement is based on a three-color gray scale with a copy of the original (or the same proportion of data) for color separation copying and printing. The three-color ink dots are printed on the steps. If the three-color optical density is equal, then The sexual gray reproduction is accurate. If there is a high-density light, it means that the entire printing screen is biased toward its opposite color.

If gray balance scales or test strips cannot be set on the printing plate, and the gray level of the original document requires faithful reproduction and reproduction, you can also select the level closest to gray (not necessarily the same as the neutral gray of the three-color optical density) on the original document. The three-color density, if the three-color optical density is not completely equal, can calculate the percentage of the optical density of a certain color in the three-color average density. Then, to measure the three-color optical density at the same level on the printed matter (the parameters of the color filters of the two densitometers must be the same), the percentage of the color cast density in the three-color average density is also calculated and compared with the original. The grey balance reproduction status of the print can be determined. However, in the printed products, due to the black cover, it is not as accurate as the measurement of the three-color gray scale.

If the original itself has a color cast, gray balance adjustments have been made during copying. To verify the accuracy of the reproduction of the color balance and gray balance reproduction, only the gray point of the original scene should be selected in the print screen. Measuring the three-color optical density, according to the error of the three-color optical density data, determine the accuracy of the second balance adjustment to reproduce and reproduce.

When the gray balance error occurs, it can be along the three-color ink layer in the field density, three-color dot printing increase, printing color sequence, three-color ink stacking printing efficiency, three-color dots in the middle process evolution, and electronic color separation screening gray Balance the data and such clues and order, and find out where the deviations are.

2. Color reproduction evaluation. The color characteristics of the printing ink and the density of the ink layer that can be achieved by the printing restrict the color rendering range of the printing screen, and also restricts the difference or closeness between the printing reproduction color and the original color. To make a more objective color re-evaluation of the product, it is necessary to first measure the chromaticity characteristics of the three-color ink and the density of the printing ink layer, and paint the color rendering range that it can achieve. If its color rendering range is large, the color of its print screen may be close to reproducing the original color. If the color rendering range is small, the difference between the color of the printed picture and the original color must be large, and the color reproduction error is also large. Also, through the test rendering of the color rendering range, it is possible to predict the degree of reproduction of some typical colors on a print, as well as overprinting color effects, gloss effects, and the like. There is a gap in the actual measured ink layer color, there is a preliminary objective evaluation criteria.