The black of the past four-color printing did not replace the gray part of the three-color composite, but a black color was superimposed on top of the three primary colors. This kind of printing method can not adapt to the conditions of today's four-color high-speed offset press under wet and wet printing conditions in the case of monochrome or two-color offset printing presses, and the printing speed is not too fast. . In the dark tone area, four inks are overprinted, and the coverage of the total dot area can be as high as 360% or more. In particular, the third and fourth colors of the postprint are printed after the ink of the previous color is still not fixed. When going up, when the printed blanket and the printed sheet are imprinted, the printed ink on the printed sheet is likely to be transferred from the surface of the printed sheet against the direction of the ink transfer to the blanket printed on the latter printed sheet. There was a serious disturbance of the ink in the ink, resulting in contamination of the picture color, which is called reverse transfer. The high coverage of the four color inks is superimposed on today's high-speed four-color wet-humid conditions, which forces people to re-examine the role of the black plate. The under color removal technology is promoted in this case.

First, the background removal definition

The so-called background color does not specifically refer to the triad-colored overprint of black, but also includes the black component contained in the color. From the printing point of view, any color, as long as its brightness is not the highest, or it has a certain degree of darkness, then it must contain a certain black component, the black component is the background color. Any background color that is overprinted by three primary colors is replaced with black, which is the background color removal, expressed as UCR (under color removal).

According to the subtractive color mixing law, any two primary colors of the three primary colors yellow, magenta, and blue are superimposed on one another, and the resulting hue of the intermediate color must be the complementary color of the third primary color. According to the complementary color law, once the third primary color is overprinted with the previous one, it will produce a neutral dark gray. If the dot coverage of the three primary colors is not equal, then three equal trap rates equal to the minimum of the three equal amounts can also be separated. The result of these three equal amounts of synthesis must be a neutral black gray, which is the background color.

Second, the analysis of the color removal

In the case where the three primary dot coverages are not equal, the background color analysis diagram in the color synthesis of the three primary colors is shown in FIG.

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First, the coverage of the dot area of ​​25% for each color version in the three primary colors is equal to the gray. The remaining 65% of the yellow and 55% of the magenta were synthesized orange-red. The synthetic color is equivalent to the superposition of the orange red and 25% of the gray, so the brightness of the orange color of the three-color overprint is not very high.

There are also cases where the coverage of the dot areas of the two primary colors is equal and lower, and the dot coverage of the other primary color is higher, as shown in FIG. The area coverage of yellow and magenta dots is both 50% and 90%. It can be analyzed in this way that 50% of the dot area is also taken from Qingzhong, when yellow, magenta, and blue all form a medium-dark background with 50%, and 40% of the cyan and the background are left. Together, the final result is a dark blue.

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