（1） Black plate effect

        The ideal color reproduction only needs the superposition of Y, m and C primary color inks to combine thousands of colors and achromatic with different brightness. This is the three primary color reproduction theory. However, the three primary color ink can not print the ideal neutral gray and can not reach the required density (shown in Figure 4-4-2). Printing Ash Balance Adjustment

      The field density of four-color printing is higher than that of three-color printing, which is due to the use of black plate compensation. In multi-color printing, it is in the state of wet pressing and wet printing.Printing Ash Balance Adjustment

       When the ink layer is thick, it is not easy to dry, and it is easy to produce poor overprint. The amount of dark tone three primary color ink must be subtracted, so it needs to be replaced with black ink.Printing Ash Balance Adjustment

      Black plate is used to compensate ink hue defects and meet the needs of multi-color high-speed printing press, so as to make printing go smoothly, which plays an important role in improving product quality.Printing Ash Balance Adjustment

① It can enhance the density contrast of the image. According to the current conventional printing conditions, the density after the superposition of Y, m and C inks is 1.6 ~ 1.7, while the visual resolution can generally reach 1.8 ~ 1.9.Printing Ash Balance Adjustment

       Therefore, the black plate with appropriate tone is selected to strengthen the absorption capacity of the paper, so as to increase the total density range of the image, increase the dark tone contrast and three-dimensional sense of the image, and improve the product quality.Printing Ash Balance Adjustment

2. Stabilize the tone that is transferred to dark tone. Theoretically, yellow, magenta and green ink can be superimposed on the required color according to the proportion of the corresponding outlets. Printing Ash Balance Adjustment

       However, the printing volume is large, the printing conditions are not stable, such as the difference of paper ink and other properties, and the variability of machine precision. Printing Ash Balance Adjustment

       The adjustment to the area of the dark tone, especially 70% to 100% dots, is restricted. Therefore, most of the products are darkened to tan, and the color is not stable.Printing Ash Balance Adjustment

         In fact, most of the originals are dark and cold, which is inconsistent with the requirements. Appropriate black plate superposition can stabilize the color and overcome the dark and color deviation.Printing Ash Balance Adjustment

      Third, we should strengthen the mediation level. In color reproduction, the general rule is to maintain bright or slight emphasis, keep the middle tone or slightly compressed, the level of the dark tone is compressed to many, and the level of the dark tone has a flat phenomenon.Printing Ash Balance Adjustment

        On the other hand, only the yellow, magenta, and green primary colors are superimposed, and the outline of the middle to dark tone is void, and the layers can not be pulled out.Printing Ash Balance Adjustment

④ Improve printability and reduce cost. With the emergence of high-speed multi-color printing press, the overprint time of each color is very short, and too thick ink brings many faults to printing.Printing Ash Balance Adjustment

        Therefore, it is necessary to reduce the amount of color ink in the neutral hair area of the dark tone part of the picture and replace it with black ink, so as to reduce printing faults and improve printing suitability. At the same time, the price of black ink is low and the cost is reduced.Printing Ash Balance Adjustment

⑤ Solve the problem of developing text printing and color reproduction to both pictures and texts, such as picture description, advertising introduction, pattern label and calendar. Printing Ash Balance Adjustment

       The text of calendar shall be copied in black. From the visual requirements, black characters have the best effect. It is unrealistic to overprint words with three color plates.Printing Ash Balance Adjustment

（2） Background color removal (UCR)

Background color removal, also known as UCR (under color remove), refers to a process of appropriately reducing the amount of ink in the dark tone composite color area of printed matter and replacing it with black ink.Printing Ash Balance Adjustment

The background color removal process arises from the requirements of wet pressing and wet printing of modern high-speed printing press and the reproduction of dark tone levels of the original.Printing Ash Balance Adjustment

        It has the advantages of rapid overprint, saving expensive color ink (i.e. removing a certain amount of Y, m and C ink and replacing it with a small amount of black ink) and neutral gray balance and neutral gray reproduction.Printing Ash Balance Adjustment

1. Principle of background color removal

Theoretically, if the ideal y, m and C inks are dissolved in the same amount, an ideal neutral gray can be formed, that is, the minimum of the three colors composed of Y, m and C inks is the neutral gray component, as shown in figure 4-4-3.Printing Ash Balance Adjustment

        In other words, the composite color overprinted by the three primary color inks of Y, m and C has the same visual effect as the composite color overprinted by the two color inks of Y, m and C and black ink.Printing Ash Balance Adjustment

        The reason is that black ink can correctly reproduce neutral gray, so as to remove the color ink constituting neutral gray in Y, m and C and replace it with black ink, so as to achieve the same visual effect for the reproduction of the same color.Printing Ash Balance Adjustment

With the development of modern desktop prepress system and image reproduction technology, the background color removal function has been continuously improved and developed, and a new process pattern of coexistence of background color removal and ash replacement process has been formed.

         Ash replacement can not only complete all the work of background color removal, but also replace y, m, C and other color inks with black ink to the greatest extent, and maintain the high quality of printed copies.Printing Ash Balance Adjustment

2. Purpose of background color removal

(1) Improve the printability and the dryness of the ink. In printing, the neutral and color are usually reproduced through y, m and C overprint. In order to compensate for the lack of level density from the middle to the dark part, the black plate is added. Printing Ash Balance Adjustment

       However, for today’s multi-color high-speed printing with fast printing speed, the ink of the next color must be overprinted before the ink printed first is dry, Printing Ash Balance Adjustment

      resulting in that the post printing ink cannot be completely transferred to the pre printing ink, resulting in inaccurate overprint, uneven ink color, and even the ink printed first is taken away by the ink printed later, Printing Ash Balance Adjustment

       They are especially obvious in the dark part with a large amount of superimposed ink in four-color wet press wet overprint. Today’s multi-color printing presses generally require dark tone parts with good printing adaptability, and the total area of overprint dots must be limited to 250% ~ 270%.Printing Ash Balance Adjustment

       Therefore, in order to meet the requirements of the printing process and improve the above adverse conditions, the background color removal process must be used to reduce the overall amount of ink, speed up the ink drying time and improve the printability.Printing Ash Balance Adjustment

(2) Reduce production costs. The prices of Y, m, C and BK four-color inks used in printing vary. When the background color is removed, a total of about 50% ~ 80% of the dots can be removed, and only about 15% of the black plate needs to be added, which can replace about 50% ~ 80% of the yellow,Printing Ash Balance Adjustment

        magenta and cyan printing effect, so as to save a lot of color ink. For mass printing, its economic benefit is very obvious. Generally, 50% more background color removal can reduce the ink cost by 10% ~ 15%.Printing Ash Balance Adjustment

(3) Color deviation compensation in the original copy of color image, even if the gray balance from highlight to middle tone is good, it often loses balance in the dark tone part and presents a certain color deviation.Printing Ash Balance Adjustment

        If it is necessary to meet the conditions that the color composition remains unchanged and the color deviation can be compensated, the method of removing the background color is very effective to solve this problem.Printing Ash Balance Adjustment

       As shown in figure 4-4-3, there is an original with dark tone and partial red color. The composition of dark tone color is Y7, M9 and C6. To remove the dark tone and partial red color, the background color can be removed, and the color composition can be changed to achieve gray balance reproduction. Printing Ash Balance Adjustment

      For example, if 2 of Y, m and C are removed in the same amount, the color composition becomes Y5, M7 and C4. Obviously, 7:9:6 ≠ 5:7:4. The color deviation can be corrected as long as the appropriate proportion composition of Y, m and C is determined.Printing Ash Balance Adjustment

(4) Accent solid background

      Removal only affects the neutral hair area, so changing the setting of dark tone through background color removal can achieve the purpose of using background color removal to emphasize solid color.Printing Ash Balance Adjustment

      Generally, when emphasizing the solid color as a whole, it is better to use the background color removal. If only one or two colors are emphasized, it is better to use the color correction method. Figure 4-4-3 also illustrates the emphasis of the background color removal on the solid color from another angle.Printing Ash Balance Adjustment

(5) Keep the balance of neutral gray stable. In ordinary color printing, y, m and C are often used to reproduce neutral gray. However, there are many unstable factors in the printing process, such as different ink concentration, different ink volume, inaccurate overprint of various colors and so on. Printing Ash Balance Adjustment

       When the original with gray tone reproduction such as machines and cameras as the main content, it is difficult to copy and easy to cause local color deviation. Printing Ash Balance Adjustment

        Therefore, by removing an appropriate amount of background color, removing the ink amount of Y, m and C superimposed with gray from y, m and C, and increasing the black plate to make up for the lack of background color, we can obtain satisfactory results and keep the neutral gray balance stable.Printing Ash Balance Adjustment