Printing gray balance adjustment
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(1) Black version
The ideal color reproduction color, only need Y, M, C three primary color ink overlay, you can combine thousands of colors and different brightness achromatic, this is the three primary color reproduction theory. However, the three primary inks do not produce the ideal neutral gray, which does not reach the required density (shown in Figure 4-4-2). The solid density of the four-color print is higher than that of the three-color print, which is the result of the black version compensation. In the case of multi-color printing, it is in a wet-wet printing state, and when the ink layer is thick, it is not easy to dry, and it is easy to cause a phenomenon such as overprinting. The amount of the dark primary ink must be subtracted, so it is necessary to replace it with black ink.
The black version plays an important role in improving product quality by compensating for ink hue defects and adapting to the needs of multi-color high-speed printing presses.
1 can enhance the density contrast of the image. According to the current printing conditions, the density of the Y, M, and C three-color inks is 1.6 to 1.7, and the visual resolution is generally 1.8 to 1.9. Therefore, the black version of the appropriate tone is used to enhance the absorption capacity of the paper, thereby increasing the total density range of the image, increasing the contrast of the image, the stereoscopic effect, and improving the product quality.
2 Stabilize the color to the dark tone. Theoretically, the yellow, magenta, and cyan inks according to the proportion of the corresponding dot can be superimposed with the required color, but the printing amount is large, the printing conditions are unstable, such as the difference in performance of the paper ink, the variability of the machine precision, etc. The dark adjustment, especially the area of the 70% to 100% dot is limited, so most products have a dark brown color and unstable color. In fact, most of the originals are dark and dark, which is inconsistent with the requirements. The appropriate black version can be used to stabilize the color and overcome the dark color cast.
3 strengthen the middle to the dark level. In color reproduction, the general rule is to maintain a bright tone or a slight emphasis, to maintain a medium tone or a slight compression, a dark tone level compression is more, a medium shadow level has a flat phenomenon, on the other hand, only yellow, goods The red and blue primary colors are superimposed, and the contours of the middle to the dark are imaginary, and the layers cannot be opened. The black outline is clear, the layers are distinct, and the dark details are clearly separated.
4 improve printability and reduce costs. With the advent of high-speed multi-color printing presses, the overprinting time per color is very short, and excessively thick ink brings many failures in printing. Therefore, it is necessary to reduce the amount of color ink in the neutral hair area of the dark portion of the picture, and replace it with black ink, thereby reducing printing failure and improving printability. At the same time, the black ink is inexpensive and the cost is lowered.
5 Solve the problem of text printing and color reproduction. The texts such as picture description, advertisement introduction, pattern label and calendar calendar are all copied in black. From the visual requirements, black characters work best. It is unrealistic to overprint text in three color versions.
(2) Background color removal (UCR)
Subcolor removal, also known as UCR (Under Color Remove), refers to a process in which the amount of ink in the dark-adjusted composite color area is appropriately reduced, and replaced with black ink.
The background removal process results from the requirements of wet-on-wet printing and original dark-level reproduction of modern high-speed printing presses. It has the advantages of facilitating rapid overprinting, saving expensive color inks (ie by removing a certain amount of Y, M, C inks and replacing them with a small amount of black ink) and facilitating neutral gray balance and neutral gray reproduction. .
1. Principle of background removal
In theory, if the ideal Y, M, C inks are mixed in equal amounts, an ideal neutral gray can be formed, that is, the minimum of the three colors of the Y, M, and C color inks is Among them, the ash component is shown in Figure 4-4-3. In other words, the composite color of the three primary color inks of Y, M, and C and the composite color of the two color inks of Y, M, and C have the same visual effect. The reason is that the black ink can correctly reproduce the neutral ash, thereby removing the neutral ink constituting the neutral gray in Y, M, and C and replacing it with the black ink, achieving the same visual effect for the same color reproduction.
With the development of modern desktop prepress systems and image reproduction processes, the background color removal function has been continuously improved and developed, and a new process pattern in which the background color removal and ash replacement processes coexist has been formed. Ash replacement not only completes all the work of background color removal, but also maximizes the use of black ink instead of Y, M, C and other color inks, and maintains the high quality of printed copies.
2. The purpose of background removal
(1) Improve printability and ink dryness. In printing, neutral and color are usually reproduced by Y, M, and C overprinting. In order to compensate for the lack of layer density in the middletone adjustment, black is added. Version. However, for multi-color high-speed printing with fast printing speed today, it is not necessary to wait for the ink to be printed first, and it is necessary to overprint the ink of the next color, so that the post-printing ink cannot be completely transferred to the first printing ink, resulting in no overprinting. The ink is uneven, and even the first printed ink is taken away by the ink printed on the back. They are particularly noticeable in the large dark-adjusted portion of the four-color wet-wet overprinted ink. Today's multi-color printing presses generally require a dark-adjusted portion with good printability. The total area of the overprinted dots must be limited to 250% to 270%. Therefore, in order to meet the requirements of the printing process and improve the above-mentioned undesirable conditions, it is necessary to adopt a ground color removal process to reduce the total ink amount, speed up the ink drying time, and improve the printability.
(2) Reducing production costs The prices of Y, M, C, and BK four-color inks used in printing are not equal. When the background color removal is used, a total of about 50% to 80% of the dots can be removed, and only about 15% of the black plate needs to be added, that is, it can replace about 50% to 80% of the yellow, magenta, and blue overprint. The effect is to save a lot of color ink, and the economic benefit is very obvious for high-volume printing. Generally, a 50% increase in background color removal can reduce ink costs by 10% to 15%.
(3) Color-shift compensation In the original of the color image reproduction, even if the image has a good gray balance from highlight to midtone, it is often out of balance in the dark-tone portion and exhibits a certain color shift. If it is necessary to satisfy the condition that the color composition is constant and the color cast can be compensated, it is very effective to solve the problem by using the ground color method. As shown in Figure 4-4-3, there is a dark reddish original, the dark tone color is composed of Y7, M9, C6. To make the dark tone color removal, the background color can be removed to make Y, If M or C is equal or unequal to remove a part, the color composition can be changed to achieve gray balance reproduction. For example, if the Y, M, and C are removed by an equal amount, the color composition becomes Y5, M7, and C4. Obviously, 7:9:6≠5:7:4, as long as the appropriate ratios of Y, M, and C are determined. Correct the color cast.
(4) Emphasis on solid color background
The removal only acts on the neutral hair area, and thus the background color removal is used to remove the emphasis on the dark color by the background color removal.
Generally, when the solid color is emphasized as a whole, it is better to use the background color removal. If only one or two colors are emphasized, the color correction method is preferred. Figure 4-4-3 also illustrates the removal of the ground color from another angle. Emphasize.
(5) Stabilizing the neutral gray balance In the ordinary color printing, the neutral gray is often reproduced by the Y, M, and C three colors, but there are many different ink concentrations and ink sizes in the printing process. Various color overprints are not allowed to be unstable. When an original that is reproduced as a main content by a gray tone such as a machine or a camera is difficult to copy, it is likely to cause partial color cast. Therefore, an appropriate amount of background color is removed to remove the ink amount of Y, M, and C which are grayed out by Y, M, and C, and the black plate is added to make up for the lack of the base color, so that a satisfactory effect can be obtained, and the neutral gray balance is maintained. stable.