Gray balance control issues throughout the print workflow
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The process of producing a visual neutral gray scale from three printed primary inks involves the problem of gray balance, which can be confirmed by film and proof during the color separation process. In the process of proofing and printing with reference to the SWOP density standard, the dot sizes of the three printed primary colors of the printed gray are usually different, and the cyan dots are usually larger than the dots of the yellow and the color. In the proofing and printing production process, the dot enlargement must be controlled to maintain the same dot size as the original film to maintain the gray balance.
The GATF/SWOP proof control strip has the following values:
Gray balance
Black cyan color yellow
75% 75% 63% 63%
50% 50% 39% 39%
25% 25% 16% 16%
SWOP proofing test strip
Total Area Coverage (TAC)
This parameter specification specifies the total percentage of cyan, color, yellow, and black in the black areas of the printed image. The theoretically darkest area should be printed with a maximum total ink volume of 400%. However, in four-color wet overprinting, this can cause problems in the actual production operation of the printing press. The main problems will be the following:
1. Inverse Transfer of Ink - The ink is transferred from the printed web to the next printing unit of the inking system, contaminating the ink on these printing units.
2. Drying problem of the ink - The temperature in the drying device is unusually high, a large amount of solvent is evaporated, and a large amount of moisture in the paper may be removed, resulting in foaming of the surface of the paper.
3. Difficulties in Overprinting - The first printed ink refuses to print other inks on its top, so that the printed ink cannot be properly transferred. The post-printed ink can be transferred to the surface of the unprinted paper and cannot be transferred to the surface that has been inked. The greater the coverage of the ink, the more pronounced the problem caused by poor ink overprinting.
According to the printing industry, the maximum TAC value that can be printed in the half-tone area is 300%, and only one color can be printed on the ground. Black objects can be successfully copied under such conditions. In some small image areas with no important details, a slightly higher percentage of TAC may also be acceptable. But in this case, even small areas (often unimportant) have TAC values below 325%. The TAC value should be measured in the heaviest (ie, the darkest) area of the film or electronic file, and each color should be read under the same actual conditions. The measurement points taken by the film inspectors of the publisher and their measurement methods should be determined.
Gray Component Replacement (GCR) and UnderColor Removal (UCR)
In the traditional three-color process, to copy a saturated color, the chromaticity is often produced by two main subtractive colors, and the gradation is increased by the third color. Although this method of color separation has been quite successful, there are still some inherent difficulties. First, if the gray component is not precisely controlled on the press, the color will be out of balance. Second, when copying complex colors such as brown, purple, magenta, flesh and gray, any slight change in the primary color of the print will cause some obvious changes. So if there are conflicting colors in the online object of the same press, obtaining the necessary color balance in one area will result in a color shift in the other area.
GCR is a color replacement technique that replaces some or all of the cyan, magenta, and yellow colors in the color separation process with gray. Therefore, the GCR process does not need to rely on the third main color to make the color gray or black, but mainly through the black ink to form the general outline and partial details of the image, which greatly reduces the possibility of color change. Smaller ink coverage also means smaller problems, and variations in black printing units can only cause a hue shift, which is much better than a change in chromaticity due to other changes in the ink. Therefore, the black ink has a function of controlling color stability. For example, a black ink may be used instead of the cyan ink of the magenta color region, the typographic ink of the green region, and the yellow ink of the purple region.
Traditionally, prepress service providers often use UCR to reduce the total amount of ink printed in neutral shaded areas. UCR has the least effect on grayscale and shadow depth, and has a greater influence on the second color.
The recommended safety range for GCR is now between 50% and 80%. The 50% GCR setting removes 50% of the ash component typically printed by color ink and adds an equal amount of black ink compensation. When using GCR of 80% or higher, prepress service providers and print customers must pay special attention to the following points: When using these GCR grades, the percentage of colored dots on the film is relatively small, which limits the color during editing. variation range. At the same time, the higher UCA also reduces the gloss of dark shadows and black areas, reducing the thickness and reducing the detail. UCA increases the amount of color in the black area of the graphic.
SWOP recommends the minimum TAC value used in the neutral black area (which can be used as a function of the dot value of the black printing unit) as follows:
The minimum TAC value of the GCR.
The percentage of dots in the black printing unit The minimum coverage in the tri-color area (must be neutral gray).