Gray balance control: color management connection between printing and prepress

The color management system is controlled by printing. On the contrary, the printing control can only be obtained if the substrate color, the ink color of the three primary colors, the ink overprint, and other printing variables are relatively stable and balanced.

There is no doubt that tricolor ink printing has made great progress from original painting development to scientific imprinting, from the development of photoengraving to full digital printing. However, printers must also improve the variability of inks and substrates that are directly related to the final printed product. These variability, especially in the variability of containing multiple types of substrates, have a great influence on the color balance. Unbalanced colors will cause overall color casts in printed reproductions, typically when the overall color of a print containing a neutral grey area is color cast. Color imbalances can occur at multiple stages of printing, regardless of when the image is passed to another format. For example, when converting from RGB to CMYK, the output to the proofer and printer (or printer) will result in color changes.

The digital color management system reduces the occurrence of color imbalance in the prepress portion to a greater extent by adjusting the tonal values ​​of different devices. But as far as printing is concerned, color imbalances still prevail, especially paper prints. This article describes the application of three-color color management for gray balance control in printing.

Color transfer To understand how to control color cast, you must first look at the color transfer system. In characterizing the copying process and its subsequent printing process, two important operators determine the color balance of the entire tonal range of the three primary colors. One is the printing operator, who decides the control of the ink imprint sequence, the embossing value, and the color density of the three primary colors; the other is the prepress operator, who sets the percentage of ink in the imprinting color of the halftone area in the printing, such as high-profile and intermediate tone. setting.

The overall color balance of a printed copy is determined by the intensity (density) and color (hue and saturation) of the three primary colors. Only when these printing factors remain stable can the mixing percentage of color dots in printing be determined. In the printing and printing and pre-press color transfer and copying, if there is no strict control, the quality control system can not guarantee the quality of the print. Practice shows that controlling ink density and gray balance through experience is one of the most important parts in color management for halftone color printing.

Gray balance control is not a new concept. Before the computer era, color separations based on photography were established, and standard SWOP density guidelines have been established. However, there have been several things that have changed. One is that flexographic printing has occupied an important seat in the printing of three primary colors. Second, due to the appearance of new ink pigment technologies, the basic ink primary colors have also undergone great changes. Moreover, digital artwork has emerged.
At a time when the new technology is excessive, although the talk about gray balance control seems to have lost its significance, to emphasize it, we must make efforts to make color management more effective.

The color management system is controlled by printing. On the contrary, the printing control can only be obtained if the substrate color, the ink color of the three primary colors, the ink overprint, and other printing variables are relatively stable and balanced. In fact, the balance of these important parameters will change constantly with the printing, such as to match the changing basic materials, inks and substrates. Prepress artists use all the controls, color management and measurement systems, but there is still no guarantee that color-balanced halftone products will be obtained. The reason for this is that small changes in the color of the substrate, the opacity of the ink, or the overprinting are not detected by the printer.

The gray balance and trichromatic inks are overprinted in the print. The three primary colors of the ink are mixed in different proportions to obtain the desired color spectral tone values. The correctness and balance of the three primary inks printed on the substrate are directly related to the printer. As a guide to obtaining the correct ink contrast, the density balance is to be operated in accordance with FIRST (Flexographic Image Reproduction Specifications and Tolerances).

Printing operators can only measure and set ink densities within a limited range. These ranges are described in various publications such as FIRST and SWOP. However, these density ranges allow for a wide range of overprinted ink colors regardless of whether the ink is pure and transparent, or whether the substrate is pure white. In general, the actual application of the prepress color management system is not ideal for the control of printing color.

The ink density range according to the instructions can be used for any ink overprint sequence, ie YMC, YCM, CMY, CYM, MCY or MYC. But does each order produce the same color? No.

Even if the density of the field is controlled to an absolute value, each overprint sequence will produce different overprint colors of red, green, and blue, because each printing ink formulation has its own different properties, such as the opacity of the ink that affects the hiding rate. Therefore, the surface energy of the previously printed ink dried film layer will affect the adhesion characteristics of the later printed wet ink.

The yellow pigments have poor coverage, and the magenta and cyan pigments have good coverage. However, the yellow pigment ink has a high surface energy.

The first step in color management in printing is to determine the best ink overprint sequence and density balance that can provide a high color rendering range. The GATF association developed a simple color hexagonal color map many years ago to visualize the ratio of different colors. Densitometers can display color differences by measuring hue deviation, grayscale, and overprint colors and compare them with the three primary colors.

For example, in the three corners of the hexagonal color diagram, the ideal three-primary ink (YMC) is on the three corners, and the overprinted composite color (RGB) is on the other three corners. The center of the hexagon is the neutral color (white to gray). To black). The greater the color strength of the ink, the closer it is to the corners and the farther away from the neutral color.

When the actual printing color and the composite color are distributed to the hexagonal color map, the density, hue difference, and gray value of the three primary color inks (CMY) and the composite color (RGB) are presented. Because the ink is not ideal, the distributed hexagons (irregularities) will not be the same size as the entire ideal hexagonal color map, but it can be used as an optimized print. Therefore, the use of the GAFT method for irregular hexagonal distribution to display the three primary colors and composite color information, can be the best density and overprinting sequence ⊥ü饬垦     虼笮 虼笮 虼笮 虼笮 龆 龆 龆 龆 龆 龆 龆湍 湍  芎 芎 ∷ ∷ (13)   员   慷龋 慷龋 慷龋 慷龋 慷龋 蛟 蛟 蛟 蛟 蛟 蛟 笤 笤 笤 笤?

Other density values ​​can also be distributed as part of the measurement sequence, such as the 25% dot, 505 dot, and 755 dot density values ​​of the color. If the distribution of these other hue values ​​is concentrated, it means that printing is under control. On the contrary, if the distribution is not concentrated, it means that printing is out of control and necessary adjustments should be made.