Adjustment of gray balance in print reproduction process

First, the concept of printing gray balance
The copying process of the printed matter is to ensure the gray tone of the original and the reproduction of the tone, color and sharpness. Colorology divides colors into two systems: gray systems, ie, black, white, and gray systems (double colors or triple colors also belong to the gray system); second, color systems, ie, yellow, orange, red, green, and blue. , blue, purple and so on.

The gray system of the three-dimensional color space has no hue or saturation. Only the brightness of the one-dimensional variable is only a white point on the chromaticity diagram. The color system has three attributes, namely, hue, saturation, and lightness. All colors contain these three attributes. These variables are different for each color.

We know that the three primary colors of R, G, and B are the transitions to the white field, and that the primary colors of light are mixed with the same amount and become gray. The three primary colors C, M, and Y of the color material transition to the black field, and the three primary colors are combined and superimposed according to different dot area values, and can completely reproduce different neutral ash. This is known as the three primary colors C, M, and Y in the industry. Neutral gray balance, the three-primary color density to obtain a neutral gray balance is called the equivalent neutral gray density.

The role of gray balance in replication
One of the key technologies of color reproduction is how to correctly reproduce the gray in the original, which is the basis for achieving tone reproduction and color reproduction.

1. Gray balance is a measure of the tonality of prepress image processing, the scale of the dot and ink changes in color reproduction and color printing, and the entire process of printing and printing throughout the entire printing process. This is to realize the color management, standardization, and dataization of print reproduction. The unified standard of quality management is also a common guideline for all processes.

2. Gray balance is also a criterion for checking whether the grey tone of the scanning color separation sheet is correct. It is also the basis for judging and correcting whether or not the original and the printed matter are color cast.

From the perspective of copying technology, the color correction based on gray balance reproduction is the key to determine whether the hue can be reproduced correctly. Only the gray tone in the original is reproduced accurately, and the color reproduction can be accurate. If the proportions of the gray scales of the three primary dot areas of C, M, and Y are inaccurate, then the color reproduction will be biased and it is possible to copy the Xihong Hang color in orange. Therefore, the premise of color reproduction is: First of all, to establish a gray balance curve of C, M, Y three primary colors with different dot area value combinations and their parameter sets, the production is adjusted well before scanning color separation, as the internal set value.

The author summarizes a law in practice. As long as the percentage of the gray balance dot area value established by the scanner, the scanner is correct, and the standard shading color correction amount established on this basis is correct, then the gray tone in the image is adjusted. Can be reproduced, at the same time, all complex colors in the image (trichromatic, composite colors) can also be reproduced, do not need to be adjusted in the scan or in the post-processing of the three-color version of the complex color dot area ratio, because the complex color It belongs to the gray system.

In terms of chromatics, the grey tone refers to neutral gray (pure gray); the second refers to the composite color containing gray, and the composite color is formed by combining three or more colors. Yellow ash, blue, gray, blue ash, red ash and other tens of thousands of color gray, all belong to the gray tone. According to scientists, of the more than 100,000 colors that are discernible, more than 80,000 are ash-containing colors.

Practice has proved that: as long as the power balancer, scanner set the gray balance curve and its dot area ratio is correct. Then, after correct scaling of the black and white fields, the grey tone in the original, as well as the thousand or 10,000 kinds of complex colors contained in the image, can be reproduced. The same color belongs to the gray system and belongs to the category where the scan should be restored.

Some of the original gray, complex color need to be adjusted, the first is to adjust the original tone dark, dark defects, this adjustment should use four-color level curve to do at the same time to deepen or reduce the processing; second is to adjust the original gray tone bias Color, using a monochrome level curve to do correction.

Gray balance dot area combination
1. Definition of printing balance
Printing gray balance refers to the combination of C, M, Y three primary color versions with different dot area percentages to generate neutral grey in printing. Specifically, under certain printability conditions (paper, ink, printing presses, etc.), C, M, and Y primary color editions are overprinted from shallow to deep according to a certain dot area percentage, and different shades of gray are obtained (white. The process of light gray, medium gray, dark gray and black is called printing gray scale.

2. Grey balance
The combination of gray balance means that the neutral gray scale has been scanned and separated. Whether it is the RGB mode or the CNYK mode, it has its own ratio value. In the RGB mode, from the theory of color light addition, R, G, and B lights become gray when mixed in the same amount. As shown in Table 1.

However, in the CMYK mode, C, M, and Y primary inks that are overprinted with the same amount of solid density do not give a neutral black, but a reddish, warmer black that can be overprinted with the same amount of halftone dot area but cannot be changed. Instead of gray, it is reddish and warm gray.

Theoretically speaking, the overprinting of the maximum saturation of the three primary inks should be black, and the same amount of three original inks with different saturation levels should be overprinted, and should show different shades of gray.

Why, then, do the three primary inks have the same amount of overprint but do not get neutral grey and become reddish warm gray? Mainly affected by two factors.

(1) Ink error
Since the three primary color inks themselves have different absorption and reflectance of light, there are color shifts and ash components, that is, components with other ink hue in an ink. Such as: C ink with some of the ingredients of the magenta, M ink with some yellow ingredients, which caused the color cast and gray content. As shown in Figure 1.

The author through the DIC-GRAF-G ink proofing test calculations, the color is the least. Therefore, the combination of gray-balanced three-primary dot area percentages is obtained through experiments based on the characteristics of the ink to be used and printability conditions such as paper. Practice has proved that as long as the printability of ink, paper, etc. is stable, the gray balance dot area percentage can be fixed.

(to be continued)