How to deal with color reproduction neutral gray balance in print copying
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First, the concept of neutral gray balance
The copying work of the printed matter is color restoration, tone reduction, texture restoration, and resolution reduction. The so-called tone is that the color is classified by brightness, the color is three-variable, the neutral gray is a three-dimensional color space, the neutral gray is composed of three colors of Y, M, and C, and the three variables of color (hue, saturation, brightness) Hv/c means that neutral gray has no hue, saturation, and only the brightness of one-dimensional variables.
We know that the three primary colors R, G, and B of the color light transition to the white field, and the three primary colors Y, M, and C of the color material are transition to the black field. In addition, four-color printing is limited by two conditions: one is to use only four colors of Y, M, C, and K; the other is that the ink layers on each sheet are evenly distributed.
Neutral gray refers to a series of colors such as Y, M, and C. They absorb the R, G, and B colors in equal amounts, and only the change in brightness has no change in hue and chroma.
When the three primary colors Y, M, and C of the color material are overprinted in an appropriate ratio, the neutral gray scale can be completely reproduced. Then, the three primary colors of Y, M, and C are said to reach a neutral gray balance. The monochromatic density of the three primary colors that achieve gray balance is called the equivalent neutral gray density.
Second, emphasize the significance of neutral gray balance
1. Copy the gray of the original correctly.
2. At the time of plate making and printing, the neutral gray balance is used as a measure to determine whether the ratio of the three primary colors of ink in color separation and printing is appropriate.
Third, the background color removal and neutral gray balance
1. The concept of background removal. The background color removal (UCR) refers to the removal of a part of the neutral color from the three primary colors of Y, M, and C, that is, the approximate black color formed by the superposition of the three primary colors Y, M, and C, and the black ink. instead.
For example, if a certain color is overprinted by Y, M, and C, the area of Y dot is 80%, the area of M dot is 80%, and the area of C dot is 100% (in the field). If it is balanced according to neutral gray, 80% Y and 80% of M can overlap with 90% of C to form a black ink equivalent to 90% dot area, then:
C = 80% Y + 80% M + 100% C = 80% Y + 80% M + 90% C + 10% C = 90% K + 10% C
2. The necessity of background removal.
1 Improve the printability: Under normal circumstances, it is used in the dark tone. If the three primary colors Y, M, C are used in the dark adjustment area, the dot area is often very high. In high-speed multi-color printing, process problems are easy to occur. If the base color is removed, the amount of ink is greatly reduced, and the printability is improved.
2 Conducive to controlling neutral gray balance during printing.
3 is conducive to the drying of the ink, reducing the amount of overprinting of the three primary color inks of Y, M, C.
4 Because the amount of Y, M, C color ink is reduced, the amount of black ink is increased, thereby reducing the cost, and the price of black ink is relatively cheap.
5 After adopting high-speed multi-color printing machine, the biggest problem affecting the speed and quality improvement of high-speed printing machine is ink drying problem, which means that the speed of ink drying will directly affect the speed and quality of high-speed multi-color printing machine. There are many ways to solve this problem, such as changing the composition and structure of the ink, improving the drying method of the printing machine, and improving the process, improving the concentration and gloss of the black ink, and reducing the Y, M, and C primary color inks. The amount of ink in the non-colored areas, thereby effectively thinning the ink layer to accelerate the drying speed, which is the root cause of UCR in color reproduction.
3. Points to note when removing the background color.
1 Background color removal generally works in intermediate, dark-adjusted neutral gray and near-neutral gray areas.
2 Correct control of the removal amount.
3 Master the control of the removal point, which is to remove the scope of control.
Fourth, the relationship between the depth of the black version and the amount of background removal
The depth of the black plate and the amount of background color removal are closely related, the amount of background color removal is large, and the black plate should be deep; conversely, the amount of background color removal is small, and the black plate should be shallow. At the same time, the length of the black tone should be determined according to the length of the dark density of the original. The dark tone density range is long, the black version should be long, the dark tone density range is short, and the black version should be short. Neutral gray balance is an important condition for color reproduction and restoration of originals. It is also a technique that is difficult to master in plate making and printing, because Y, M, and C three-color inks have a certain difficulty in reproducing neutral gray balance.
5. The main factors affecting the neutral gray balance control of neutral gray balance
1 color characteristics of the ink.
2 The ratio of the three primary colors.
3 printed color sequence.
4 paper performance.
5 process operations, etc.
1. The color characteristics and variation of ink, the control of neutral gray balance, and the factors affecting neutral gray balance, we must first understand the law of color change.
1 Three primary color inks can be mixed in black in a certain ratio.
2 After the three primary color inks are mixed, they are approximated black, and with different proportions of the thinner, then neutral ash of different colors is obtained.
3 The three primary colors are mixed in different proportions to obtain inter-color or complex color of different hue, and the hue is biased to a large hue.
4 The two primary colors of ink are mixed in equal amounts to obtain a standard color.
2. Neutral ash balance requirements for ink are:
1 High transparency: When the ink is applied into a uniform film, the degree of coloration of the underlying color of the substance that withstands the ink film, and the clearer the background color is displayed, the higher the transparency of the ink.
2 fine pigment particles: printing high-grade refined products to fine pigment particles.
3 strong tinting strength: the tinting strength of the ink is an amount indicating the color strength or concentration of the ink. For the ink of the same hue, the tinting strength depends on the dispersion of the pigment on the binder, and the higher the degree of dispersion, the stronger the tinting strength; It also depends on the content of the pigment, and the higher the content, the stronger the coloring power.
4 Lightfastness: Under the action of light, the color of the pigment will change color and fade to varying degrees, which is lightfast.
5 color efficiency is high: color efficiency is a comprehensive indicator of ink color difference and gray level.
6 Chemical stability: The color of the ink will change under the action of some chemicals. Therefore, the ink is required to have good chemical stability, water resistance, acid and alkali resistance, and ethanol resistance.
3. The relationship between paper performance and neutral gray balance. The quality of the printing company's products is greatly affected by the raw material paper. The ideal paper whiteness is 100% reflectivity and the total reflection is 380-700 nm. In the process of plate making and printing, we should consider "different from paper", and use the appropriate process content to adapt to the paper used, especially in the thickness of the dot, the color of the color, the size of the contrast.
High-grade and exquisite offset printing papers include domestic coated paper, offset paper, newspaper and imported coated paper, egg-pattern paper, cloth paper, etc., due to paper tightness, whiteness, paper absorption, smoothness, and flexibility. The curve that produces the reproduction tone reproduction is also different. The level of printing on the coated paper is very rich, and the level of printing with offset paper is relatively dull. The same is the 157g/m2 coated paper. Due to different origins, there is a certain gap between domestic paper and imported paper.
The high-quality paper with low absorption, high whiteness and high smoothness of paper has an ideal reproduction curve, the amount of ink is thin, and the color is still vivid.
4. Printing color sequence and neutral gray relationship. The monochrome offset printing machine adopts the sequential order of Y, M, C, and K. Firstly, the ink with the lowest transparency is printed in the first color, and then the ink with high transparency is printed. This color sequence is arranged according to the transparency of the ink. The size is arranged. At the same time, the viscosity of the ink is also considered. Since the viscosity of the Y ink is small, the back surface of the printed matter can be prevented from being sticky; the light color is placed on the front side, and it is easy to clean the ink roller; it is also convenient for four-color overprinting accuracy. The multi-color offset printing machine adopts the inverted color sequence K, C, M, Y. Since the multi-color machine is wet and wet, the printing ink is overprinted without the oxidized conjunctiva drying, and another color ink is overprinted. The viscosity of the four-color ink is K>C>M>Y, which satisfies the requirements of the four-color machine. The color sequence of the multi-color machine is gradually lowered from high to low, and the viscosity is high. Printing, stacking fastness is better.
In addition, the multi-color machine color sequence arrangement takes into account, the transparency is the lowest, the black ink has the lowest transparency, that is, the darkest, as the outline of the picture is thicker, so it should be placed on the front to make the transparency high. C two colors are placed on the back to make the whole picture vivid.
Different color order arrangements have a greater impact on the neutral gray balance. Different color sequences will directly affect the quality of the printed products and the color tone. In the actual production, comprehensive analysis should be carried out, and some variables involved in the printing process should be considered a little more, and the color sequence should be based on different types. The original manuscript, the customer's request for the printed effect to arrange the color sequence, in order to achieve the perfect reproduction of the printed matter to copy the original.
Usually, the color cast of several inks after overprinting should also be considered: M, C, and Y are easy to be green after overprinting, C, M, and Y are reddish after overprinting, and C, Y, and M are easily reddish after overprinting, M. Y, C is brownish green after overprinting, Y, C, M is superimposed after printing, and Y, M, C are blue-green after overprinting.