Research on Printing Grey Balance Control Law (I)
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Summary
The improvement of print quality requires the establishment of a concept of data, in order to control the stability of the reproduction. The measurement of gray balance is often an important basis for judging whether a replica is color cast, so gray balance dataization can improve the quality and stability of printed matter. The gray balance control law of data is discussed in this study.
I. Introduction
Gray balance is an important basis for judging copying when performing color reproduction. After the color reproduction, in terms of the concentration meter, the ink amounts of the three primary colors are equal to each other, and the neutral gray is achieved. Neutral gray produces a color cast that is easy to detect because people are particularly sensitive to gray. When the original is copied, his neutral color is the same as the neutral color of the copy, so the color balance of the color reproduction can be considered correct. Therefore, the color copying person regards the gray portion as a reference for whether the color cast and the print quality are used for color overprinting.
Since the yellow, magenta, and cyan inks contain motley colors other than themselves, it is impossible to overprint the ink at the same concentration to get the concentration you want. Therefore, in order to prevent the color change of the printed matter, it is necessary to find the respective concentrations of the appropriate three primary colors to achieve the gray balance. In the color reproduction process, the main control work of the gray balance is to control the area of the dots of the three primary colors, rather than the thickness of the ink. The gray color printed by the three primary colors of different sizes is required to balance the gray color. The gray color block is measured by the concentration, and when the concentration values of clear, magenta, and yellow are the same, the gray balance is reached.
Because there are many different inks in a printing plant, the dot size of the three primary colors combined by different inks may be different, not to mention the errors produced in different environments or under different requirements and conditions. The factors affecting the gray balance include the amount of ink, paper, dot area, , , , , and so on, and the balance value needs to be applied to the CIE uniform color space and the color difference formula. Therefore, this paper hopes to sort out some methods for obtaining the gray balance concentration value, describe these methods in detail, compare the advantages and disadvantages of these methods, and the applicable environment, and then obtain the rule of gray balance control method.
Second, the theoretical basis
The theoretical basis is mainly to facilitate the reader to understand the internal text, so it is divided into two points. First, the noun explanation 2. CIE uniform color space and color difference formula
First, the term explanation
(1). Gray Balance (Gray Balance)
After the color reproduction, in terms of the concentration meter, the ink amounts of the three primary colors are equal to each other, and the neutral gray is achieved. In general, a black version will be added to increase the level of the dark part.
(2). Chroma (Chroma)
The color has a neutral color component. The higher the value, the farther away from the neutral color; the lower the value, the closer to the neutral color.
(3). Colorimetry
According to the human eye visual sensing, a science of color measurement is a comprehensive science involving cross-disciplinary fields such as physical optics, visual physiology, visual psychology, and psychophysics.
(4). CIE color system (CIE International Lighting Commission Commission International del' Eclairage)
The stimuli of color and the feeling of color are caused by a source, an attenuator, and a detector.
These three elements form the basic structure of the CIE color system.
(5). CIE standard illuminants (illuminants)
From the standpoint of spectral energy distribution, it does not necessarily make a physical light source for human visual observation and color measurement.
(6). CIE standard light source (sources)
The CIE standard illuminator is made from a CIE standard illuminator to simulate its relative spectral energy to correct the CIE standard source.
Second, CIE uniform color space and color difference formula:
(1). CIELAB color space:
CIE uses the "1976 L*a*b* color space" referred to as CIELAB.
a*: stands for red and green coordinates, with red at the timing and green at the negative.
b*: stands for the yellow-blue coordinate, the timing is yellow, and the negative is blue.
L*: is the brightness value, 0 is the ideal black, and 10 is the ideal white.
(B). CIELUV color space:
CIE recommends a second uniform color space, "CIE 1976 L*u*v* color space" referred to as CIELUV. u*: is a red and green coordinate, with red at the timing and green at the negative.
v*: yellow-blue coordinates, yellow with timing and blue with negative.
L*: Brightness.
(3).CIELCH color space
Deriving other representations from CIELAB and CIELUV, called CIELCH (L* C* h°)
C*: for chroma
h°: is the hue angle, representing the hue
(4). CIELAB color difference formula:
(5). CIELUV color difference formula:
Third, the factors affecting the gray balance
1. The amount of ink:
The yellow, magenta, and cyan inks all contain motley colors other than themselves, so it is not possible to overprint the ink at the same concentration to get the concentration you want. This is the impurity property of the ink. The change in the amount of ink affects the density of the dots, so the amount of ink on the layout needs to be managed, and the method of managing the amount of ink is managed by printing the full plate density.
2. Paper:
Because the surface material of the printed matter will be different, the percentage of dots on the printed matter will change, and usually the dot will expand. Because light is transmitted through the paper, it is affected by four factors:
(1). Absorption of ink layers
(2). First surface reflectance
(3). Paper reflection (paper reflectance)
(4). Multiple internal reflectance
3. Full Ink Density-SID:
The so-called full-page concentration refers to "the highest concentration of paper coloring materials". That is, the R, G, B three color filters are used to measure the highest concentration of C, M, Y ink. When the full plate concentration exceeds a range, the dot gain is more powerful and the print contrast is lower. This will result in a reduction in the picture level, which will make the picture lack stereoscopic.
Ds=-logRs Rs=10-Ds
Ds: full version concentration
Dr: Reflectivity of full-print ink
Rs: reflection concentration
Dr=-log(R)
When printing, we need to control the full-page concentration. In the process of printing, if CMYK's full-page concentration is not set, then one of the four colors will produce a concentration difference, which will cause errors in the finished product. The printing factory sets a fixed set of four primary colors to effectively control the gray balance.
4. Dot Area-DA
(1). Film dot area (Film Dot Area-FDA)
FDA=1-Tt Tt: Transmitter transmittance of flat screen Dt: integrated concentration of flat net film
(2) Effective Effective Area (EDA)
EDA=FDA+(ODG+PDG)
ODG=Optical Dot Gain
PDG=Physical Dot Gain
(3). Actual dot area (PDA) (Practical Dot Area-PDA)
PDA=FDA+PDG
When color separation is made, the area of the appropriate three-color dot is determined, and the appropriate dot area on the overlay varies depending on the printing conditions. Ps: 尔尔尼尔 equation a = PDA, Murray Davis equation a = EDA
5. Overprint:
The gray color of the three primary colors of yellow, magenta and cyan in the subtractive method cannot match the original, and the dark part lacks contrast. The black ink in the four-color overlay can make the three-color overlay increase its dark concentration. Gray-scale replacement (GCR) can be used to reduce costs and make it easier to achieve gray balance. However, the addition of the black version will reduce the contrast, so it should be considered according to different needs.
According to different replica requirements, the black version can be divided into three types:
(1) The first type of black plate is used to extend the maximum concentration of the replica because the density of the three primary colors is not sufficient.
(2) Extending the first type of black plate, but increasing the UCR effect, so that the color ink of the three primary colors reaches only a certain level; and the amount of ink of the black plate is increased to replace the three primary colors of the overprinted portion.
(3) The full-level blackboard can be divided into two forms, the difference is that the two use UCR.
6. Screen Ruling
The number of dots in the unit length of the printed matter, the larger the number of screen lines, the finer the print performance; the smaller the number of screen lines, the opposite. The screen can be divided into a glass screen, a contact screen and a word dot screen, but a contact screen is the mainstream. The shape of the contact screen is further divided into a chain type dot and a square type dot.
Different replicas have different requirements, so there are different screen lines, which can usually be divided into: 65.100.120.150.175.200.300 (lines/inch). The quality of printing is not the larger the number of screen lines, the more the screen lines are finer, the more serious the dot enlargement may be. Therefore, Mr. Zhang's master's thesis "Study on the balance of screen lines and gray balance" discusses the neutral balance of the neutral colors expressed by different screen lines.