Research on Printing Grey Balance Control Law

Research on Printing Grey Balance Control Law

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1.  the gray balance and the guide copy of the guide

The Rochester Institute of Technology (RIT) developed the Tone reproduction and Nentral Determination (TRAND) to determine the neutral concentration of the gray balance. TRAND has six color blocks, five of which are colored, and each color block contains dozens of color circles. The printing staff can select the color circle closest to the original gray from the guide table, and compare the value of each dot of the color circle, so that the gray balance dot values of the three primary colors can be obtained.

Its characteristics are the convenience of estimation, and the need for gray balance in the visual selection of neutral patches. Because it is estimated by the human eye to gray, it may cause errors or conditional colors due to light sources of different color temperatures. Therefore, it must be fixed at the same source to make a more accurate value.

2.  the number of screen lines to get a better gray balance

In the article "Research on the balance of screen lines and gray balance", the purpose is to find the change of chroma of different screen lines in four-color and three-color overprint, and analyze the four-color overprint and three-color overprint on printing. The difference in neutral color (the number of screen lines of 85, 120, 150, 175, 200 is the experimental range).

After printing the printed samples, the full-scale concentration test is carried out, and the quality is satisfactory, and the measurement is started. Using a colorimeter as a measurement tool, the color change of the level of the sample is measured with a D50 light source and a 2° viewing angle. The measurement item is the a* value and the b* value of the CIELAB color space. After measurement, it is converted to C*ab value and Chroma in the CIELCH color space.

After statistical calculation, the analysis results are available.

Conclusion: The chroma value correlation between the number of lines in the three-color overprinting is only the second-fifth order correlation, and it is negatively correlated, which means that the higher the number of screen lines, the smaller the chroma value, the gray balance condition. The better the value. The four-color overprint is similar to the three-color overprint, and only the chroma of the twenty-fifth order shows that the 175 line can obtain a better neutral color. As for the chroma values of the three-color overprint and the four-color overprint, on the whole, the 150-line, 200-line black version incorporates a neutral color rendering with improved three primary color ink overprints.

3.  the equation for color correction:

The color correction equation is a linear one-time equation:

C, M, and Y represent the amount of cyan, yellow, and magenta inks required for copying, while Dr, Dg, and Db are the concentrations measured by the originals in red, green, and blue color filters. A11 to a33 are coefficients relating to specific inks and specific printing conditions. Due to the non-additive nature of the ink, the replication effect of this formula is not very satisfactory, so the empirical equation based on the color correction equation is proposed. (Note: Li Taizhen, Discussion on the Mathematical Model of Gray Balance in Lithography, P16)

4.  the empirical equation:

Both Clapper (1961) and Pobboravsky (1962) published an empirical equation at TAGA:

Cr, Mg, and Yb represent the amount of cyan, yellow, and magenta inks required for copying (concentration measured by a color filter). Dr, Dg, and Db are the concentrations (chromaticity density values) measured by the color filter of the original, and a11 to a39 are correlation coefficients. The difference between the empirical equation and the color correction equation is that it considers the ink concentration to be disproportionate and non-additive. The squared term in the formula is used to correct the disproportionate nature of the ink, while the cross product is used to correct the ink's non-additive properties.

Because the empirical equation is based on a halftone sample, but because of the development of the field of colorimetry and spectrophotometry, the empirical equation based on concentration is relatively low. (Note: Li Taizhen, Discussion on the Mathematical Model of Gray Balance in Lithography, P16-P17)

5. Neugebauer Equation:

The Normberg equation uses the data of the full version of the color block to predict the halftone value.

The three primary colors are overprinted into eight primary colors, which are:

C, M, Y... one color

R, G, B... secondary color

Bk ...............three colors

W ...............four times color

The greater the percentage of the dot area of the eight replicated primary colors, the greater the color effect seen by the human eye. X, Y, and Z in the Norbert's equation are the tristimulus values of the replicated colors, while X1, Y1, Z1 to X8, Y8, and Z8 are the tristimulus values of the eight primary colors, and f1 to f8 are the respective eight primary colors. The percentage of dot area. The Norberber equation for gray balance is as follows:

First, assume the C, M, and Y three-color version of the dot area (c, m, y), and then use the Demichel Equation to find the percentage of the dot area occupied by each copy of the primary color.

After calculating f1 to f8, the tristimulus values (X1, Y1, Z1 to X8, Y8, Z8) of the replicated eight primary colors are measured using a colorimeter or a spectrometer, and the two equations are substituted into the Normberg equation to obtain the replicated three. The stimulus value (X, Y, Z), then convert the tristimulus value into the chromaticity coordinate value (X, Y) and compare it with the chromaticity coordinate value of the observed light source. If it is within the allowable range, it means the copied color combination. In neutral gray.

The dot area combined with this neutral gray color is the desired halftone dot area. The corresponding concentration values are then determined using the Murray Davies Equation.

In this way, the neutral gray dot area and its corresponding density value can be obtained. (Note: Li Taizhen, Discussion on the Mathematical Model of Gray Balance in Lithography, P17-P18)

6. Modify the Modified Neugebauer Equation:

It is corrected that the combination of the Norwegian equation and the Normberg equation is the same as the neutral color dot area, and the difference lies in the equation to which the corresponding concentration value is applied.

Since light is affected by the passage of light, Yule and Colt proposed an n value in 1951 to correct the Murray Davis equation, which is called the Yule Nelsen Equation.

The general equation of the Neil is considered to be the value of n, so a in the equation (the halftone dot area of each color version) is regarded as the actual dot area. The size of the n value is affected by four points (n values between about 1.5 and 3.05):

a. Paper type

b. Halftone screen line number

c. Halftone dot size

d. full version concentration

(Note: Li Taizhen, Discussion on the Mathematical Model of Gray Balance in Lithography, P21-P22)

7. n-value Modified Neugebauer Equation:

This equation is a further correction to the Normberg equation, because the modified Normberg equation only considers the effect of the n value on the calculated concentration, but does not consider the scattering, reflection, etc. of the light in the paper. The tristimulus values of the eight primary colors produce errors.

8. Comparison of Wu and control methods:

Because the number of screen lines is controlled by using different screen lines (85 to 200) to compare which lines can get better quality, it is different from the way of calculating the better gray balance by using the color reproduction equation. The two can't compare their advantages and disadvantages, so I control the number of screen lines first to compare the advantages and disadvantages of three-color overprinting and four-color overprinting on different screen lines. Then compare the three-color overprint and the four-color overprint on each screen line. The pros and cons of the number.

(1) the control method of the number of screen lines:

First, compare the advantages and disadvantages of three-color overprinting and four-color overprinting on different screen lines, and then compare the advantages and disadvantages of three-color overprinting and four-color overprinting on each screen.

(2) the comparison of color reproduction equations:

Compare a, color correction equation b, empirical equation c, Nagbert's formula d, correct Nagbert's equation e, n value to correct the accuracy and application range of the five equations of the Nagbar equation.

Since print color reproduction has evolved toward the field of colorimetry and spectrophotometry, the concentration-based color correction equations and empirical equations are less practical in the future than the Normberg equations. The Normberg equation is to determine the concentration of the neutral gray dot area, but there is one point to consider, that is, the dot enlargement and the shape of the dot. Whether the subsequent study can solve the Norwegian equation The error caused by the expansion of the dot.

9. conclusion

The rule of gray balance control is to be able to find the correct amount of ink by the equation under different bars. The management of traditional print quality requires a data standard to control the stability of the copy. The early method was to establish the concept of dataization by the coordination of the gray balance and the copy of the guide, the concentration meter and the empirical equation. With the development of colorimetry and spectrophotometry, the standard of dataization is now established by the colorimeter and the n-value modified Normberg equation. When the standard of data is established, the actual printing must pay attention to whether the printing machine is The amount of ink required can be printed stably, so that the printing quality can be improved. The Normberg equation is the density value corresponding to the area of the neutral gray dot, so there are several points to consider, that is, the dot enlargement and the shape of the dot, so it is very important that the equation can solve the error.

The future printing trend "digital printing machine" has a very different control method of gray balance than traditional printing. Traditional printing mainly controls the area of the dot to achieve the gray balance requirement, while the printing method of the digital printing machine has a version. Printing and non-plateless pressure printing, etc., may control the gray balance mode somewhat, and the color complex equation can be applied. Therefore, I hope that in the future, there will be research on related aspects of digital printing standard rules of digital printing, and the application of color complex equations in digital printing in the future.