肆, the way to control the gray balance
First, the gray balance and copy version of the guide
The Rochester Institute of Technology (RIT) has developed a gray balance and tonal reproduction (TRAND) for gray balance neutral density values. TRAND has six color blocks, of which five color blocks, each color block contains dozens of color circles. The printing staff can select the color circle closest to the original document from the guide sheet, and compare the color dot values ​​of each version, so that the gray balance point values ​​of the three primary colors can be obtained.
Its characteristic lies in the ease of estimation, and the need for gray balance is achieved by the visual selection of neutral patches. Because it is using the human eye to make an estimate of the gray color, errors or conditions may occur due to different color temperature sources. So you must be fixed in the same light source to do the estimation to get the correct value.
Second, the screen line number to get a better gray balance
In the article “Study on the Number of Screen Lines for Gray Balanceâ€, the purpose is to find the change in the chroma of four-color and three-color overprint of different screen lines, and analyze the four-color overprint and three-color overprint on the printing. Neutral color difference (with screen lines of 85, 120, 150, 175, and 200 as the experimental range).
After printing the printed proofs, the quality of the printed sheets was tested in full format, and the measurement was started. Using a colorimeter as a measurement tool, the D50 light source and a 2° viewing angle were used to measure the color change of the series. The measurement item is the a* value and b* value of the CIELAB color space. After the measurement, it is converted into the C*ab value and Chroma in the CIELCH color space. The formula for the other chroma value is converted to:
After statistical calculation, the analysis results can be obtained.
Conclusion: The correlation of chroma values ​​between lines of three-color superimposition is only the correlation of the twenty-fifth order and is negatively correlated. It means that the higher the number of screen lines, the smaller the chroma value, and the gray balance. The better the value. Four-color overprinting is similar to three-color overprinting. Only the twenty-fiveth-order chroma value can be known, and 175 lines can obtain a better neutral color. As for the comparison of the three-color overprint and the four-color overprinting chroma values, on the whole, the 150-line, 200-line black plate incorporates a neutral color rendering with an improved three-color ink overprint.
Third, the color equation:
The color correction equation is a linear one equation and its formula is as follows:
C, M, and Y represent the amounts of cyan, yellow, and magenta inks required for copying, and Dr, Dg, and Db are the concentrations measured by the original with red, green, and blue filters. A11 to a33 are coefficients relating to a specific ink 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 equations derived from the color correction equations are proposed. (Note: Li Tiejun, Discussion on Gray Balance Mathematical Mode in Planographic Printing, P16)
Fourth, the empirical equation:
Both Clapper (1961) and Pobboravsky (1962) publish an empirical equation in TAGA. Its formula is as follows:
Cr, Mg, and Yb represent the amounts of cyan ink, yellow ink, and magenta ink (measured with a color filter) required for copying. Dr, Dg, and Db are the density (chroma density value) 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-additional. The square term in the formula is used to correct the ink's non-proportional characteristics, while the cross product term is used to correct the ink's non-additional properties.
Because empirical equations are based on the measurement of halftone samples, but because of the development in the field of printing towards colorimetry and spectrophotometry, the empirical equation based on concentration studies also has a relatively low value. (Note: Li Tiejun, Discussion on Gray Balance Mathematical Mode in Planographic Printing, P16-P17)
V. Neugebauer Equation:
The runkob equation predicts the halftone value with the data of the full color patch. The runkob equation is as follows:
The three primary inks are overprinted into eight primary colors, which are:
C, M, Y ... one-time color R, G, B ... secondary color Bk ............... three-time color W ............... four times color eight kinds of copy original color dot area percentage is greater, the person The greater the color effect seen by the eye.
The X, Y, and Z in the runcock equation are the tristimulus values ​​of the copied color, and X1, Y1, Z1 to X8, Y8, and Z8 are the tristimulus values ​​of the eight primary colors, and f1 to f8 are the respective primary colors of the eight primary colors. Percentage of outlet area. The method of obtaining the gray balance from the runcke equation is as follows:
First, assume that C, M, Y three-tone dot area (c, m, y), and then apply Demeter equation (Demichel Equation) to calculate the percentage of dot area occupied by each primary color.
Demic equation format:
After f1 to f8 are calculated, the tristimulus values ​​(X1, Y1, Z1 to X8, Y8, and Z8) of the eight primary colors are measured using a colorimeter or spectrometer, and the two formulas are substituted into the Norbert equation to obtain the duplicated threes. Stimulus value (X, Y, Z), then tristimulus values ​​are converted into chromaticity coordinates (X, Y) and compared with the chromaticity coordinates of the observation light source. Neutral gray.
The area of ​​the dots that combine this neutral grey color is the halftone dot area required. Then use the Murray Davies Equation to find the corresponding concentration value.
The Murray Davis equation is as follows:
In this way, the neutral gray dot area and its corresponding density value can be obtained. (Note: Li Tiejun, Discussion on Gray Balance Mathematical Mode in Planographic Printing, P17-P18)
6. Modified Modified Neugebauer Equation:
The modified Neuberg color equation and the Norkerberg equation are the same as the neutral color color dot area method, and the difference lies in the equation to which the corresponding concentration value is applied.
As light passes through the paper, Yule and Colt proposed an n value in 1951 to correct the Murray Davis equation. This equation is called the Yule Nelsen Equation. The equation is as follows:
Euler
Since the Neeles equation takes into account the value of n, a (halftone dot area of ​​each color plate) in the equation is regarded as the actual dot area. The size of the n value is affected by four points (the n value is approximately between 1.5 and 3.05):
a. Paper Type b. Halftone Screen Lines c. Halftone Dot Size d. Full Plate Density (Note: Li Tiejun, Discussion on Gray Balance Mathematical Mode in Planographic Printing, P21-P22)
Seven, n value modified Neugebauer Equation (n-value Modified Neugebauer Equation):
This equation is a further correction to the Norbert equation, because the modified Runbeck 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.
Therefore, Yule proposes to change to the following formula:
(to be continued)