Detailed Prepress Operation - Separation Operation Part 2 (Bottom)

3. Correction of dark-shaded originals

Dark color cast refers to a condition where the spectral densities of R, G, and B in the dark tone that should be neutral gray are inconsistent. Normally, even the color originals that have been properly photographed and correctly photographed have a certain degree of color cast in their dark areas. In fact, it can be assumed that all color originals have dark spots or color imbalances, as shown in Figure 6.11. , shown in 6.12.

There are four methods for color correction of dark originals. (1) Use the AUTO/AN EB function, that is, use the AUTO/AN EB function to set the black field calibration value of the selected black field to a standard neutral gray value during black field calibration, but after calibration Care should be taken to check its effect on midtones. (2) Using the MAN EB function, the shaded color to be selected is treated as a neutral gray at the time of calibration, and the percentage values ​​of the three color dots of C, M, and Y are directly adjusted to neutral gray. Shadows affect the color of the image area and must be handled carefully. (3) With the CAST function, after the calibration is applied, there is still a color shift at the dark tone, and the CAST function can be used to shift the color separation curves of the three primary colors of C, M, and Y at the dark tone. Until the color shift of C, M, Y is eliminated. The CAST function is performed in different regions. In the dark tone portion, only the dark tone plays a significant role and has little influence on other regions. (4)Using the UCR or UCA function, it can be seen from the foregoing that the UCR and UCA can eliminate dark tonality, as shown in Figure 6.13. When UCR/UCA is used, the Y version should be used as the reference, and the starting point of UCR/UCA should be selected at the same time. The slope, removal amount/gain amount should be combined with the process and controlled within a suitable range.

In summary, the correction of color-originating manuscripts should be considered as the analysis of the color cast state of the manuscript and the main body of copying, and the white-field/dark-field calibration, gradation correction, CAST, UCR/UCA, and color correction functions should be flexibly applied to adjust , And check the corrected image status at any time to ensure the correct color reproduction.

(to be continued)