Separation Options

Next, choose one of two methods for generating CMYK values and creating CMYK separations from RGB values: Under Color Removal (UCR) or Gray Component Replacement (GCR). Again, input from your printing company is important here. Theoretically, the gamut conversion should be RGB to CMY. We add black (K) to our CMY images for two reasons: 1) so that we can create a good high-contrast black (which is difficult to accomplish by using just CMY inks), and 2) to reduce ink volumes, and particularly in the threequarter-tone to shadow regions of our images, so as to allow us to maintain shadow detail when printing our images.

In general, UCR concentrates black (K) substitution from the three-quarter tone to the shadow, while GCR has more black (K) substitution through a larger tonal range of the image. GCR separations tend to be lighter and higher contrast, while UCR separations tend to have better color saturation. Discuss these trade-offs with your printer or experiment with both on your output device. If you select either UCR or GCR, you will then want to fill in the black ink and total ink limits:
Black Ink Limits will typically vary from about 80 (newsprint) to 90–100 for commercial printing.
Total Ink Limits will vary from about 220 (news print) to 290–340 (coated stock). Total ink limits may exceed 340 when printing with stochastic (FM) dots. Consult your printing company for guidance.

If you select GCR, you will then want to select a Black Generation level from Light to Maximum. The higher the black substitution you choose, the more black will be substituted for CMY. When GCR is selected, we have the opportunity to add extra color (10% added here) in the shadow region with Under Color Addition (UCA). This added color can be used to increase ink density in shadow regions, but be wary of its ability to plug up shadows. Use UCA carefully, because adding too much ink can destroy shadow detail. Aprofile name is automatically generated and placed in the Name field at the top of the Custom CMYK dialog box—in this example, SWOP (Coated), 20%, GCR, Medium.) Accept or change
this name as you like.