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Chapter 28. Halftones, Screen Frequency,... > Separations for Offset Printing

Separations for Offset Printing

The preceding discussion is concerned strictly with black-and-white images. Color images create a whole new set of issues.

A Separate Piece

Process color printing, as you recall, uses four colors—cyan, magenta, yellow, and black. When a color image is to be printed, it first must be separated into four pieces: the cyan portion, the magenta portion, the yellow portion, and the black portion. So you need four halftones. But here's the rub: You'll recall that black-and-white halftones orient the screen at a 45-degree angle. When you have four halftones, each color has to be at a different angle; otherwise, the dot patterns interfere with each other and create an unpleasant optical effect called a moiré (pronounced MWAH-ray) pattern. (The term comes from a type of woven fabric that has a “watery” design.) Unfortunately, it's easy to create a moiré pattern; keeping each screen more than 30 degrees from the others keeps moiré at bay, but 30×4 = 120 degrees, and technically there are only 90 degrees to work with. Over the years, however, printers have found a set of angles for each color that works (see Figure 28.5).


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