Products may be printed in two or more colors by almost all of the processes described earlier. The simplest method of color reproduction is flat-color printing. A separate plate is prepared for each color desired. Plates are then printed in sequence on a single sheet of paper. Each plate is covered with the desired color ink and then printed in register with all of the other color images.
The process-color printing method is used to reproduce full-color pictures such as the colored continuous tone photographs. Process-color activities include: color separation, color correction, and color reproduction.
A beam of white light passed through a glass prism results in rainbow of color. This is because white light is really a mixture of the colors of light. Each color refracts or bends differently as it enters and leaves the prism, Figure 2-19.
Three color light--red, blue, and green--can be used to reproduce white light, Figure 2-20. Where all three colors of light overlap, white produced. Red, blue, and green are called additive primary colors because added together they form white light.
Where any two additive primary colors of light overlap, a third color is formed. Red and blue combine to form magenta. Red and green combine to produce yellow. Magenta, cyan, and yellow are called subtractive primary colors because they subtract colors from white light to form black. The subtractive process is shown in Figure 2-21.
Color separation is done by photographing the original color copy three times. The procedure for doing this is similar to producing a halftone negative. There are two important differences, however, the use of filters and the position of the halftone screen.
FILTERS. A filter is a piece of colored glass or gelatin material which allows only light of its own color to pass through. Other are absorbed by the filter. Filters are used to record, on three separate pieces of photographic film, the amounts of red, blue, green contained in a color illustration.
Each of the three negatives is produced through a different color filter. Because different amounts of light are absorbed by each filter, exposure times for each negative will be different.
Photographing through a red filter produces a negative recording of the red in the original. Red light passes through the filter and exposes the process film. Blues and greens (cyan light) cannot pass through the red filter. They are not recorded and are represented by clear areas in the negative. The resulting negative is known as a red separation negative.
Photographing through a blue filter produces a negative recording of the blue in the original. Blue light passes through the filter and exposes the process film. Reds and greens (yellow light) are not recorded and become clear areas in the negative. The negative that results is called a blue separation negative.
Photographing through a green filter produces a negative recording of the green in the original. Only green light passes through the green filter. Blues and reds (magenta light) are not recorded on the film. Blues and reds are represented by clear areas in this negative which is called a green separation negative.
HALFTONE SCREEN. Each separation negative is also a halftone negative. Each negative must be exposed through a halftone screen and will contain a halftone dot pattern. Dots must not interfere with each other when the job is printed. If they do interfere, an objectional pattern, called a moire (pronounced mwa'ray), will result.
moire patterns are avoided by changing the angle of the halftone screen for each separation negative. Revolve the screen 30° after each exposure to reduce the possibility of a moire pattern developing.
color correction can compensate for color deficiencies of process inks. Manual etching photographic masking, and electronic scanning are techniques that can be used. Color correction allows less ink of certain colors to be printed in selected areas of the reproduction.
ELECTRONIC SCANNING One of the more modern methods involves color seperation and correction by electronic scanning. The color original is exposed through separation filters. The separated light is picked up by photocells and analyzed by the computer. The computer can color correct for the inks and paper to be used.
Subtractive primary color inks are transferred to the paper when the plates made from the separation negatives are printed. Magenta (blue + red) ink is printed by the plate made from the green separation negative. Cyan (blue + green) ink is printed by the plate made from the red separation negative. Yellow (red + green) ink is printed by a plate made from the blue separation negative.
Process-color prints are made by printing yellow, magenta, and cyan plates in perfect register on a single sheet of paper. Figure 2-21 also shows how additive primary colors are created by overprinting pairs of subtractive primary color inks. By controlling the amounts of yellow, magenta, and cyan inks, you can produce all other colors of the visible spectrum.
In theory, only the three subtractive primary colors of ink are necessary to reproduce all colors of the spectrum. This is not true in actual practice, however. A fourth ink ,black is needed to improve the grays and shadows in the print.