Multicolor Printing

a method of producing color impressions (reproductions) by successive printing on paper or other material from plates on a machine or press. Color reproductions can be made by any method (relief, plane, or gravure printing). The production of a color print with a certain number of colors is common to all methods. The number of plates from which the prints are made corresponds to the number of colors used.

Color reproduction appeared at the dawn of printing (prints from wood or metal engravings were colored by hand). Multicolor printing came into use after the invention of lithography in the late 18th century, when a separate plate was made on the lithographic stone for each color of the original. Color lithography came to be called chromolithography. In the late 19th century the development of photosensitive emulsions and other advances in photographic technology (better optics, light filters, and powerful light sources) led to the replacement of manual methods of preparing plates for color reproduction by photomechanical methods.

The main task of multicolor printing is to produce with a certain number of colored inks color images on each part of the print that are identical in color and shape to the given part of the original. According to the three-component theory of vision, the diversity of colors in a color reproduction is achieved as a result of trichromatic synthesis, which is based on the subtractive method of reproduction—that is, on the principle that a color is formed by subtracting certain rays from white light. Any color—and consequently any polychromatic original—can be reproduced with three inks: purple (bluish red), light blue (greenish blue), and yellow. Each of the colors has maximum absorption in one zone of the spectrum and maximum reflection in the other two zones. Because of the transparency of inks when applied in equal quantities, virtually no black can be obtained. This deficiency is overcome by using a fourth ink—black. Therefore, the use of four-color rather than three-color synthesis is recommended. The results of color synthesis in multicolor printing depend on the color range of the set (triad) of inks—that is, on the maximum number of color tones that can be produced by combining the inks in various quantities—and on the surface properties of the paper or other material used. When the main set of inks does not allow reproduction of a certain color that is subjectively important for a given original, another colored ink, such as green, violet, or gold, is used in addition to the main ink triad.

The process of producing a color reproduction consists of three primary stages. The first, analytical stage (or color-separation) can be accomplished by photographic or electronic means. The second, transitional stage (or shading process) consists in the production of shades of the color-separated image and includes the making of color-separated half-tone or screen negatives and diapositives and plates. The third, synthetic stage is the production of color prints.

One-color, two-color, or multicolor presses are used for multicolor printing. When one-color and two-color presses are used, a one-or two-color print is obtained after each printing cycle, and to produce a four-color print the printing process must be correspondingly repeated four or two times to imprint subsequent colors. The use of multicolor presses on which all four colors are printed successively in one printing cycle on one or two sides of the paper sheet is most promising.