Photocomposition, also called photoimagesetting, is any of several methods of assembling types and illustrations on photographic paper or film, or on a printing plate. It is used in the printing of books, newspapers, posters, signs, and various other materials.
Photocomposition machines use computers to create photographic images of type characters and of photos and other illustrations, also called graphics. The material to be set is either typed on a keyboard, created with computer graphics software, or scanned electronically. A computer then stores this material as digital (numeric) information in its memory, which also contains the instructions for each character in a font. A font is a set of all the letters, numbers, and other characters of one style of type.
The photocomposition machine uses the digital information stored in the computer to reproduce the images of the type and illustrations as a pattern of dots. A laser in the machine sends out light pulses to form the images on photosensitive paper or film, or directly on a printing plate.
Photocomposition has many advantages over the older method of typesetting, which used metal type. It is much faster, and the use of computers simplifies many operations. For example, computer programs can automatically justify lines—that is, they can adjust the space between words, and hyphenate words when necessary, to line up the right-hand margin. In addition, changes in a line or page of text can be made on a computer without rekeying the entire line or page.
