FOTOMECHANICAL ARTS. — This name designates those processes that permit the printing of images by means of a plate which, instead of being worked by hand (v. INCISIONE), is obtained by means of light. The process for engraving the plate is based on the property of gelatin treated with potassium bichromate of becoming insoluble after exposure to light; thus, a metal plate, for example, with its surface suitably prepared and exposed to light beneath a photographic negative, will present, after washing, an image suitable for reproduction by a mechanical process.
There are, naturally, various systems for printing, employing various forms of plates. The most commonly used reproduction, because it permits images to be printed together with the text, is the phototypographic process (also called zincotypy, zincography, and photozincotypy, because it uses a zinc plate). This system, however, would permit only so-called line reproductions, that is, reproductions in black and white without gradations or halftones, were it not for other devices, such as the screen, which breaks halftones down into dots. It is obvious that the fidelity of the reproduction is thereby linked to the quality of the screen, which may be more or less dense and therefore more or less visible. In this process the images consequently consist of raised reliefs on which the ink adheres; in other processes, on the other hand, the image on the plate is not raised but recessed, and receives the ink, while the background is not inked. This is the so-called intaglio printing used by rotary presses and therefore also called rotogravure: it employs a copper cylinder on which, in addition to the images, the text may also be reproduced by means of the screen, and it is consequently much used today for printing illustrated newspapers. Reproductions that, by contrast, do not display the characteristic screen proper to the cliché used in the typographic process are those obtained by means of planographic processes, including phototype, photolithography, and the Offset process. These employ the procedures proper to lithography for transferring the image onto a flat surface, so that the plate is neither raised nor recessed but smooth, with parts suitably prepared to accept the ink and others that reject II. This is the system that permits the most faithful reproductions of halftones. For the reproduction of colors, mention should be made of three-color printing, based on the superimposition of the primary colors (yellow, red, and blue), and four-color printing, in which black is added for deep shadows (phototypographic process); chromophotolithography (planographic process); and rotogravure chromolithography (intaglio process).