PHOTOMECHANICAL ARTS. — This term refers to those processes that enable the printing of images by means of a matrix which, instead of being worked by hand (v. ENGRAVING), is obtained through the action of light. The process for etching the matrix is based on the property of gelatin treated with potassium bichromate of becoming insoluble after exposure to light; thus, a metal plate, for instance, with its surface suitably prepared and exposed to light under a photographic negative, will, after washing, present an image capable of being reproduced by a mechanical process.
Naturally, there exist various systems for printing that employ different forms of matrices. The most commonly used reproduction, because it allows images to be printed alongside text, is the photopaque process, also known as zincography or photozincography (since it uses a zinc plate). However, this system would only allow so-called line reproductions, i.e., in black and white without halftones, unless other measures, such as a screen, are employed to break down the halftones into dots. It is evident that the quality of the reproduction is thus dependent on the quality of the screen, which can be more or less fine and therefore more or less visible. In this process, the images consist of raised areas onto which the ink adheres; in other processes, the image on the matrix is not raised but recessed. The ink is received in these recessed areas while the background remains uninked. This is the so-called intaglio printing, one of the rotary press machines of the rotogravure type: it uses a copper cylinder onto which, in addition to the images, text can also be transferred by means of a screen, and is therefore widely used today for printing illustrated newspapers. Reproductions that, on the contrary, do not exhibit the characteristic screen of the cliché used in typographic printing are those obtained through planographic processes, which include phototype, photolithography, and the offset process. These employ the methods proper to lithography for transferring the image onto a flat surface, so that the matrix is neither raised nor recessed but smooth, with parts that, being prepared in opposite ways, either accept ink or reject II. This is the system that allows the best reproductions in terms of the fidelity of halftones. For color reproduction, mention should be made of trichromy, based on the superimposition of the primary colors (yellow, red, and blue), and quadrichromy, which adds black for deep shadows (phototypographic process); chromophotolithography (planographic process) and rotogravure in color (intaglio process).