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one of the main techniques for printing text and illustrations, in which the printing and nonprinting elements of the plate lie in virtually the same plane. The division of printing and nonprinting elements is based on various physicochemical properties of the surface: the printing elements are soaked with a greasy ink, whereas the nonprinting elements are moistened with water. This is done by preliminary chemical treatment of the plate’s surface, which results in the formation of adsorptive and mineral coatings, with the corresponding molecular surface properties. Since grease and water do not mix, the previously moistened nonprinting sections of the plate do not absorb the ink, and the ink covers only the printing elements; when the plate is moistened, the water does not soak the layer of ink but is absorbed by the nonprinting elements. In the printing process the plate is alternately moistened with water and rolled with ink, and then the paper or other material is brought into contact with the plate under pressure, thus producing an impression.
Lithography, the technique of contact printing from flat plates, which was invented in 1798, is not very efficient and has a number of shortcomings. Therefore, it has been replaced by offset printing, which is the transfer of an image from a flat plate to the paper with the aid of an intermediate rubber surface. Offset printing is used to reproduce text, line drawings, and black-and-white and color halftones. The versatility and technical and economic advantages of the technique have brought about its wide use for all types of published matter, such as newspapers, books, magazines, maps, and artistic materials, as well as for the printing of packing materials.
In planographic printing, as in gravure printing, a gradual transition of tones from light to dark is achieved by breaking the image down into dots of various sizes, depending on the tone. The resulting image is a halftone. The printing of color originals also involves color separation and the production of separate plates for each color.
Planographic printing techniques also include photogelatin printing, or collotype, whose distinguishing feature is the reproduction of halftone originals without using a screen. Photogelatin printing is used for facsimile reproduction of black-and-white and color originals. The technique is not very efficient and is comparatively costly; therefore, it is used only to print art reproductions and complex illustrations in medical and technical publications.
A. L. POPOVA