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means of producing reproductions of written material or images in multiple copies. There are four traditional types of printing: relief printing (with which this article is mainly concerned), intaglio, lithography, and screen process printing.
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(offset lithography), a method of printing in which the ink impression is transferred from the printing plate to an intermediate rubber-coated surface, or blanket. The image is then transferred from the blanket to paper or some other printing material. Offset was first used in the United States, where the first offset press was built in 1905.
The term “offset” usually designates those printing processes that use planographic plates and that are based on the coating of the image, or printing, areas with ink and the non-image, or nonprinting, areas with an aqueous solution that is not ink receptive. During printing, the plate is alternately coated with an aqueous solution and roller-coated with ink. The plate then makes contact with the surface of a rubber blanket, which in turn is brought into contact with paper to produce a print. Thus, a double transfer of the image occurs without the paper ever coming into direct contact with the plate. This lessens significantly the amount of pressure required during printing and, thereby, increases the wear resistance of the plate. In turn, productivity is higher, as is the quality of reproduction.
Offset involves photomechanical and electronic processes during platemaking, as well as mechanized and automated processes during the preparation of the plates and printing. The plates are aluminum or zinc sheets that vary in thickness from 0.35 to 0.8 mm. Their surface is grained to obtain a uniformly mat surface. The printing and nonprinting areas on the surface of the plates are produced by light-generating films, which differ in terms of molecular surface properties and which are receptive to either water or ink. Aluminum plates are subjected to a complex electrochemical preparation in automated lines to increase their adsorption capacity and wear resistance. Composite plates consist of two metals, one of which is extremely ink receptive and forms the printing areas (such as copper), while the other is naturally water receptive and forms the nonprinting areas (nickel, chromium, or stainless steel). Composite plates are used for long-run printing in high-speed presses owing to the high hydrophily and wear resistance of the nonimage areas. Composite plates are usually produced on an aluminum or steel base, and galvanic methods are used to coat the entire surface with copper film having a thickness to 10 μ or with chromium film with a thickness of 1–3 μ. The printing image on either monometallic or multimetallic plates is produced photochemically by projecting an image through a photographic negative or positive onto a light-sensitive coating of the plate. Such a coating consists of high-molecular compounds (albumin, gum, polyvinyl alcohol), chromium salts (diazo compounds), and film-forming substances or photopolymers. When exposed to light, chromium salts harden. Thus, the exposed areas of the coating harden and are rendered insoluble in water. The exposed areas of the coating are shielded by the opaque parts of the negative or positive. The coating is subsequently removed, with the printing image fixed on the plate.
More widely used are coatings based on diazo compounds, in which light causes photochemical decomposition on the exposed portions, leading to the removal of the coating from these areas of the plate during developing. The areas of photopolymeric coatings exposed to light polymerize and become insoluble in water. The coating is removed from the nonilluminated areas of the plate during developing. Coatings consisting of both diazo compounds and photopolymers may be applied in a thin layer on monometallic and multimetallic plates; the layer remains unchanged for a long period of time (more than a year). This makes it possible to prepare the metal and presensitize the plates in special plants.
In the production of presensitized plates, the printing areas on monometals are produced on the coating, which has been shielded during copying by the opaque portions of the photographic positive and which is retained after development of the copy. The coating on multimetallic plates is removed from the printing areas after developing and remains as a temporary protective layer for the nonprinting areas. Chemical or electrochemical etching of the top metal (nickel or chromium) is done down to the copper layer; the protective coating is then removed from the nonprinting areas. In this case, the printing areas are produced on the copper, and the nonprinting areas on the nickel or chromium. All methods of platemaking require that, after the production of the printing areas, the nonprinting areas be treated with a water-receptive solution. This imparts stable water-receptive properties to the areas.
Operations required for the processing of monometallic plates (developing, washing, and drying), are performed separately by mechanized equipment. The production of prints and the preparation of multimetallic plates are carried out on continuous production lines.
Offset prints are produced by offset presses. Each working cycle of a press consists of coating the printing plate, rolling the ink onto the printing areas, feeding the paper into the press, printing, and removing the finished print from the press and placing it on the receiving table.
Offset is popular owing to the mechanization of platemaking and the high productivity of the presses. It makes possible the reproduction of all types of publications.
REFERENCESSiniakov, N. I. Tekhnologiia izgotovleniia fotomekhanicheskikh pechatnykh form. Moscow, 1966.
Nikanchikova, E. A., and A. L. Popova. Tekhnologiia ofsetnoi pechati. Moscow, 1966.
Zakharov, A. G., and D. A. Fufaevskii. Ofsetnye mashiny i rabota na nikh. Moscow, 1972.
A. L. POPOVA
E.g. the Perl function splice(ARRAY, OFFSET, LENGTH, LIST) replaces LENGTH elements starting at index OFFSET in array with LIST, where offset zero means the start of the array.
For an Intel x86 processor with a segmented address space the offset is the position of a byte relative to the start of the segment.
offset(1) See offset press.
(2) The distance from a starting point, either the start of a file or the start of a memory address. Its value is added to a base value to derive the actual value. An offset into a file is simply the character location within that file, usually starting with 0; thus "offset 240" is actually the 241st byte in the file. See relative address.
(3) In word processing, the amount of space a document is printed from the left margin.