xerography(redirected from xerographs)
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xerography(zərŏg`rəfē'), also called electrophotography, method of dry photocopying in which the image is transferred by using the attractive forces of electric charges. A beam of light, usually from a laser, is made to strike the original material, e.g., a white page with black lettering. Light rays are reflected off the white areas onto a photosensitive plate over which electric charges have been spread. Charges are neutralized from the areas struck by the rays. Since no light rays are reflected from the lettering, charges are retained on the plate in areas corresponding to the lettered areas of the original. A plastic powder called toner is introduced that sticks to the charged areas. A sheet of paper is then passed between the plate and another charged object that draws the powder from the plate to the paper, forming an image of the original; the powder is fused to the paper with heat. The process has image resolution that is sufficient for printed or written materials, and certain pictorial materials are fairly well reproduced. As the image on the drum is a projected one rather than one made by contact printing, it is possible to produce a copy that is smaller or larger than the original. Variations of the xerographic process are used in such devices as computer laser printerslaser printer,
a computer printer that produces high-resolution output by means of a process that is similar to photocopying. In place of reflected light from an image (as is used in xerography), a laser printer uses data sent from a computer to turn a laser beam on and off
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in communications, system for transmitting pictures or other graphic matter by wire or radio. Facsimile is used to transmit such materials as documents, telegrams, drawings, pictures taken from satellites, and even entire newspapers.
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See study by D. Owen (2004).
one of the most widely used processes for reproducing documents and for making enlargements from microfilm.
Xerography is based on the photoconductivity of semiconductor materials that are applied to a special backing made of, for example, paper or metal and on the ability of the materials to hold dyed particulate by means of electrostatic force. Xerography was patented in the United States in 1938, and the first xerographic machines were commercially available in 1950. The popularity of xerography is due to the high quality of the copies it produces, its ability to make copies from virtually any original, its high copy speed (more than 7,000 copies per hour), and its ability to produce offset masters (seeOFFSET PRINTING and ELECTROSTRATOGRAPHY). In the 1970s, xerographic processes were developed in which multicolored copies could be made from continuous-tone originals.
There are two types of xerography—direct xerography and indirect, or transfer, xerography. In direct xerography, copies are made directly on electrophotographic paper. In transfer xerography, copies are produced by means of an intermediate information carrier, or agent, which consists of a polished metallic sheet (usually aluminum), a cylinder, or a flexible tape. The agent is coated with a photoconductive layer that consists of such materials as amorphous selenium, cadmium selenide, or cadmium sulfide.
Figure 1 presents a diagram of the direct xerographic process. The photoconductive layer of the paper on which the copy will be printed is charged in the dark to a potential of several hundred volts by means of, for example, a corona discharge. When an image of the original is projected onto the charged layer, the charges from the illuminated, or white, portions of the layer leak onto the electrically conductive backing. The portions that are not exposed, that is, the portions corresponding to the dark lines of the original, retain their charge. As a result, a latent image of the original is produced in the photoconductive layer in the form of a charge pattern. The image is usually developed by means of a dyed powder, called the toner, the particles of which have a charge whose sign is the opposite of that of the charge pattern. Attracted to the charge pattern, the particles form a visible image, which may be fixed by heating the powder to its melting point and thereby bonding the particles to the paper’s backing.
In transfer xerography, the latent image is produced in the light-sensitive layer of the agent. Developed by means of an electrified, dyed powder, the image is then transferred onto, say, plain paper or tracing paper. The image fixing process is the same as that in direct xerography.
Xerography is performed on machines that use intermediate information carriers and that produce copies on plain paper, as well as machines that make copies on electrophotographic paper. Xerograpic copiers are differentiated by the methods they use for exposure, development (liquid or dry toner), and fixing of the image. They may also differ according to the size of the originals they can copy, the size of the copies they make, and the degree of automation.
Exposure in transfer copiers that have a plate for an agent is accomplished frame by frame. Machines whose agent is a cylinder or a tape use dynamic methods in which the original, the optical system, and the surface of the agent are continuously shifting with respect to each other. Exposure time depends on the illuminance of the original, the light-sensitivity of the photoconductor, and the quality of the optical system. The Soviet-made ER-620R nonportable rotary xerographic copier, for example, can make copies of design documents on paper rolls 620 mm wide at a rate of nearly 3 m/min.
REFERENCESSlutskin, A. A., and V. I. Sheberstov. Kopiroval’nye protsessy i materialy reprografii i maloi poligrafii i Moscow, 1971.
Protsessy i apparaty elektrofotografii. Leningrad, 1972.
Alferov, A. V., I. S. Reznik, and V. G. Shorin. Orgatekhnika. Moscow, 1973.
Ivanov, R. N. Reprografiia. Moscow, 1977.
A. V. ALFEROV