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the procedures used to obtain photographic images on light-sensitive electrophotographic materials. The materials consist of a photoconductive layer that has a high dark resistivity and that is applied to an electrically conductive base, or backing. In order to form an image, the backing is grounded, and the photoconductive layer is sensitized by being charged with ions that are usually produced in a corona discharge. The electrophotographic materials, which are now uniformly charged, are exposed to light. Some of the charge “leaks” from the exposed portion of the photoconductive layer onto the backing. The higher the illuminance, the greater the leakage. A latent photographic image is produced in the form of a charge pattern; that is, a distribution of the electrostatic potential corresponding to the distribution of illuminance recorded in the image is formed on the surface of the photoconductive layer. The latent image is then converted into a visible image. Thus, when electrophotographic materials are sensitized, an electric double layer is formed by the surface charge and by the screening charge that occurs in either the body of the photoconductive layer or in the electrically conductive backing. The photoconductivity of the layer causes subsequent local modulation of the layer’s stability, which is a product of the surface-charge density and the thickness of the double layer.
Several forms of electrophotography exist; they differ in the method used to render the latent image visible. In classical electrophotography, the latent image is converted into a visual image by charged, dyed particles of powder that are used either in a dry state or in a liquid solution. In some cases the particles are subsequently transferred onto a nonphotosensitive base.
Xerography is the name often given to electrophotographic processes in which dry powder is used to convert the latent image. It is possible to produce negative and positive black-and-white, monochromatic, and multicolor images by changing the sign of the charge and the color of the powder. In some forms of electrophotography, the latent image is scanned element by element with optical, electron-beam, or electrostatic detectors.
Electrophotography based on the thermoplastic recording on a photosensitive layer usually allows for thermoplastic conversion of the latent image by converting the charge pattern into a surface relief pattern; the conversion is based on the thermomechanical properties of the electrophotographic materials (see alsoTHERMOPLASTIC RECORDING and PHOTOPLASTIC RECORDING). In a form of electrophotography that uses photoelectrets (see) as electrophotographic materials, the latent image forms when the stable electric polarization of the photoconductive layer is partially destroyed upon exposure to light. In some cases, such as in electrophotography in which the latent image is scanned with detectors, the connection of external power sources permits an intensification of the latent image. To a certain extent, such intensification is similar to the intensification that occurs in the classical photographic process. In other cases, for example, when powder is used to render the image visible, no intensification occurs.
The photosensitivity of the most widely used electrophotographic materials is rated at 1–2 GOST (State Standard) units for amorphous selenium layers that are developed with dry powder and that have a resolution of 40–60 mm–1. The sensitivity is 0.2–0.3 GOST units for dye-sensitized layers of zinc oxide dispersed in a binder (the resolution is 60–100 mm–1 or higher, and the layer is developed with a liquid solution) and for layers based on an organic photoconductor, such as polyvinyl carbazole. When an electron beam is used to scan and intensify the latent image, the photosensitivity of electrophotographic materials can be as high as 500 GOST units.
The sensitivity of electrophotographic materials ranges from the X-ray region to the near-infrared region of the spectrum. The sensitivity at the long-wave boundary of the range can be extended in photoconductors through sensitization based on the internal photoelectric effect. In addition to the usual optical sensitization, structural and injection sensitization are also used in electrophotography.
In structural sensitization, the molecular structure and texture of the photoconductor as well as the macrostructure of the photoconductive layer are altered. This method is used both for organic photoconductors, such as vinyl polymers and organic polymer complexes with a polyvinyl carbazole base, and for inorganic photoconductors, primarily those based on selenium and its alloys with such elements as tellurium, arsenic, thallium, cadmium, and germanium. Structural sensitization includes the formation in electrophotographic materials of an electron-hole heterostructure (seeSEMICONDUCTOR HETEROJUNCTION) or a structure of the photoconductor-dielectric type. Photoinjection of charge carriers into photosemiconductors may be used for sensitizing polyvinyl carbazole films with selenium. The method is known as injection sensitization (seeSEMICONDUCTOR: Nonequilibrium charge carriers; Photoconductivity).
Certain characteristics and combinations of characteristics are often essentially unattainable in other photographic processes. They include real-time processing, that is, processing that is simultaneous with rather short-term processes; the possibility of lengthy preservation of the latent image, sometimes even when the image is exposed to light; multiple rerecording of information, if desired; and economy. As a result, electrophotography has found wide application in the small-scale rapid reproduction of texts and graphics, which is known as reprography. Electrophotography is also used as a recording and research tool in such fields of science and technology as roentgenography, holography, spectroscopy, and semiconductor physics.
REFERENCESSchaffert, R. M. Elektrofotografiia. Moscow, 1968. (Translated from English.)
Grenishin, S. G. Elektrofotograficheskii protsess. Moscow, 1970.
Protsessy i apparaty elektrofotografii. Leningrad, 1972.
IU. A. CHERKASOV