Theory Of Photo Conductivity, Xerography,

Theory Of Photo Conductivity, Xerography,  electrostatic dry-printing process for the reproduction of images or documents, widely employed in commerce and industry in copying machines (see Office Systems).

The process was invented by the American physicist and patent attorney Chester F. Carlson in 1937 and first commercially developed in 1950. It makes use of the principle of photoconductivity, that is, that certain substances resist passage of an electric current except when struck by light. Silicon, germanium, and selenium are poor conductors of electricity, but when light energy is absorbed by some of their electrons, the electrons are able to pass from one atom to another, thus allowing a current to flow when a voltage is applied. When the light is removed, their conductivity again becomes low. Xerography employs a photoconductive insulating layer, such as selenium, on an aluminum or other conductive metal support.

The layer is charged electrostatically, either with positive or negative ions (see Ion), the polarity of the charge depending on the type of photoconductive insulating layer selected. When the plate is exposed, in a camera or photographic machine, those areas of the coating subjected to light lose a varying portion of the charge, depending upon the intensity of the illumination. Thus, the variation of the amount of charge retained on the coated metal plate is established as an electrical or electrostatic pattern of the image. The image is rendered visible by sprinkling over the exposed plate a special, charged powder, which carries an opposite charge to the initial charge applied to the plate and insulating layer. The powder adheres to those areas that have retained their charge. The print is obtained by covering the plate with paper, then applying a charge over the back of the paper of the same polarity as the initial charge applied to the photoconductive insulating layer. In this way the opposite charged powders are transferred to the paper surface. The powder image is then fused onto the paper by exposure to solvent vapors or heat to make the image permanent.

The entire xerographic process can be carried out, in high-speed mechanized equipment, in less than 5 seconds, and it is comparatively inexpensive to make these images because the photoconductive insulating layer can be recycled many thousand times. The process has found its primary usefulness in copying office documents and in low-volume duplication of data.

The xerographic method also permits the making, quickly and cheaply, of paper offset masterplates for low-to-medium-volume runs on office offset-printing presses (see Printing Techniques). The method has also been applied to the production of X-ray images in a technique that is known as xeroradiography and is used in mammography for early detection of breast cancer. Xerography is applied in industrial nondestructive testing.

A variety of automatic xerographic machines are available today that make office copies on the push of a button and that are almost unable to fail. Xerographic machines can reproduce half-tone photographs and can enlarge or reduce copy. They can be set to print on either one or both sides of paper, to produce automatically a large or small number of copies without further intervention by the operator, and to sort and collate. A development of the late 1970s is color xerography. The economy, versatility, and flexibility of the xerographic process have created legal problems relating to copyright and helped stimulate numerous changes in the copyright laws in the U.S. and elsewhere.

Another machine, called Copyflo, developed in the late 1950s, can be used to reproduce enlarged images from microfilm and has demonstrated that an entire book can be printed from microfilm in from 3 to 5 minutes. Copyflo is used primarily to make copies of out-of-print books. See also Facsimile Transmission.