photocopying

photocopying process whereby written or printed matter is directly copied by photographic techniques. Generally, photocopying is practical when just a few copies of an original are needed. When many copies are required, printing processes are more economical. However, when a printing process is used, the master or stencil required can sometimes be produced by photocopying. Principal photocopying processes include silver halide, transfer, plan, thermographic (see thermography ), and electrostatic (e.g., xerography , which has become so widespread that the process is popularly almost synonymous with photocopying). Two well-known silver halide processes, photostating (see photostat ) and microfilming, use cameras to make photographic copies of an original. Microfilming generates copies that are from 1/12 to as little as 1/100 the size of the originals, allowing great economy in space and materials when long-term storage is necessary. Microfilms are read by either projecting them or photographically printing them as enlargements. In transfer processes the original is placed in contact with negative paper and exposed to light. In the diffusion transfer the negative is developed while in contact with the positive. During development the chemicals forming the image in the negative diffuse to the positive, producing an image there. In gelatin transfer the negative is developed and then pressed against positive paper. A dyed gelatin on the negative is picked up by the positive, producing an image on it. Transfer methods are less expensive than silver halide processes, but the image produced by the former deteriorates with time. Plan copying is used to copy materials such as architects' drawings and engineers' plans. In one variety of plan copying known as the whiteprint process, an original is made on translucent paper. The paper is placed over a sheet coated with a diazo compound and exposed to a source of ultraviolet light. The compound covering the area that is exposed to the light decomposes. The compound shielded from the ultraviolet rays by the dark areas of the original can be developed to form a positive image. The blueprint process, another method of plan copying, has been largely superseded by whiteprints, which are of better quality and cost approximately the same.

photocopying Reproduction of words, drawings, or photographs by machine. In a photocopying machine, a light shines on the item to be copied, and an optical system forms an image of it. Various techniques may be used to reproduce this image on paper. In a modern plain-paper copier, the image is projected onto an electrically charged drum, coated with the light-sensitive element selenium. Light makes the selenium conduct electricity, so bright areas of the drum lose their charge. The dark areas, which usually correspond to image detail, retain their charge, and this attracts particles of a fine powder called toner. Electrically charged paper in contact with the drum picks up the pattern of toner powder. A heated roller fuses the powder so that it sticks to the paper and forms a permanent image.