photocopying machine, also called Photocopier, any device for producing copies of text or graphic material by the use of light, heat, chemicals, or electrostatic charges. The need for a process other than wet photographic reproduction for copying documents stimulated the invention of various techniques, notably the diffusion-transfer and dye-line processes, during the early 1950s. In the diffusion-transfer process a master copy is made on a translucent sheet, which is placed on light-sensitized negative paper and exposed to light. The negative is then placed in contact with a sheet of positive transfer paper and fed into a developer. When the two sheets are peeled apart, the image is transferred to the positive paper. The dye-line process also requires a translucent original but only one sheet of sensitized paper. This process uses ammonia fumes rather than liquid to develop the image, obviating problems of paper shrinkage.
Another photocopying method that became available in the early 1950s uses the heat of infrared light. In this process, sometimes called thermography, sensitized copy paper is placed in contact with the original and both are exposed to infrared rays. The original absorbs the rays in areas darkened by print or by the lines and shades of an illustration, and thereby transfers the impressions to the surface of the copy paper.
The method most widely used by modern office photocopiers is called xerography (from the Greek words meaning “dry writing”). Although developed by the U.S. physicist Chester F. Carlson in 1937, the process did not become available for commercial use until 1950. Xerography, which involves the application of electrostatic charges and heat, is extremely versatile and can be employed to produce copies of all kinds of written, printed, and graphic matter. The basis of the process is photoconductivity, an increase in the ability of certain substances to allow an electric current to flow through them when struck by light. The chemical element selenium, for example, is a poor electrical conductor, but when light is absorbed by some of its electrons and a voltage is applied, these electrons are able to pass more freely from one atom to another. When the light is removed, their mobility falls. Xerography typically uses an aluminum drum coated with a layer of selenium. Light passed through the document to be copied, or reflected from its surface, reaches the selenium surface, onto which negatively charged particles of ink (i.e., the toner) are sprayed, forming an image of the document on the drum. A sheet of copy paper is passed close to the drum, and a positive electric charge under the sheet attracts the negatively charged ink particles, resulting in the transfer of the image to the copy paper. Heat is then momentarily applied to fuse the ink particles to the paper. The copy paper itself originally provided the treated surface, but the substitution of the selenium-coated drum permitted the use of ordinary paper. Other improvements were introduced, making it possible to print on both sides of the paper, sort and collate, automatically produce a predetermined number of copies, and enlarge or reduce the image reproduced from the original. Xerographic machines capable of duplicating coloured materials became available in the 1970s.