facsimile or fax, 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. The surface of the material to be sent is traversed by a light-beam and a photodiode that translates the light and dark areas of the material thus scanned into electric signals for transmission. A receiving station reproduces the transmitted material by a variety of means. Newspapers and television stations have long transmitted and recorded news photographs using a process in which the received electric signals activate a variable lamp that is used to scan a photographic film.

A modern office fax machine scans a page to make an electronic representation of its text or graphics, compresses the data to save transmission time, and transmits it to another fax machine (or computer emulating a fax machine). The receiving machine decrypts the signal and uses a printer (usually built in) to make a facsimile of the original page. Because of the adoption of Group 3 digital standards in 1980 by the CCITT (International Telegraph and Telephone Consultative Committee), facsimile devices have become extremely prevalent in offices. These machines work over the public telephone network; they use digital modems and transmit at data rates up to 9600 bits per second. Images are produced with a resolution of 200 dots per inch. Personal computers can emulate Group 3 facsimile machines if they are equipped with a fax modem, printer, and appropriate software. Facsimile machines that produce higher-resolution images or color and gray-scale images are also available.

in full facsimile

Device for the transmission and reproduction of documents by digitized signals sent over telephone lines. Fax machines scan printed text and graphics and convert the image into a digital code: 1 for dark areas, 0 for white areas. The code is transmitted through the telephone network to similar devices, where the documents are reproduced in close to their original form. Though the concepts for fax technology were developed in the 19th century, widespread use did not occur until the 1970s, when inexpensive means of adapting digitized information to telephone circuits became common.

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