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thermography(thûr'mŏg`rəfē), contact photocopying process that produces a direct positive image and in which infrared rays are used to expose the copy paper. In a specially designed machine the original is placed in contact with a copy paper containing a heat-sensitive substance. As the infrared rays produced in the machine impinge on the dark or printed regions of the original, the heat generated is transferred to the adjoining areas of the sensitive paper. These areas become dark as well, creating an image of the original. As infrared rays are difficult to focus, the image formed is lacking in sharpness, resolution, and detail. The copies remain sensitive to heat, so that they may darken further with age. The advantages of this process are that it is low in cost and fast and can be performed with simple equipment.
(1) In the broad sense, the recording of the heat field of objects—that is, the field of their infrared radiation.
(2) See (in photocopying).
(in photocopying), a copying process based on the ability of heat-sensitive materials to change their state under the action of heat—in this case, infrared radiation. In the case of direct, or thermochemical, copying, copies are made by the contact method on thermal copy paper, using transillumination or reflected radiation. In transfer copying, copies are produced on a copy base, using thermal copy paper or film. In both cases, copies may be made from originals in india ink or black pencil or from typewritten or typeset material; the elements of such originals are capable of intense absorption of heat.
Upon exposure to infrared radiation, the light—that is, blank—portions of the original reflect most of the rays, whereas the dark portions absorb the rays and thus are heated. In contact thermography, heat from a heated element of the original causes a chemical reaction in the sensitive layer of the thermal copy paper, with which it is in contact, resulting in the formation of a dark contrast substance. In transfer thermography, the sensitive layer of a thermoplastic film or thermal copy paper melts under the action of heat and is transferred to the copy base.
Copies on thermal copy paper darken with time as a consequence of the action of heat and light on the nonprinting elements, which remain heat-sensitive; therefore, the storage life of such copies is limited. Thermoplastic copying makes possible the production of printing plates to be used in small-job printing; in addition, it makes possible the production of copies for prolonged storage.
REFERENCEAlferov, A. V., I. S. Reznik, and V. G. Shorin. Orgatekhnika. Moscow, 1973.
A. IA. MANTSEN