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In physics, the transmutation of heat rays into others of higher refrangibility; a peculiar transmutation of the invisible calorific rays, observable beyond the red rays of the spectrum of solar and electric light, into visible luminous rays, by passing them through a solution of iodine in bisulphide of carbon, which intercepts the luminous rays and transmits the calorific. The latter, when brought to a focus, produce a heat strong enough to ignite combustible substances, and to heat up metals to incandescence; the less refrangible calorific rays being converted into rays of higher refrangibility, whereby they become luminous.

The process of transformation of rays of low refrangibility (non-luminous rays) into those of higher refrangibility (luminous rays) by the interposition of a body to be heated, such as a piece of platinum or carbon.