Abstract: The measurement of the temperature of a body such as a wire, bar or tube, or a metal sheet, is effected in relation to a black background by providing either a swinging mirror or two flat mirrors and either a concave spherical mirror associated with the flat swinging mirror, or two concave spherical mirrors associated with the two flat mirrors, as well as an infrared radiation detector whose sensitive element receives alternately and successively, after reflection by the flat mirror(s) and collection by the spherical mirror(s), on the one hand, the infrared radiation of the wire and of the black background which surrounds it and, on the other hand, of the black background alone.