Radiometer, sighting device for an apparatus and process infrared.

Sighting device for an apparatus IR (1) for the visible marking a measuring surface, comprising: at least three light sources (244), so that each of said light sources (244) emitting a beam visible light and said light sources (242) are arranged such that said beams generate bright points at the edge of said measuring surface; and a control circuit (252-260) for switching said at least three light sources (244), said control circuit (252-260) being connected to each of said light sources and being constituted such that at most two light sources are connected simultaneously; wherein the control device performs control connection and disconnection of at least three light sources (244) so ​​that said points are illuminated in a predefined order at a frequency reaches 20 Hz, so that the user has the visual impression that a point moves around said measuring surface, being that frequency in a monotonous relationship with the magnitude of the time deviation of the temperature measured by said infrared apparatus.