a system for photographically displaying the magnitude of a physical variable . in one embodiment , a sensor mounted on a movable probe produces an electrical signal indicative of the physical variable . the electrical signal then drives a lamp which is also mounted on the movable probe . a photographic time exposure of the lamp made while the probe scans an area provides a two - dimensional map of the physical variable . in another embodiment , a position transducer measures probe movement to provide an output indicative of the position of the sensor . the transducer output drives a positioning mechanism on which the lamp is mounted so that movement of the lamp corresponds to the movement of the sensor .