Abstract: The invention provides a surveillance arrangement, and in particular an airborne surveillance arrangement, in which a television display of a scene is brightened up, by means of infra-red detectors, to emphasize heat emitting targets.In one example of airborne surveillance system, a linear array of infra-red detectors is arranged to view part of a scene viewed by a television camera, with the same horizontal field of view at a desired target range. In front of the array of detectors a light chopper is provided so that the output of each detector is a modulated carrier. The output of each detector is applied to a store, via individual impedance transformers, narrow bandpass filters and demodulators. Between each demodulator and the store, individual sharp out-off low pass filters and threshold detectors may be provided to provide a degree of discrimination against spurious responses.