Apparatus for detecting the emission of infrared radiation

Apparatus for detecting the emission of infra-red radiation eg from pedestrians waiting at a pedestrian crossing, comprises optical means such as a lens 14 for projecting an image of a three-dimensional zone to be detected into the apparatus, an array of heat sensors 15 within the apparatus for detecting the emission of infra-red radiation from each of a plurality of sub-zones of the zone to be detected, and electronic circuit means 16, 17 responsive to the outputs of the sensors 15 for producing a signal indicative of the presence and/or absence of an infra-red emitter significantly different from the level of background infra-red emission from the detected zone. The sensors may be thermopiles and their output used to control traffic lights.