Infra-red monitoring system

An intruder monitoring apparatus 1 comprises a series of passive infrared detectors 2 for detecting infrared radiation within region to be monitored, and providing first signals containing information representing the temperature of objects emitting infrared radiation. The passive infrared detectors 2 are arranged in pairs and each pair of detectors 2 sends output signals to a processor 3 which determines whether an object emitting infrared radiation lies within a predetermined temperature range and occupies a region exceeding a predetermined size. In such a situation, the processor 3 sends an output signal to alarm means 4 which sends an alarm call to a remote location via a radio antenna 5 or the public telephone network 6.