An intrusion detection system and a method therefor

A dual sensing intrusion detection system comprising a passive infrared radiation detection sensor (12) which generates a first output signal (32) in response to the detection of an intruder in the volume of space, a second detection sensor (6) which is directed to the same volume of space and generates a second output signal (46) in response to detection of the intruder and a switch (38) which activates the second detection sensor in response to the detection of the intruder by the infrared radiation detector. Logic circuit (36) receives the first and second output signals and produces an alarm signal in response thereto to indicate the detection of the presence of the intruder in the volume of space.