Intruder detector

Claim 1

An infra-red intrusion detector for monitoring any unwanted disarming thereof and/or any approach of an intruder into the monitored area comprising: a housing having an infra-red transparent window for the passage therethrough of infra-red radiation into the housing interior, an infra-red radiation detector within the housing, an optical means within the housing for providing an incidental output onto the detector by focussing the infra-red radiation arising from the approach of the intruder a modulated infra-red radiation generator means mounted externally of the housing for directing radiation through the window and for providing a normal recognizable radiation output onto the detector, signal processing means for monitoring the normal recognizable and incidental outputs on the detector insuring against disablement by any rendering of the window opaque to infra-red and for generating a signal in the event of the normal recognizable output from the detector falling below a pre-set value due to insufficient radiation reaching the detector from the modulated radiation generator and/or a change in the radiation reaching the detector commensurate with the approach of the intruder.