Abstract: An intruder detection system employs two infra-red sensors 5,60 within the same housing 4. Both sensors have substantially the same field of view 6 and one 5 of the sensors detects an intruder by sensing the latter's emitted IR. The other sensor 60 detects near-IR. In order to detect against the presence of IR masking material in the field of view (which might be used to mask the presence of an intruder) remote units are provided, each emitting near-IR to sensor 60. The output from the remote units may be coded to distinguish them from the ambient IR. A warning is produced if a mask (such as mask 40) blocks one of the remote units.

Abstract: An intruder sensor contains a microwave detector and a passive infra-red detector in the same housing. A panel in the housing defines a series of Fresnel lens segments which define the zones for the infra-red sensitive element. The panel also forms the radome for the microwave detector. In the twin horn microwave detector, the infra-red sensitive element is mounted on a circuit board between the horns.