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: A system for locating zones defined by Fresnel lens segments or mirror facets of an optical arrangement in a passive infra-red security sensor uses a prism sheet which deflects light emitted by an LED mounted to the circuit board adjacent the infra-red sensitive detector so that it follows the path of infra-red radiation that would be focused onto the element of the detector without the prism sheet present.

Abstract: A passive infra-red sensor includes a window in a housing which also houses an infra-red detector. The window defines a plurality of infra-red transmitting strips which are separated by Fresnel lens segments or infra-red opaque strips. The infra-red transmitting strips provide short range sensitivity. The Fresnel lens segments where provided give longer range sensitivity.

Abstract: A security installation at one Site has remote sensors which detect intrusion, fire, etc. and transmit corresponding signals by radio to a master station. Similar installations are provided at other Sites. The sensors of each Site precede their data signals with an access code which is particular to that Site and prevents their signals being received by the master station of any adjacent Site within radio range. When a new installation is to be installed at a new, adjacent Site, an arbitrary value is initially selected for its access code and the master station there transmits a special code to all adjacent Sites. In response to this special code, the master station in each such adjacent Site transmits its own access code back to the new installation, where it is compared in value with the arbitrarily selected access code. The latter is automatically changed until it becomes different from that of any and all adjacent Sites. The remote sensors of the new installation are then set to this value.