Abstract: Arrays of pyroelectric elements are used in surveillance systems by focusing the radiation from a scene on to them and examining the output from the array. If an object is moved into the scene and left stationary, it will hinder the subsequent operation of the system by masking part of the scene from the field of view of the array; this fault condition may be detected by the following procedure. At intervals arrangements are made to move the image of the scene to and from across the array using a suitable transducer and the outputs from the array are examined. The outputs from the array when the scene is in its normal condition and the image is moved across the array comprise a set of signals corresponding to a reference image, which may be compared with the corresponding outputs from the array when the image is moved across the array on a subsequent occasion.

Abstract: Arrays of pyroelectric elements are used in surveillance systems by focusing the radiation from a scene on to them and examining the output from the array. If an object is moved into the scene and left stationary, it will hinder the subsequent operation of the system by masking part of the scene from the field of view of the array; this fault condition may be detected by the following procedure. At intervals arrangements are made to move the image of the scene to and from across the array using a suitable transducer and the outputs from the array are examined. The outputs from the array when the scene is in its normal condition and the image is moved across the array comprise a set of signals corresponding to a reference image, which may be compared with the corresponding outputs from the array when the image is moved across the array on a subsequent occasion.

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.

Abstract: A security installation comprises a plurality of sensors in and around a building and which transmit digital information to a central station by radio, in each case preceded by an access code specific to the particular installation; signals inadvertently received from the sensors of an adjacent installation are rejected. The information is transmitted by encoding a multiple bit word incorporating the access code, a code indentifying the particular sensor, and the actual data. The sensor transmitters may drift over a wide bandwidth. To avoid using a wide band receiver in the master station, the receiver bandwidth is narrow but swept over the wide bandwidth. As soon as a signal is detected, the sweep is halted until the recognition process has been completed.

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.

Abstract: A security installation has four, for example, normally-closed sensor switches which open in the presence of an intruder. Each switch is in series with a respective resistor and these series combinations are connected in parallel across a pair of bus lines which connect to a central monitoring point. There, normally open test switches are connected across the bus lines in series with respective resistors which have values corresponding to the resistors of the sensor switches. Constant current is applied to the bus lines, and a voltage monitor senses the rise in voltage when one of the sensors opens, and this initiates a scanning unit which closes the test switches in sequence. When the test switch corresponding to the opened sensor is closed, the line voltage reverts to the predetermined value, and the closed test switch identifies the operated sensor.