Abstract: An infrared radiation sensor is provided comprising an infrared optical system and a pyroelectric radiation detector for receiving infrared radiation from the optical system and generating an output signal. The optical system has a lens (6) arranged to feed source radiation through an aperture (9) into a reflective radiation cavity (10), the lens and the aperture defining a radiation sensitive angular zone width and direction (2,3,4,5) for the sensor. The pyroelectric radiation detector comprises a film (11) of pyroelectric plastics material within the cavity. The film area can be made large within the cavity without affecting the angular resolution of the sensor which is controlled by the ratio of the optical system focal length to the aperture width.

Abstract: An array of lenses is provided for focusing thermal infrared radiation onto a detector from a set of arcuately displaced directions 7, 8, 9, 10, 11 to detect intruders. The lenses are formed as fresnel lenses, typically, in a sheet 17 of radiation transmissive material as two planar leaves 18, 19 transverse to one another, one horizontal, the other vertical. With the detector 6 set to receive focused radiation from both leaves, the radiation at each lens is more nearly normal to the lenses, greatly reducing transmission losses from directions almost under the array and from horizontal directions. Also, when moulded as a single sheet, the intersecting edge 20 of the two sheets greatly increases the mechanical rigidity of the array. Being flat, a lip 32 can be provided around the sheet edge for easy mounting and good dust sealing.

Abstract: Array of Fresnel lenses moulded in a single sheet of plastic 29 are used in conjunction with a thermal radiation detector 1 to provide intruder alarm apparatus. Each lens defines one detection direction in a fan of such directions 26,27,28. Such arrays have been moulded as a curved surface 22 centered on the detector. To reduce moulding costs while maintaining the optical efficiencies of all lenses in the array, a flat array of lenses is provided in which each lens is moulded as an angled facet 30,31,32. The poles 36,37,38 of the lenses lie in a plane.