GB1264809A - - Google Patents

Abstract

1,264,809. Photo-electric detection apparatus. PILKINGTON PERKIN-ELMER Ltd. May 27, 1969 [May 29, 1968], No. 25759/68. Heading G1A. [Also in Divisions G2 and G4] In an intruder detector, the intensities of regions of preferably infrared radiation, polarized in orthogonal directions are compared at 5, and an alarm signal is produced upon entry of an intruder into a polarized region with consequent diminution of intensity in that region as measured at 5. As shown, infrared radiation in the range 0À1-15 microns, produced at 6, is directed through a lens 8 in the form of a mosaic of polarizing areas having mutually orthogonal optical axes to produce the orthogonally polarized regions. A rotating, uniformly polarizing, lens 9 modulates the intensities of the orthogonally polarized regions in anti phase at 15Hz. Concave mirrors 2 mounted on a tower or framework 3 reflect the radiation over a space to be protected to a photo-electric detector 13 which normally detects equal amounts of the polarized regions to produce a constant signal. Entry of an intruder results in a fluctuating signal which is amplified at (14), Fig. 5 (not shown); fed through a bandpass filter (15) with a centre frequency equal to the intensity modulation frequency; amplified further in an amplifier (16) having a resistive negative feedback path whose value may be varied to vary the sensitivity of the detector; and fed to a Schmitt trigger (17) to actuate a relay (18) and an alarm circuit (19), which produces audible and or visible signals. If an intruder enters the protected space close to the detector 5, equal amounts of the polarized regions may be obscured. The detector may therefore be arranged to produce an alarm signal in response to a drop in the total detected intensity. In an alternative embodiment, two sources with associated reflectors are disposed at opposite ends of the space to produce a rectangular "curtain". (For Figures see next page)