GB1320002A - Detector systems - Google Patents

Abstract

1320002 A warning device PILKINGTON P E Ltd 3 Sept 1971 [27 June 1970] 31268/70 Heading G1A In a warning device, suitable for detecting the presence of an obstruction in a predetermined space or area such as a level crossing, radiation from a source is split by a modulating and separating means into two beams, passing through the area, the beams being modulated in a complementary fashion Fig. 2 (not shown) such that the total intensity of the radiation is constant, and whereby variation related to the modulation arises in a detector receiving the beam when an obstruction is present. In one embodiment, Fig. 1, light from a source 1 is split up into cylically modulated beams B 1 B L , a reference and a test beam, by a rotating disc 3 which is partly reflecting and partly transparent (see dotted lines). The beams are received by a photo-sensitive detector 11 when no obstruction is present in the test beam and the detector emits a D.C. signal but when an obstruction is present some of the radiation does not reach the detector resulting in the detector producing an A. C. signal which triggers an alarm 12. The apparatus shown in Fig. 1 is schematic and in the level crossing application the source and the detector are housed in a single unit Fig. 4 (not shown) which sends the three test and three reference beams in opposite directions the beams being reflected back by six retro-reflectors spaced equi-distantly on either side of the unit. In a further similar embodiment, Fig. 3 the separating and modulating is achieved by directing light from a source 13 via a polarizer 15, and polarization direction modulator 16 (electro optic or magneto optic) to a polarization separator 17. The separator 17 is adapted to transmit light polarized in one plane and to reflect light polarized in the orthogonal plane. Alternately the polarizer 15 and the modulator 16 may be replaced by a rotating polaroid disc. In another embodiment, Fig. 6, light from a source 22 is directed as shown via a mask 25, a circular polarize 26, and rotating quarter wave plate in the form of a disc 27 to impinge on a polarization selective beam splitter 30, thereby forming the two beams B 1 B 2 which are directed via quarter waveplates 32, 33 to retro-reflected screens 20, 21. The beams are reflected back, and since they pass through the quarter wave plates 32, 33 twice they are able to pass back through the beam splitter 30 along a different path as shown, and be received by a detector 36. A rotatable analyser disc 37 is included to permit adjustment of the balancing of the system. A logic circuit Fig. 7 (not shown) for processing the detector output is disclosed.