GB1232112A - - Google Patents

Abstract

1,232,112. Photo-electric absorptiometers. MULLARD Ltd. May 17, 1968, No.23632/68. Heading G1A. [Also in Division G2] Beam switching in an alternating type of twopath absorptiometer is effected by a Faraday element switching the plane of an incoming beam of polarized light between two positions and a device which selectively directs light of these planes of polarization along the two paths. Monochromatic plane polarized infrared light from a source 1 is switched between planes of polarization at 90 degrees to one another by a yttrium iron garnet Faraday cell 3 and falls on a germanium plate 7 at its Brewster angle. The beams of different polarization pass through a test cell 11 and a reference cell 12 to a detector 14 connected to a phase sensitive rectifier 21. The windows of cells 11, 12 are at the Brewster angle to enhance the polarization selection of the plate 7. The error signal from rectifier 21 corresponding to the difference in absorption in the cells 11, 12 controls a Faraday cell 17 which together with a Brewster angle reflection polarizer 10 constitutes an attenuator in the reference path. A meter 20 measuring the error signal indicates the difference in attenuation. A polarizer 16 reduces the intensity of the residue of wrongly polarized light transmitted by plate 7 and permits equalization of the light fluxes in the paths. The absorptiometer may be used to measure the alcohol content of a driver's breath.