GB1188479A - Optical Filter - Google Patents

Abstract

1,188,479. Light modulation apparatus. MULLARD Ltd. 18 June, 1968, No. 28928/68. Heading H4F. [Also in Division G1] The temperature of a body X is measured by passing the radiation emitted from the body through a polarizer P, a Faraday effect device FE and an analyser A, operating the device FE so as firstly to reject radiation at a first wavelength and then to reject radiation at a second wavelength, and detecting the intensities of the radiation transmitted by the analyser in each case, and then comparing the detected intensities. The ratio between the intensities of radiation at two different wavelengths is different at different temperatures and so by the use of calibration curves, the ratio measured may be used to determine the temperature of the body. In order to reject the two different wavelengths the device FE is chosen to be of a kind that rotates the plane of polarization in dependence on wavelength of the radiation passing through it, as well as on the magnetic field strength applied to the deivce. A suitable material for device FE is Yttrium iron garnet (YIG) or such garnets doped with Gallium (for the iron) and Terbium (for the yttrium). In the example described, the two wavelengths rejected are 1À15Á and 1À9Á (in the infra red region). By application of a square wave alternating voltage and a D.C. voltage from battery 3 to coil 1, two different levels of field strength H 1 , H 2 are applied to the device F.E. With the analyser A and the polarizer P in alignment, and by suitable arrangement of the field strengths H 1 , H 2 , it is arranged that on the half cycles when field H 1 is applied, radiation at 1À15Á. is not transmitted by the analyser A, since radiation at this wavelength has been rotated through 90 degrees with respect to the polarizer (and analyser). Radiation at 1À9Á however is only rotated through 45 degrees and so is partially transmitted. Intermediate wavelengths are transmitted in varying proportions between these levels. All radiation above a certain wavelength near to but higher than 1À9Á is absorbed by a filter F(g). On half cycles when the field H 2 is applied, 1À9Á radiation, having been rotated through 90 degrees, is cut out, while 1À15Á radiation rotated through 180 degrees is passed. To detect the two different intensity levels and compare them, either a single detector D is used, the output of which is switched alternately along lines 5, 6 to a ratio-measuring device 7, or separate detectors individually sensitive to the two wavelengths 1À15Á and 1À9Á are used to avoid the need for switching. If the A.C. source is not a true square-wave (e.g. sinusiodal) peak detectors are used in the detector circuitry. Two manually switched D.C. sources may be provided to provide the levels H 1 , H 2 of the magnetic field as required. The detector(s) may be of the germanium or lead sulphide type.