Patent GB-1436238-A (GB)
Title: Light analyzing apparatus
IPC: G01J3/44
Filed: 1973-07-24
Abstract: 1436238 Photo-electric spectrometers ALLIED CHEMICAL CORP 24 July 1973 [14 Aug 1972 25 June 1973] 35310/73 Heading G1A Light having spectral components periodic in frequency is analyzed by means of an interferometer arranged to provide a plurality of regularly spaced transmission frequency windows which are adjusted to match the spacing of the periodic spectral components which are thus passed to form a detectable fringe, scanning means for varying the transmission windows, and means for modulating the phase difference between interfering rays and for synchronously detecting the fringe signal. As shown, Fig. 2, a gas sample 40 is illuminated by a source 36 which may be a pulsed laser and the rotational Raman scattered radiation is applied by a collimator 12 to a secondary Falary-Perot Interferometer (FPI) 24 which passes spectral lines corresponding to the major components of the gas (e.g. O 2  or N 2 ) and reflects that corresponding to the other components to a primary FPI 16 which transmits spectral lines corresponding to a selected component. A corner reflector 34 is arranged to pass the transmitted radiation back through the interferometer to increase the contrast while maintaining a high throughput. The FPI has a spectral transmission characteristic in the form of a "comb" and periodic spectral lines corresponding to a selected gas component are passed, these lines adding to form a fringe which is detected by a photo-multiplier 50. The tuning of the FPI 16 is varied by a small amount to produce intensity modulation of the detected beam and thus permit synchronous demodulation to be used, the resulting signal being applied to an oscilloscope and a chart recorder. The signal may be derived by counting the pulses produced over a selected period or number of modulation pulses. The tuning of the FPI 50 is also scanned across the spectrum. As shown, Fig. 3, the primary FPI comprises a tubular member 60 supported by air bearings 56, 58 and carrying one mirror 62. The position of the member is adjusted by means of a lead screw arrangement driven through gear-box 76 by a digital stepping motor 74. The other mirror 66 of the FPI is mounted on a Piezo-electric ceramic cylinder 92 to which an alternating voltage can be applied to longitudinally oscillate the mirror 66. Instead of the Piezo-electric cylinder a known arrangement can be used in which the pressure of the air between the mirrors is varied to produce the modulation of the tuning. A fixed etalon can be used instead of the adjustable arrangement shown, tuning being achieved by means of temperature control. A plurality of fixed secondary etalons can be used to extract various periodic spectra produced by different gas molecules from the light beam. The light may be passed several times through the interferometer, the contrast increasing with the number of passes. If the radiation source is polarized a polarizer may be included in the analyzed light beam path to reduce the effect of Rayleigh scattering. The gas sample may be located a large distance (e.g. 5 miles) from the detector and the use of a pulsed laserbeam enables the distance to be determined by radar techniques. The outputs of the primary and secondary interferometers may be compared to increase concentration detection accuracy.