GB1318626A - Apparatus for analysing optical radiation - Google Patents

Abstract

1318626 Photo-electric spectral analysis MULLARD Ltd 16 Nov 1970 54431/70 Heading G1A In a double beam apparatus for measuring the spectral distribution of energy in a radiation beam 1, in which the beam is passed cyclically and alternately through different pass-band filters 11, 15 and the radiation from both paths 8, 9 is subsequently incident on a photo-cell 14, the path switching is achieved by causing the plane of polarization of the beam 1 to alternate between two mutually perpendicular values and using a polarization sensitive device 7 to direct the beam along one of paths 8, 9 according to its instantaneous polarization direction. The beam, e.g. of infrared radiation, is first plane-polarized by its passage through a Brewster window 2. The beam then passes through a Faraday effect element 3, comprising a YIG crystal or a derivative thereof, located in a magnetic field modulated by a square wave source 4. The field strength is adjusted so that the polarization plane of the emerging beam 1" alternates between two perpendicular values. Further Brewster windows 7, 12, which are reflective or transmissive according to the polarization of the incident beam, first separate and then recombine the beam components in cyclic sequence. The fluctuating photo-cell output provides an indication of the relative amounts of energy in the passbands #1(II) and #2(15) in the original beam. By using a synchronous detector 17, this indication may be derived at a terminal 19. It is often advantageous to have a zero detector output for incident radiation having a predetermined spectral content. To achieve this, the bandwidth of one of the filters may be varied, or a variable aperture or neutral filter may be placed in one path. Alternatively, a band-rejection filter 18 may be provided, whose cut-off wavelength is a function of its inclination to the incident beam. This cut-off wavelength is arranged to be adjustable within one of the pass-bands # 1 , # 2 , hence allowing selective attenuation of that band. In a further embodiment, Fig.3 (not shown), compensating adjustment of one beam is achieved by having a further Faraday effect device (25) in one path. By altering the magnetic field across a YIG crystal, the polarization of the beam 8, and hence the transmissivity of the window 12 is adjusted. The Faraday device may be included in a negative-feedback null-balance control loop so that the magnetic field is automatically adjusted to give zero photo-cell output. In this case, the current output from the magnetic field generator gives the spectral distribution of the radiation. In all the embodiments, the Faraday devices may be replaced by electro-optical devices such as Kerr cells or Pockels crystals, and the focusing lens 13 may be replaced by a mirror. The apparatus is preferably mounted in a dustfree enclosure 32. Applications. The apparatus may be used as a two-colour # 1 , # 2 ) radiation pyrometer to measure the temperature of a furnace or other incandescent body. It may be used to detect the presence of a jet aircraft in the presence of reflected sunlight. In this case, a null or reference output is achieved using incident radiation from the sun. An aircraft crossing the field of view represents a black body at a substantially lower temperature, and hence changes the spectral content of the incident radiation. The apparatus can also be used in the absorption analysis of a gas sample, Fig.4 (not shown), by investigating the differential absorption of the wavebands #1 and #2 within the gas. In this application the gas is irradiated with a collimated beam of radiation.