Patent GB-1157086-A (GB)
Title: Improvements relating to Spectrophotometers
Inventor: FORD MICHAEL ALAN
IPC: G01J3/08
Filed: 1965-08-11
Abstract: 1,157,086. Photo-electric spectrometer. PERKIN-ELMER Ltd. July 22, 1966 [Aug. 11, 1965], No.34398/65. Heading G1A. In a double-beam spectrometer, a first beamswitching device 6 directs radiation from a source S alternately along sample and reference beam paths, a second beam-switching device 21 directs the beams from the sample and reference paths alternately at full aperture to a detector 41, which produces an output including components in quadrature representing the sample and reference beams, and the re-radiation (i.e. radiation emitted by sample, reference, or path elements due to their temperature) from the sample and reference beam paths is cancelled out before the ratio of the sample and reference components is determined. The optical system comprises mirrors 1-5 focussing radiation from an infrared source S on to a first rotary beam-switching device 6. The deivce 6 has reflecting and transparent portions so that the beam is directed alternately along the reference path 17-20, which may or may not include a reference object 26, and along the sample path 17<SP>1</SP>-20<SP>1</SP>, including test sample 25, to second beam-switching device 21. The device 21 has similar reflecting and transparent portions, but rotates at twice the speed of device 6. It is arranged that the radiation reaching the detector 41 via mirror system 32-35, mono-chromator 37, filters 39 and mirror 40, comprises, in sequence: firstly, radiation passing through the sample path (S + S o , where S o  represents reradiation in the path); secondly, radiation through the reference path (R + R o ); thirdly, re-radiation from the sample path (S o ); fourthly, re-radiation from the reference path (R o ). The detector (a thermocouple or Golay cell) produces an output representative of this sequence which is demodulated by a special commutator system, Fig. 12 (not shown). The amplified detector output is fed direct to one commutator (62), turning in time with beam-switching device 6, and via a phase inverter (63) to a second commutator (69) which is mounted with its segments in opposition to the first. The combined commutator outputs are produced as separated reference and sample signals on respective conductors (78, 79). A servo-potentiometer is used to produce a ratio of the integrated reference and sample signals, which is recorded (91). Various alternative constructions of beamswitching devices 6, 21 are given. They may be both the same, a disc divided into reflecting and transparent halves, Fig. 3 (not shown). Alternatively, the splitter 6 only may be of the latter type, the device 21 comprising one transparent quadrant and one reflecting quadrant, the other half of the disc being opaque (and non-reflective), Fig. 2 (not shown). If a beam-switching device 21 comprising four quadrants, alternately transparent and reflective, were used, this would be rotated at the same speed as the beam-switching device 6. A combined device, Fig.4 (not shown), the outer portion defining the first beamswitching device and the inner portion the second, and formed with alternate transparent and reflecting quadrants, may be used instead of two separate devices 6 and 21.