Patent GB-1193281-A (GB)
Title: Spectrophotometer
IPC: G01J3/12
Filed: 1967-06-06
Abstract: 1,193,281. Photo-electric spectrometers. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AEROSPATIALES. June 6, 1967 [June 16, 1966], No. 26041/67. Heading G1A. [Also in Division G2] A double-beam spectrometer, of the type using grid-like input and output optical devices, is used to determine the variation in the absorption properties of a sample with wavelength, and includes a vibrating optical member to oscillate an image of the input device across the output device, a tuned A. C. amplifier being provided in the circuit of the photo-electric detector sensing the spectrometer output. In one embodiment, Fig. 1, the input device 31 comprises a plate having reflecting and tranparent zones. Radiation received by the device 31 from a source 10 and beam splitter 14 via the sample path 16, 33, is reflected from the reflecting zones to oscillating, parabolic mirror 37. The mirror oscillated about axis 38 is arranged to project an image, dispersed by grating 39 on to an output device 41, corresponding in pattern and orientation with plate 31, such that when mirror 37 is at its mean position, no radiation from this sample path is passed by the plate 41. However, a maximum amount of radiation from the reference beam path 15, which passes through the transparent zones of plate 31, passes through plate 41 at this time. At other positions of the mirror 37, more and less radiation from the sample and reference paths, respectively, is passed by plate 41. The output of the spectrometer monitored by detector 22 is thus the sum of the radiation intensities from the sample and beam paths passed by the plate 41 and represents the attenuation by the sample at the wavelength set by the position of adjustment of grating 39. In fact, this output is an A. C. signal having an amplitude corresponding to this sample attenuation and a frequency equal to the frequency of oscillation of mirror 37. By the use of an A. C. amplifier tuned to this frequency the attenuation of the sample is measured. When no sample is present, the A.C. output is zero. Further, radiation due to other wavelengths than that of interest is detected as a constant amplitude level, which is thus removed by the use of the tuned amplifier. The grating 39 may be slowly (in comparison with the oscillation of mirror 31) rotated to give a wavelength scan. In an alternative arrangement of this embodiment, the superimposition of the image of the input device onto the output device occurs when the mirror 31 is at one extreme of its oscillation period, the output of the detector having twice the frequency of the mirror oscillation frequency and requiring a suitably-tuned amplifier. The output of the detector may be used to control an attentuator comb (53), Fig. 7 (not shown) in the reference beam path 15. A null method is used, the attenuator (53) setting being measured. In a second embodiment, Figs. 8, 9 and 11 (not shown), the input and output devices of Fig. 1 are replaced by an array of four such devices, Fig. 10, and a mirror system for directing reference and sample beams towards this array. The sample beam (63) is directed to one pair of surfaces 72, 75 from which radiation is reflected (by the reflecting zones) to the spectrometer (the oscillating mirror and the grating). This returns to the radiation to the array, radiation from surface 72 being reflected back on to surface 75 and vice versa. Similarly, the reference beam is directed at surfaces 73, 74. The orientation of the patterns on the four zones is such as to produce a light output ranging in accordance with sample attenuation as before. A typical grid pattern for an input/output device of Fig. 1, or a facet of Fig. 10 is illustrated in Fig. 12 (not shown).