Patent GB-896686-A (GB)
Title: Improvements in and relating to infra-red analysers
Inventor: MARTIN ALBERT EDWARD
IPC: G01N21/31
Filed: 1959-11-20
Abstract: 896,686. Infra-red analysis of fluids. GRUBB PARSONS & CO. Ltd., HOWARD. Nov. 10, 1960 [Nov. 20, 1959], No. 39480/59. Class 40(3). In order to use robust energy detectors of the photo-conductive or photo-e.m.f. type in an infra-red absorption fluid analysis device interference type filter means are interposed in the path of the radiation to limit the spread of wavelength falling on the detector. The invention is concerned with two-path systems in which the radiation is transmitted over the two paths alternately and so arranged that the wavelengths transmitted in one beam path differ from those of the other. The wavelengths in one path require to be those intensely absorbed by the substance of interest in the sample and test samples are placed at least in this radiation path. In one embodiment a glowing alloy wire A radiates energy towards a rotating disc B provided with two sets of angularly alternate radial slots, passing radiation alternately to the upper and lower parts (as shown) of collimating lens D. The upper radiation beam passes through interference type filter E and the lower beam through filter F before both beams traverse sample holder G and are directed by lens H to detector J. The A.C. output from the detector is amplified at K and its value indicated by meter L. The filters are arranged to pass preferably adjacent spectral regions one of which is strongly absorbed by the substance of interest and the apparatus is initially adjusted by means of shutter M so that there is zero output with no sample present. In this embodiment the sample should absorb little or no radiation in the spectral region transmitted along the reference path. 'In a modification it is proposed that the spectral contents of the reference path should lie in two separate bands on each side of the band of wavelengths passed to, and suffering absorption by, the sample. This arrangement may be designed to stay in balance in spite of any temperature variation of the source. In another modification (Fig. 2, not shown) identical interference filters are used for the filters E, F but they are set at different angles to the beams passing through them so that their spectral pass-bands differ. Fig. 3 shows a system using a single filter E and a single lens H. The variation in the angle of incidence of the rays from the source on the filter provide in effect two adjacent pass-bands for the beams above and below the optical axis. These beams are fed alternately through the system by rotating shutter C and only one of them is absorbed by the sample substance at G. Spherical mirrors instead of lenses may be used to direct radiant energy through the system and the order of the elements in the system can be varied. Alternation of the beams may be achieved by a vibrating member with two staggered apertures instead of by the use of a rotating shutter. In other variations the reference beam does not pass through the sample, two sources, one for each beam path, may be exposed alternately, or the A.C. output may be used to adjust an optical attenuator in the reference path until the radiation falling on the detector is constant.