Patent GB-1056566-A (GB)
Title: Optical interferometers
Inventor: MARTIN ALBERT EDWARD
IPC: G01J3/453
Filed: 1964-02-26
Abstract: 1,056,566. Spectral analysis using interferometer. SIR HOWARD GRUBB PARSONS & CO. Ltd. Feb.16, 1965 [Feb.26, 1964], No. 7933/64. Heading G1A. [Also in Division G2] An optical interferometer is used for recording the spectral characteristics of a sample and involves the combination of two radiation beams, one being of variable path length, to form a single beam for reception by a detector, and the alternate positioning of the sample in and out of the single beam for discrete intervals of time (by movement of either sample or beam), to produce concurrently two interferograms. The optical system is an adapted Michelson interferometer Fig. 1 (not shown), having a number of fixed mirrors and one movable one M 8  to change the path difference between the two beams. These are combined, and then focussed, by a condensing system M 4 , M 5 , on to the test point F. A further condensing system M 6 , M, focusses light from point F on to the detector D which may be a Golay cell. Radiation, especially in the range 20-1000Á, from the source S is initially chopped to give an A.C. signal from the detector, which is rectified and smoothed. In one embodiment the test sample and a comparison sample are mounted in cells V 1 , V 2  on a sector shaped member R Fig. 2 (not shown), which is moved about a pivot H by alternately energized electromagnets E 1 , E 2 , to bring either sample or comparison substances into the single beam at the point F, where it is held for sufficient time for the signal intensity to be recorded on a punched tape. The values recorded thus represent alternate values of intensity, for the sample and the comparison substance, as the path difference between the two interferometer paths varies as a result of movement of mirror M 8 . The mirror may be moved in discrete intervals but is more conveniently moved continuously at constant speed. The interferogram Fig. 4 (not shown), of a comparison substance, having a black body spectral characteristic Fig. 3 (not shown), tends to drop off quickly in amplitude, while that, Fig. 5 (not shown), for a sample whose absorption spectrum exhibits fine structure has appreciable amplitude over the range of path differences considered and errors may arise due to the wide amplitude differences when these interferograms are obtained together. It is therefore necessary to include a background absorbing substance, exhibiting a fine structure spectrum, continuously in the beam path, to provide comparable amplitudes. The apparatus is usually operated in an evacuated chamber to avoid the very intense absorption of radiation by water vapour, but small amounts of this may be introduced to act as the background absorbing substance into a special sample changer compartment which can be separately pumped down. Other methods of introducing water vapour or other background substances, are described. In another arrangement, the comparison substance may be a specimen of the sample material, but of different thickness or concentration, as appropriate. The comparison substance may be one of known spectrum with a fine structure, in which case a background absorbing substance is not necessary. Interferograms may also be produced with a sample cell and an evacuated cell, whose atmosphere constitutes a comparison material. In another embodiment, the sample and comparison material may be fixed and the beam switched to pass through either alternately. Various considerations are again necessary as above in respect of water vapour and any inequalities in absorption between the two paths. The two interferograms produced on punched tape are dealt with by a digital computer and can provide details of the absorption or transmissivity of sample and comparison substances over a wide range of wavelengths.