GB817829A - Improvements in or relating to optical, or like measuring instruments - Google Patents

Abstract

817,829. Photo-electric measuring apparatus. AITKEN, I. M. E. Nov. 23, 1955 [Nov. 23, 1954], No. 33890/54. Class 40 (3). In a measuring instrument for the determination of physical characteristics of a sample, an initial beam of radiation from source L is divided by means of a system of mirrors M1, M2, M3 into two secondary beams which are thereafter subjected to oppositely phased sinusoidal variations of intensity by the passage through masks K1, K2 having apertures A1, A2 occluded alternately by a rotating disc D formed with peripheral apertures S1, S2, S3, &c. The radiation may comprise visible light, or infra-red or ultra-violet radiation. As applied to absorption measurements, one secondary beam is passed through the sample contained in a tube T whilst the other passes through an adjustable optical wedge W. The emergent beams are combined by a second system of mirrors M4, M5, M6 and impressed on a photo-electric cell P which controls an indicator I through an amplifier A. Inequality in the amplitudes of the two beams causes a variation in the light intensity impinging the photo-electric cell and results in a sinusoidal output at the frequency of the cyclic variation imparted by disc D and of a phase determined by which beam is of greater amplitude. For manual operation, wedge W is adjusted to obtain equality as shown by zero or a minimum indication on indicator I. For automatic operation, the output from amplifier A is applied to a motor to control the position of wedge W, a phase-reference signal tc being provided from the rotating disc, by means of a commutator on the shaft or a photo-electric cell receiving light through the disc, so that the desired direction of movement for the wedge may be determined. The apertures A1, A2 are shaped to produce a sinusoidal variation of light intensity, a practical shape which closely approaches the theoretically correct shape being illustrated in Fig. 5 (not shown). In an alternative arrangement, Fig. 1 (not shown), the two apertures are arranged closely adjacent in a common mask and are exposed alternately through the same slit. In another arrangement, the masks may be disposed at diametrically opposed points on the periphery of the disc which is formed with an odd number of apertures. The assembly of masks and disc may be arranged in the apparatus after the sample. The invention may be applied to polariscopes, saccharimeters, absorptiometers, turbidimeters and colorimeters. As applied to a polariscope of the type described in Specification 540,876, [Group XX], the two secondary beams are polarized in planes at 90 degrees and both passed through the sample under test. The emergent beams are then passed through an analyser (e.g. a Nicol prism) before combination and application to the photo-electric cell.