GB755900A - Improvements in or relating to photometric apparatus - Google Patents

Abstract

755,900. Photo-electric measuring apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Aug. 18, 1954 [Aug. 18, 1953], No. 22788/53. Class 40 (3). In apparatus for determining the concentration of suspensions of particles in a liquid medium a photo-electric comparison is made of the light transmitted by the suspension and that scattered in a forward direction by the suspension. As shown, light from a source 1 having a straight helical filament passes to photo-electric cells 2, 3. The path to cell 2 includes a compound plano-convex lens 9, having a filter 18, rotatable optical wedge 13, rectangular slit 12, compound plano-convex lens 10, including between the two components a cell containing the sample under test, and a slide 14 having an opaque rectangular stop 16 and a rectangular aperture 17. The slide may be so positioned as to allow either the transmitted or scattered light to reach cell 2. In the position shown, scattered light only is collected by lens 15 and passes through a variable compensating stop 20 to cell 2. Cell 3, with filter 4, diffusing screen 5 and iris diaphragm 6, and an optical wedge comprising thumb-screw 7 constitutes an optical comparison arrangement. In use, the slide 14 is positioned so that opaque stop 16 is aligned with the optical axis of the system and the light from slit 12 is focused on to opaque stop 16 by lens 10. Initially wedge 13 is set at its position of minimum density and iris diaphragm 6 adjusted until the outputs of the two photocells, as determined by a null circuit comprising a microammeter, are equal. Slide 14 is then shifted so as to align aperture 17 with the optical path. Wedge 13 is then re-adjusted and its position at balance indicates the ratio between the transmitted and scattered light. To compensate for the effect of stray light a shunt light path (Fig. 3) comprising mirrors 21, 22 is provided, it being stated that if the amount of light reaching slit 12 is adjusted to p (1 -p) times the scattered light, where p is a fraction representing the amount of stray scattered light, the relationship between the wedge reaching and the concentra.- tion of the sample is restored. In the simplified arrangement of Fig. 3 (not shown), cell 3 is omitted while a pair of cells are provided in place of cell 2. One receives only scattered light while a mirror placed in the centre of the light leaving the specimen reflects transmitted light to a second cell. An optical wedge is included in the light path to this second cell. Thus the comparison of transmitted and scattered light is made directly.