Patent GB-1352839-A (GB)
Title: Methods of and apparatus for effecting optical measurement and analysis
IPC: G01D5/38
Filed: 1970-07-31
Abstract: 1352839 Photo-electric position measurement NATIONAL RESEARCH DEVELOPMENT CORP 23 July 1971 [31 July 1970] 37145/70 Heading G1A The position of an optical grating MG is determined by cyclically casting a shadow of the grating on to a single photo-sensor PI through each in turn of the sub-sections of a composite grid CG and comparing the phase of the photosensor output with a reference signal synchronized with the shadow casting cycle. A suitable composite grid, Fig.2, comprises sub-sections A, B, C, having the same line repeat dimension and orientation but whose lines are shifted in phase between one sub-section and the next, e.g. 120 degrees between each sub-section. In one embodiment shown in Fig.1, light from source S is passed through a continuously rotating polarizer RPF, through grating MG and grid CG to a photo-cell, a phase-identifying polarizing filter structure PIF being positioned in front of the grid CG. The directions of polarization of the sections of PIF are as shown in Fig.3, and thus on rotation of polarizer disc RPF, light is arranged to reach photo-cell PI twice during each cycle from each of the grid portions A, B, C. The reference frequency is derived from a photo-cell PZ monitoring light passing through a stationary polarizer SP after having passed through a side portion of rotating polarizer RPF, or is derived direct from the shaft of RPF. The phase difference, measured at CH, is representative of the position of MG and the amplitude of the photo-cell P1 represents the contrast of the shadow. Modifications. The Grid CG, may be opaque lines formed on a polarizing base so that the clear regions each polarize in one of three directions at 60 degrees to one another, by use of a series of polarizing base layers, any three consecutive clear portions would polarize in the three different directions. Alternatively, strips cut from polarizing sheets may be joined together. In all cases, filter PIF would not be needed in the system. Rotating filter, PIF may be mounted inside the hollow shaft of a motor, Fig.6 (not shown) or at the end of a solid one. Alternatively, a fixed polarizer is associated with a rotating half-wave plate, or a combination of fixed and rotating half-wave plates, may be used. An air turbine may be used to rotate the filter. Non-mechanical scanning arrangements. In one, Fig.7 (not shown), three light sources energized by respective phases of a three-phase supply are associated with respective polarizers oriented at 60 degrees to one another and a common diffusing screen. The rest of the system is as in Fig.l, except that the reference signal is obtained from one of the supply phases. Suitable sources are neon or other gas discharge lamps or gallium arsenide or phosphide photo emissive devices. In a second arrangement, Fig.8 (not shown), a single lamp is associated with three electrooptical modulators, controlled by a three phase supply and modulating light to three polarizers as in the Fig.7 arrangement. Instead of a polarizing arrangement for selecting the grid section to be operative, colour filtering techniques can be used in the Fig.1, 7, 8, arrangements. Alternatively, Fig.9 (not shown), lens/prism arrangements and three sources supplied by a three phase supply, illuminate the sections of the composite grid in turn. In a further alternative, Fig.10 (not shown), a holographic mixer is used. Three coherent sources placed off-axis illuminate a hologram which has sets of fringes at 60 degrees. The sources are provided with polarizers, and optical stops are provided to prevent direct illumination of grating MG by the sources.