GB1367886A - Measuring apparatus - Google Patents

Abstract

1367886 Measuring displacement TI (GROUP SERVICES) Ltd 29 Oet 1971 [29 Oct 1970] 51357/70 Heading G4H In a photo-electric displacement measuring system nth order defraction patterns from a grating are reflected back from a planar reflector on to the grating to produce further patterns co-linear with the initial light beam incident on the grating, the further patterns interfering and producing periodic maxima in intensity as the grating moves. In Fig. 1 light from a laser 11, polarized in orthogonal directions is directed by mirror 12 on to grating 13 producing first order defraction patterns 15, 16 which are reflected back on to the grating producing further first order defraction patterns, 21 and 24 of which are co-linear with the original incident laser beam. The two further patterns interfere to produce intensity maxima for increments of the grating displacement equal to one quarter grating width. Beam splitter 28 divides the beam produced by the interfering patterns on to photo-electric detectors 30, 32 via polarizers 29 and 31 whose axes are respectively perpendicular and parallel to the axis of an one eighth wavelength plate 25, introduced into beam 16, whereby mutually perpendicularly polarized beams are received by detectors 30 and 32 whose quadrature outputs therefore indicate the extent and displacement of the grating. The arrangement of Fig. 1 is used for determining the degree of flatness of a line or plane along or over which the grating is moved, any upward or downward movement of the grating indicating irregularities in the line or plane (Fig. 2, not shown). Two or three of such arrangements may be combined to give a two or three dimensional output (Fig. 4, not shown). Instead of mirrors 17 and 18 in Fig. 1 retro reflector prisms in combination with mirrors adjacent the laser source may be used (Fig. 3, not shown) thereby avoiding precise optical setting up of the apparatus.