Patent GB-1259310-A (GB)
IPC: G01B11/00
Filed: 1969-02-03
Abstract: 1,259,310. Optical detection of movement. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 3 Feb., 1969 [6 Feb., 1968], No. 5663/69. Heading H4D. [Also in Division G2] In a system to determine movement of object 6, Fig. 1, in two dimensions X and Y, Fig. 2, a two-dimensional diffraction grating 8 is fixed to the object and a beam of light from source 1 caused to impinge on the grating, whereby the thus diffracted beams are detected by means 19 and 20, means 19 detecting the beam diffracted by the grating lines in the X dimension, and, by counting the sinusoidal variations of the intensity of said beam, determining the movement of the object in said dimensions, and means 20 likewise detecting the Y dimensions diffracted and so determining movement in said Y dimension. The grating may be as shown (highly magnified) in Fig. 2, the 0 and # indicating the relative phases of light reflected from the facets of the grating. The light from source 1 is monochromatic and linearly polarized and is focused on an opening 3 in concave mirror 4. The light beam diverges and is rendered parallel by a lens 5. The parallel beam impinges normally on the grating 8 and is diffracted thereby. Only the first order diffracted beams are considered, the other orders being absorbed in the lens container (shaded) on passing back out through hole 3. Four first order beams are produced, referenced (+1, +1), (+1, -1), (-1, -1) and (-1, + 1), Fig. 3, depending on how they diverge in the X and Y directions, beams (+1, +1) and (-1, -1) being produced by diffractions by the Y aligned gratings, and beams (-1, +1) and (+1, -1) being produced by diffraction by the X aligned gratings. The (+1, +1) and (-1, -1) beams, when they fall on and are reflected by mirror 4, pass through respective -wave plates 9<SP>1</SP> and 9 such that after said reflection their plane of polarization is perpendicular to that of the (-1, +1) and (+1, -1) beams. The four beams are reflected back to the grating and undergo such diffraction thereat that they are all directed back to hole 3 to pass to a beam separating assembly 13. The two pairs of beams, giving respective X and Y movement information, have orthogonal planes of polarization and are separated out by a prism arrangement 16 and directed to respective detectors 19 and 20. Two non-polarized light sources 1 and 1<SP>1</SP> may be used, slightly spaced, the respective beams from which pass through respective holes 3 and 3<SP>1</SP> in the mirror 4. The -wave plates 9, 9<SP>1</SP> and the polarization separating means 13 are removed. A thick quartz plate is placed between lens 5 and grating 6 and functions in a similar manner to that described in Specification 997,405. Two sets of four divergent diffracted beams are produced, impinging on mirror 4 as shown in Fig. 4. The beams produced by the diffraction, by the X aligned gratings, of the light from source 1 are removed by absorbing areas F 1  and F 2 , and the beams produced by the diffraction by the Y aligned gratings of the light from source 1<SP>1</SP> are removed by absorbing areas F 3  and F 4 . Thus, all light passing back out through hole 3 carries only information regarding the X direction movement and that passing through hole 3<SP>1</SP> carries only information regarding the Y direction movement. Such light is detected by correspondingly spaced detection 19 and 19<SP>1</SP>, and 20 and 20<SP>1</SP>. The quartz plate causes the beams to have relative phase differences of #/2 and be separable by a polarization separating beam splitting prism.