Patent GB-903030-A (GB)
Title: Improvements in and relating to photographic plate or film measuring devices
Inventor: SISSON GEORGE MACFARLAN
IPC: G01J3/40
Filed: 1960-01-29
Abstract: 903,030. Photo-electric position determination. GRUBB PARSONS & CO. Ltd., Sir HOWARD. Jan. 4, 1961 [Jan. 29, 1960], No. 3333/60. Class 40 (3). [Also in Group XL (c)] Relates to means for measuring the positions of marks, e.g. star images or spectral lines, on a transparent or translucent plate or film T, Fig. 1, in which the plate or film is moved to bring each mark into precise coincidence with the optical axis of an optical system, said axis being perpendicular to the plate or film and the amount of movement being measured by known means, e.g. a micrometer control. According to the present invention the plate T is illuminated by a light source B and an image of a mark A on the plate is focused on to an apertured diaphragm E located in front of a photo-cell J, means being provided for producing a cyclic relative displacement between the image and the diaphragm so that an electrical image pulse is produced by the photocell J, each time the image of the mark falls on the aperture, said image pulse being coincident in time with an electrical reference pulse produced at a predetermined point of said relative displacement when the mark A is coincident with the optical axis of the optical system. The relative timing of the image and reference pulses is indicated by displaying the pulses on a cathode-ray oscilloscope, Fig. 4 (not shown). If it is only required to determine displacement of the mark A from the optical axis in a single direction in the plane of the plate T the cyclic relative displacement may comprise an oscillatory motion but the apparatus described allows determination of the components of the displacement in two orthogonal directions in the plane of the plate T and comprises a stationary diaphragm E, Figs. 1 and 2, having two orthogonal radial slits S1, S2, Fig. 2, the image of the mark A being caused to traverse a circular path Z, Fig. 2, on the diaphragm E by means of an inclined transparent plate F mounted in a cylindrical holder G which is rotated coaxially with the optical axis; alternatively the plate F may be replaced by a wedge or prism. Separate reference pulse trains whose timing indicates respectively datum positions of the two slits S1 and S2 are produced (1) by electrical contacts controlled by the rotating holder G, or (2) photo-electrically by means of an auxiliary light source and photo-cell(s) acting in co-operation with one or two mirrors attached to the holder G, Fig. 3 (not shown); alternatively, Fig. 5 (not shown), light from the source B is reflected on to auxiliary photo-cells by the top surface of the plate F. The two sets of image and reference pulses corresponding to displacements in the two orthogonal directions may be displayed on separate cathoderay oscilloscopes or on a single cathode-ray oscilloscope. If it is only required to deter. mine displacement in a single direction then only a single slit is required in the diaphragm E. In a modification (not shown) the plate F is fixed and the diaphragm E is rotated, the reference pulses being then synchronized with the rotation of the diaphragm.