GB742372A - Apparatus for the measurement of very small lengths - Google Patents

Abstract

742,372. Measuring length photo-electrically. SOC. GENEVOISE D'INSTRUMENTS DE PHYSIQUE. May 15, 1953 [June 11, 1952], No. 13691/53. Class 40 (3). In apparatus for measuring small lengths in which interference fringes are displaced in accordance with the lengths, an apertured grid oscillates continuously over the fringes so that the difference between successive time intervals between successive coincidences of the bright fringes with the slits is a measure of the length and is determined photo-electrically. As shown, the movement to be measured is translated into the motion of a member 10 carrying a mirror 7, which, with mirror 6 and prisms 4, 5 forms a Michelson interferometer. The fringes produced from the light source 1 are imaged by lenses 11 in the plane of a grid 14 which is continuously oscillated by an A.C.-fed solenoid 15. Depending on the movement of the fringes the coincidences of fringes and slits succeed each other at unequal time intervals, alternate intervals being equal. The light impulses passing through the grid are received on a photo-electric cell and converted into very short impulses by known electronic devices which may be as described in Specification 581,930 and fed to a measuring device III comprising a reversing switch described in Specification 686,274 and a reading instrument having considerable mechanical inertia which translates the inequality of time elapsing between the impulses emitted during the whole of period of oscillation of the grid into a measure of the displacement of the feeler 10. The impulses produced by the photo-cell may be used to illuminate a rotating stroboscopic disc as described in Specification 555,103 or may be displayed as a cathode-ray tube as described in Specification 684,435.