GB1065226A - Length measuring instrument - Google Patents

Abstract

1,065,226. Measuring length photoelectrically. KEARNEY & TRECKER CORPORATION. April 29, 1965 [June 1, 1964], No. 18186/65, Heading G1A. In an arrangement for measuring the distance away of a movable reflector (43) Fig. 1 (not shown), light from a source (49) such as a laser, is split into two beams which are ultimately recombined for monitoring in respect of fringes by a photoelectric device, one part having been reflected at the source end for use as a reference and the other part having been reflected by the movable reflector. The pulses detected are applied to a bi-directional counter to indicate the length digitally, and phase differentiating means may be introduced into one of the paths to produce two sub-beams which are 90 degrees out of phase at the point of recombination so that separate signals developed by two monitoring devices can indicate jointly whether the distance is increasing or diminishing. The beam is split by a half silvered layer (57) between two triangular prism elements (55) and the reference beam is totally internally reflected by a layer (66) on the outside of one element which layer serves also to reflect back to the movable reflector a beam from the movable reflector. The reflected beam returned toward the prism from the movable reflector is combined with the reference beam at the half silvered face so that a part of it proceeds with the reference beam to the photo-electric monitor (68). The laser may be a continuous helium neon gas laser and the detector may comprise high resistance silicon units. The movable reflector may be mounted on a carriage driven from a motor through a clutch and a lead screw, and may comprise a trihedral prism, a plane mirror, or a triangular prism. The measurement of distance may be used to determine angular rotation when the movable reflector is mounted on the end of a rotatable arm or the optical arrangement may be built into a telescopic internal feeler device described with reference to Fig.21 (not shown). The sub-beam of different phase may be produced by a transparent layer terminating at the centre of the reflected beam or may comprise a stepped layer with the step at the centre. The movable reflector may be brought into contact with the prism face for calibration purposes.