GB893650A - A device for testing straightness by means of a two-foci element - Google Patents

Abstract

893,650. Two-foci straightness testing devices. AGENCY OF INDUSTRIAL SCIENCE & TECHNOLOGY, MINISTRY OF INTERNATIONAL TRADE & INDUSTRY. April 30, 1958 [Oct. 18, 1957; Feb. 4, 1958(2)], No. 13809/58. Class 97(1). An optical element in the form of a transparent plate having deposited on it, by vacuum deposition or photographic means, a radially-varying distribution of absorbing and/ or transparent phase-retarding material (for example as indicated, respectively, by 2 and 3 in Fig. 3), gives rise to a pupil function in an associated collimator system which provides Strehl intensity maxima at two separated axial positions, so that there are, in effect, two foci, the said optical element and collimator being comprised in a straightness tester whereby the images of two targets located at the regions of the Strehl maxima may be observed in superposition by an observing telescope. As shown in Fig. 5(1) two axially separated targets A and B are viewed through a telescope T, E via a two-foci element F mounted on a support which traverses a surface M. Vertical movement of the axis of F from the line defined by A-B causes relative vertical displacement of the superposed images of A and B. In the arrangement of Fig. 5(II), the autocollimator C serves as the telescope objective, a half-silvered mirror N directing light into the eyepiece E. Two two-foci elements may also be used. The targets may be in the form of knife-edges (Fig. 6) the cross-over point being displaced horizontally as the images are relatively displaced vertically.