Patent GB-717233-A (GB)
Title: Improvements in or relating to arrangements for testing optical components and optical systems
IPC: G01B9/02
Filed: 1951-03-19
Abstract: 717,233. Optical apparatus. SMITH, F. H., and BAKER & HOLBORN, Ltd., C. March, 19, 1952 [March 19, 1951], No. 6551/51. Class 97(1) Apparatus for testing optical reflectors or systems comprises birefringent means for dividing a light beam by double refraction into two coherent beams having wavefronts with mutually different directions of propagation and/or different curvatures the parts of the wavefronts which can subsequently mutually interfere being incident on the reflector or the system at positions spaced, apart or partly overlapping, laterally of their direction or directions of propagation, the reflector, or a reflector associated with the system reflecting the beams back, to the birefringement means so that it refracts them to travel along coincidental paths, and analysing means allowing the two coincident beams to produce viewable interference effects giving an indication of errors in the reflector or system under test. As shown in Fig. 1 a beam 11, polarized by a pack of glass plates 15 is divided into beams 17, 18 by a calcite wedge 16 mounted on a glass plate 19 to give zero deviation to the beam 17. A quartz plate 21 compensates the path difference between the beams 17, 18 which are then incident on the lens 22 under test and are brought to a focus 23 thereby. At the point 23 is located the centre of a hemispherical reflector 25 having a plane reflecting surface 26 with a central aperture. The rays 17 and 18 are reflected back along their paths by the reflector 25, are recombined by the wedge 16 and pass to an analyser 31 and eyepiece 30 in which two coincident images of the lens 22 are seen, crossed by interference fringes according to the imperfections of the lens 22. In a modification the focus 23 may be formed on the upper plane reflecting surface 26, the rays not being reflected back along the same paths. If the lens 22 under test is a microscope objective the cover glass 35 with which it is to be used may be included in the system. The lens 22 may be replaced by a concave reflector to be tested or if the lens 22 is accurate the reflecting system 25 may be replaced by a convex reflector to be tested. In the system shown in Fig. 3 a light beam from a diaphragm 42 is polarized by a polarizer 43, collimated by a lens 44 and divided by a parallel sided calcite plate 46 having its sides oblique to its optic axis. The divided beams 51 and 52 are incident at spaced points on the lens 53 under test and are focused thereby on a plane mirror 54 whence they pass back through another part of the lens and are recombined by the plate 46 to pass through a semi reflecting surface 45 to be viewed through an analyser 31. A quarter wave plate 35 is provided and the spacing of the beams may be varied by tilting the plate 46. This system may be used for testing a plane or prismatic reflector, the lens 53 being absent. As birefringent means two superposed double focus birefringent lenses may be used and may be moved laterally to vary the separation of the beams. Specifications 639,014 and 710,495 are referred to.