GB826004A - Improvements relating to apparatus for the testing of optical systems - Google Patents

Abstract

826,004. Optical apparatus. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Jan. 17, 1956 [Jan. 17, 1955], No. 1381/55. Class 97(1) Apparatus for inspecting optical systems comprises means for producing two beams of light plane polarized at right angles and projecting said beams into a system to be tested in such a manner that one of the beams is distributed over the whole surface or cross-section of the system and the other beam is focused on to a small area of the cross-section of the system, and means for combining the two beams emerging from the system to produce an interference fringe pattern, the arrangement being such that irregularities of the system under test cause distortion of the fringe system. Fig. 1 shows. an arrangement for testing a concave spherical mirror 7. At the centre of curvature of the mirror 7 is placed a triplet lens 3, 4, 5 and a lens 10, the central lens 4 of the triplet being birefringent with its optic axis in the plane of the lens, the triplet having zero power for the extraordinary beam but being converging for the ordinary beam. A beam of light 1 plane polarized at 45 degrees to the optic axis of the birefringent lens is divided into two beams of equal intensity. The extraordinary beam is focused by the lens 10 at the pole 8 of the mirror and is returned to the lenses as an ordinary beam after double passage through a greater wave plate 9 and is therefore focused by the lenses at a point 11. The ordinary beam is focused by the lenses at a point 6 thence it diverges to cover the whole mirror 7 and to be returned thereby as an extraordinary beam after double passage through the photo 9 to be also focused at the point 11. An analyser is placed in the combined beams at point 11 to enable interference to take place or the reflector 2 may be multilayer to act as as analyser. Fig. 5 shows a second system for testing a concave spherical mirror 9. A narrower collimated beam 1 is divided by a polarizing reflector into a plane-polarized reflected beam incident at 17 on the mirror 9 and a transmitted beam polarized at right-angles thereto and focused by a lens 7 at the centre of curvature 8 of the mirror after reflection at reflectors 4, 5, 6. The beam focused at 8 diverges to reach the whole aperture of the mirror, is reflected back thereby to the point 8 and then focused by the lens 7 at a point 11 at which the beam reflected at point 17 is also focused, an analyser 12 being provided in the combined beams. The device may also be used to test a positive or negative lens, a suitable mirror being provided to return the light through the system.