GB741557A - Improvements relating to apparatus for measuring optical thicknesses - Google Patents

Abstract

741,557. Optical apparatus. ASSOCIATED ELECTRICAL INDUSTRIES, Ltd. Nov. 23, 1953 [Oct. 22, 1952], No. 26549/52. Class 97 (1). Apparatus for measuring the optical thickness of a transparent object comprises a microscope having means dividing the illuminating beam into ordinary and extraordinary beams so that one passes through the object and the other by-passes the object, and means for combining the beams which have respectively passed through and by-passed the object so that they interfere and an analysing apparatus for measuring the phase difference between the two beams. As shown, the illuminating beam 5 is divided by a birefringent plate 2, for example of Iceland Spar, into an ordinary beam 6 by-passing the object 1 and an extraordinary beam 7 passing through it. The planes of polarization of the beams are interchanged by passage through a half-wave plate 4 and they are combined by a birefringent plate 3 similar to the plate 2. The combined beams are reflected by a concave spherical mirror 9 and by a semi-reflecting surface 10 to form an image of the object at the pole of the mirror 9 which is observed by the microscope objective 11. An analyser in the eyepiece renders the interference effects in the image of the object visible. To measure the phase difference between the beams within the image and outside the image a halfwave plate in the form of a narrow strip 12 is located in the image plane of the objective 11 with its principal directions coinciding with the planes of polarization of the beams 6 and 7 and extending into the field beyond the image of the object 1. A quarter-wave plate 13 converts these beams into circularly polarized beams with opposite senses of rotation the result being a linearly polarized beam the direction of polarization of which depends on the phase difference between the beams 6 and 7. An analyser 14 is provided, rotatable about a vertical axis, and is rotated until the intensity within the strip is equal to that outside the strip first for the portion of the strip covering the image and then for the portion extending into the field, the difference between the angular rotations of the analyser 14 indicating the optical thickness of the object. In a modification a quarter-wave plate is placed above the objective and the strip 12 replaced by a half-wave plate in the form of a lattice, a rotatable analyser again being provided.