Patent GB-602459-A (GB)
Title: Improvements in or relating to interferometric testing apparatus
IPC: G01B9/02
Filed: 1944-11-11
Abstract: 602,459. Optical apparatus. WILLIAMS, W. E. Nov. 11, 1944, No. 22235. [Class 97 (i)] [Also in Group XIX] An interferometer for testing the accuracy of curved or plane surfaces, testing screw-threads and measuring lengths and angles, comprises a point source of 'monochromatic light such as an illuminated aperture 2, Fig. 3, an optical system 28, 29 so arranged that the light emerges normal to the reference surface 7 which is a reproduction to a different scale of the surface 8 to be tested, and a semi-transparent mirror 4 so arranged that an eye placed at 12 views the fringe produced by interference of light reflected from the surfaces 7 and 8 back through the optical system. To render the surfaces 7 and 8 parallel, a mirror 14 is provided so that images of the aperture 2 produced in its neighbourhood by reflection from the surface 8 may be viewed at 13, and the surface 8 adjusted until the images coincide with the aperture. Any fringes then visible at 12 will indicate any differences between the surfaces 7 and 8. The surface 7 may be spherical for testing spherical surfaces. A glass plate 3 provides correction for the astigmatism produced by the mirror 4. Projections may be provided at the surface 7 against which the surface 8 is located. For testing a spherical ball, a four-component lens system is used, the last lens bearing a reference surface which is a hemisphere at the centre of which the ball is located by suitable projections. Such a system may be used for testing the first components of microscope objectives. Cylinders may be tested by a similar system of cylindrical lens. Optical systems employing internal reflection may also be used. Fig. 6 shows an optical system comprising spherical surface 48 rendering the beam parallel and a conical surface 49 reflecting the beam to pass normally through reference surface 7 to be reflected back by the surface of the cylinder 51 to be tested. The surfaces 49 may be inclined to reflect the beam outwards for testing hollow cylinders. For testing a spherocylindrical surface 70, Fig. 9, such as an inner ball race, the optical system comprises a circular body having silvered ellipsoidal surfaces 71-72 which focus the beam on to a circle 62-63 which is the centre of the annular reference surface 69 and of the spherical curved portions 70 of the body 68. The beam thus emerges normal to the reference surface and is reflected back thereby and by the surface 70 to provide an interference effect. A similar system is described for testing the inner surface of an outer ball race and for testing a sphere. Screw-threads are tested by the system shown in Fig. 10 comprising a spherical surface 48 rendering the beam from the aperture 2 parallel and a conical surface 49 reflecting the beam to the reference surface 7 in the form of a screw-thread, the light being reflected from one surface of the screw to be incident normally on the other surface as shown, observations of the fringes being effected through a lens 80. Two opposed interferometers having plane reference surfaces may be used to determine the difference in length between two end gauges by adjusting them until one pair of ends is coplanar as tested by one interferometer when the fringe system seen in the other gives a measure of the difference of length. A similar arrangement permits measurement of the angle of a prism.