Improvements in or relating to length measurements by interferometer

Abstract

Abstract 367,859. Optical interferometers. TWYMAN, F. and DOWELL, J. H., 24, Rochester Place, Camden Road, London. Nov. 26, 1930, No. 35614. [Class 97 (i).] In an interferometer apparatus for measuring in terms of wave lengths of light the length between two surfaces or between predetermined points thereon, the test object carrying the said points or surfaces, and a standard object carrying points or surfaces separated by a known length are adjusted so that the lengths to be compared are truly parallel, the distance in the longitudinal direction between the point or surface at one end of the test object and that at one end of the standard object is measured or adjusted to a measured value, which may be zero, by means of interference fringes, and the corresponding distance relating to the other ends is measured by similar means. The Figure shows a form of apparatus using two interferometer light-dividing systems, but forms are also described in which a single system only is used. In the form shown a collimator tube has at one end an illuminated apertured diaphragm 11 and at the other end a collimating lens 14 whereby parallel light is incident on the light-dividing plate 31, the second surface of which is lightly silvered so as to be partly reflecting. The light transmitted by the plate 31 passes through a compensating plate 32, identical as regards thickness and kind of glass with the plate 31, and falls on the surfaces 62, 72 of the standard object 61 and test object 71 by which it is reflected back along its own path. The light reflected by the plate 31 is reflected by a reference mirror 41 back to the plate. The returned separated beams are recombined at the silvered surface and enter a viewing telescope 85. A second system identical with that above described and comprising dividing and compensating plates 33, 34 and reference mirror 42 is arranged on the opposite sides of the test and standard objects 71, 61 so that light reflected from the surfaces 62, 73, and from the mirror 42, is received in a telescope 185. When the mirrors and the objects 61, 71 are properly adjusted and the instrument is illuminated by white light, an observer sees in the telescope 85 interference bands by means of which the surfaces 62, 72 can be adjusted so as to be in one and the same plane. The second interferometer system then enables the observer to see to what extent the surfaces 63, 73 deviate from being in the same plane. The method of adjusting the several parts of the apparatus is fully described in the Specification. The apparatus may be arranged to be illuminated by monochromatic light of any desired wave length, which may be obtained, for instance, by means of a constant deviation prism.

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