Patent GB-555672-A (GB)
Title: Improvements in or relating to interferometers for determination of length
IPC: G01B9/02
Filed: 1942-02-23
Abstract: 555,672. Interferometers. HILGER, A, Ltd., and DOWELL, J. H. Feb. 23, 1942, No. 2379. [Class 97 (i)] Interferometers such as are described in Specification 367,859 for comparing or determining the lengths of objects, in which both endfaces of the objects are under observation by a single telescope, and no mechanical contact is made with any of said end-faces, are characterized in that these faces are observed simultaneously by a single telescope over their whole area without the use of separate reference mirrors. Where two objects are being compared, an end-face of one object may form a reference plane common to the other end-faces of both objects, and the optical system is so arranged that the said end faces appear partly to overlap one another in the telescope. When the two sets of white light bands formed by one face overlapping the two faces at the other end are set in coincidence, the relative displacement of the third set of white light bands formed between the other face at the first end and one at the other end will determine the required difference of length. The instrument may also be used for checking the parallelism of, or measuring the angle between, the faces of an object. In the form shown in Fig. 1, a collimator 1 provided with alternative white and neon light sources 2, 3 and an iris diaphragm 4 projects parallel light on to a dividing mirror 6 formed of a plate having a layer of silver or aluminium in contact with another plate serving as a path compensator. The reflected beam proceeds via mirror 7 to the endfaces A, C of objects 8, 9 whose lengths are to be compared; while the transmitted beam proceeds via mirror 10 to the other end-faces B, D of the objects. Both beams are reflected back along their own paths to unite at the dividing mirror 6, a part of each proceeding therefrom to a telescope having a mirror between its objective and eyepiece 12 for turning the light into a convenient direction for observation. One object 8 may be a standard with which the other 9 is to be compared, and each is held on a plate 13 or 14 as by a spring 15. The plate 14 has a carriage support having screw adjusting means enabling object 9 to be tilted about horizontal and vertical axes to set its end-faces parallel to those of object 8 ; while the plate 13 has a carriage support adjustable by screw means to set object 8 exactly midway along the optical path of the two beams so that white light bands can be obtained, and this screw means also permits fine adjustment to bring adjacent end-faces of the objects at one end thereof into the same plane. Means is provided also for tilting the mirror 10 about horizontal and vertical axes. so that the white light bands can be adjusted for position and width along the face of the object 8. Fig. 7 illustrates the image seen in the telescope when the objects have been correctly set, the faces A and C (shown vertically displaced for clarity only) appearing to overlap the faces B, D. When the central: bands J and K are in line horizontally, the faces A, C are in adjustment in one plane, and the difference in length between the objects is indicated by the displacement of the central band L from the horizontal defined by J and K, and may be computed in known manner. The Specification also describes the measurement of the absolute length of an object by a method in which the standard object 8 is replaced by two optical flats wrung together with polished faces in contact so as to provide a zero plane with oppositely-directed faces corresponding to C, D. In a modified instrument, the dividing mirror and compensating plate are separate, and one of the mirrors 7, 10 is replaced by a pair of mirrors with their junction edge vertical, reflecting the beam twice. In another, one of the mirrors 7, 10 is replaced by a roof prism, or equivalent pair of mirrors at right angles, with its junction edge horizontal. Both of these modifications of the mirror system 7, 10 may be embodied in the same instrument. The Specification describes also the use of the instruments for testing for parallelism of the endfaces of an object, and the use of the modification having three mirrors in addition to the dividing mirror to compare the angles of anglegauges or other angular objects. Reflectors.-A pair of mirrors functioning as a roof prism for double reflection comprises, Fig. 18 (not shown), a block of material with two plane faces at right angles, and a flat piece of material placed in contact with one of said plane faces and projecting beyond the angle to form with the other plain face a re-entrant dihedral angle of 90 degrees.