Patent GB-617416-A (GB)
Title: Optical instrument for testing plane surfaces and rectilinear lines
IPC: G01B11/30
Filed: 1945-05-16
Abstract: 617,416. Optical gauging-apparatus. CUNY, B. E. A. M. May 16, 1945. No. 12288. Convention date, Sept. 1, 1943. [Class 106 (ii)] [Also in Group XX] In optical apparatus for testing evenness and rectilinearity of a plane surface of the kind comprising a feeler movable over the surface, the feeler is located between two reflecting systems whereby the light rays from a sight or reticule are constrained to pass through a circuit of constant length, starting from the feeler and returning thereto after reflection on the two reflecting systems, this circuit being arranged in the path followed by the light for the formation of the final image of the sight. The reflecting systems may be each arranged to act twice on the light beam. In this apparatus the image of the sight is permanently in focus in spite of the movement of the exploring feeler. As shown in Fig. 5, the optical system comprises a separate stationary sight M having the reticule, two optical elements 01, 02 integral with a feeler P adapted to be moved over the surface L to be tested, two end reflectors R1, R2, an angularly adjustable block 1, and a stationary viewer I having an object lens Ob. The element 01 comprises a lower single and two upper plane mirrors, the element 02 is a semi-silvered mirror, and R2 is a plane mirror. Light rays emitted by the reticule follow the path shown to the viewer I. Irrespective of the movement of the feeler P the length of the optical path from and to the feeler is constant. This arrangement tests evenness of the surface L but not rectilinearity, the measurements being effected by rocking the block 1. In the system shown in Fig. 6, both evenness and rectilinearity of a surface L can be tested. The elements O1, O2 carried by the feeler P are orthogonal trihedrals each having three plane surfaces perpendicular to one another. The reticular sight M is stationary and secured to the end trihedral reflector R1, and the reflector R2 is a prismatic body having three reflecting surfaces and is rigid with the viewer I. The measuring block l is a parallelepipedic plate which can rock in two directions perpendicular to one another so that evenness and rectilinearity of the surface L can be tested. In another form the reticular sight is combined with a single object lens on the feeler, the right hand end reflector comprising a prism having three reflecting surfaces one of which is semi-silvered and spaced apart from the left hand reflector such that the length of the light path travelled from the sight M on the lens and back thereto is equal to the focal length of the lens, so that the image of the sight is at infinity. The image is observed in a viewer having a micrometric eye-piece. The latter arrangement can be further modified by arranging a separate reticule sight on the top of the object lens, replacing the right end prism by a plane mirror, and arranging in the viewer, between its object lens and viewing lens, a deflecting prism which can be tilted to give a measure of the errors in the surface L. In Fig. 11, two reticule sights M, M1 are used. The end reflectors R1, R2 each comprise a pair of mirrors with an object lens therebetween of such a focal length that the image of sight M is located at M1. The feeler P carries the sight M and eye-piece Oc. The parts M1, M, R, and Oc may be fixed in relation to each other, in which case measuring will be effected by micrometric displacement of the whole apparatus in the direction of the deviations to be measured, until the image of M coincides with M1. The measurements may also be made by micrometric displacement of M, M1, or of M or M1 in relation to the feeler. In a modification (Fig. 12, not shown) of this arrangement the end reflectors are spherical mirrors of similar focal length with their foci in coincidence.