Patent GB-236634-A (GB)
Title: Improvements in optical testing apparatus and the like
IPC: G01B9/02
Filed: 1924-04-08
Abstract: 236,634. Smith, T., and Dowell, J. H. April 8, 1924. Testing apparatus. - Apparatus for testing and finishing lenses, in which the principle of rotating the lens under test about an axis to bring different parts of the image under examination is employed, is provided with means whereby the correct disposition of the object, actual or virtual, the lens under test and the image receiver is maintained as the lens under test is rotated. The invention is more particularly described in connection with the method of testing set out in Specification 130,224, but is also stated to be applicable to other methods. In order that no appreciable aberration is introduced by any of the elements moved to secure the displacement of the object or the image, such elements are moved in the direction of their own axes and along the axis of the light incident upon them. In the examples of apparatus described an intermediate object at a finite distance from the lens under test is assumed so that the lens is examined under the conditions of a definite magnification. The lens L, Fig. 1, receives incident parallel light, producing an immediate object at a definite distance from the lens X under test, which is rotated by a rod OI about a point 0 which divides the disrod 0 I about a point 0 which divides the distance between the nodal points of the lever in the ratio of the magnification. To the rod OI is fixed a bar IC at right angles, by which the point C, the centre of curvature of the mirror M is constrained to lie in the desired image surface. The lever AB with a fixed fulcrum D, dividing it in the ratio of the selected magnification carries bars AE, BC set parallel to one another, so that movement of the pin C is communicated to the carriage carrying the auxiliary lens L. Jointed rods or cams may replace the lever system AEDBC. In a modification, Fig. 4, light from the lens under test converging to the image point P is intercepted by a lens R and is refracted to an intermediate image point coinciding with the centre of curvature of the mirror M. The lens R and mirror M are independently movable along the axis of the apparatus, and their movements are so controlled that the centre of curvature C of the mirror M always lies at the point conjugate to that of point P in the lens R. A pin or roller F on the lens carriage at the front principal focus of the lens R is maintained in contact with a bar JF rigidly attached at a point J to the rod OI. Pivoted on the pin F is a right angled lever one arm of which is constrained to lie along the bar JF and the other carries at a fixed point S a rod SG hinged at the point S and the point G on the main axis, the length SG being equal to that of the lever SF. This arrangement permits the apparatus to be of convenient size and confined to a region relatively near to the lens under test. In the case of low magnification a compound object lens E, E<1>, Fig. 5 is used, the separation of the components of which may be varied by a set of levers for each component similar to that of Fig. 1 and arranged as shown. In testing process lenses fitted with a reflecting prism, the object lens is controlled from a point P, Fig. 7 on the axis of the apparatus, I<1>P, being the reflection of the image plane. At an angle, of 45‹ with OP and rotating with it about, the point 0 is a rod QQ<1>, the points QQ<1> being the common points of two pairs of equal links, connected to the points C, P. In the testing of telephoto lenses, where the nodel points are in advance of the lens, the lens is rotated about an axis passing through the centre of the entrance pupil, rendering unnecessary modification of the method of controlling any auxiliary apparatus on the object side of the lens. To cause the axis of the mirror to lie along the axis of the emergent beam of light and its centre to be at the point where the axis meets the image surface, the arrangement shown in Fig. 11 is utilized. The axis of the incident beam meets the first principal plane, that which passes through the first nodal point N1, in the point U1 and a transverse plane JPK, at right angles to the optical axis, in the point P. The principal ray passes through the centre Z of the exit pupil and meets the transverse plane in the point K. The line PO normal to the plane JPK is of constant length for that plane and constitutes a bar of a right angled lever by which the centre C of the mirror M is constrained to move along the line ZK and be retained in the plane JPK. This arrangement may be used for testing a telescope system or any system satisfying the tangent condition. Modifications are described for testing systems obeying some other law, e.g. the sine condition.