Patent GB-498419-A (GB)
Title: Improvements in or relating to optical projection systems
IPC: G02B1/00
Filed: 1937-09-24
Abstract: 498,419. Optical-projection systems. KAPELLA, Ltd., WARMISHAM, A., and REASON, R. E. Sept. 24, 1937, Nos. 25954/37, 29572/37, and 19544/38. [Class 97 (i)] The projection of an image of an object lying in a surface inclined to the direction of projection on to an image surface also inclined to the direction of projection, is effected by means of a system of lenses so arranged as to collimate the principal rays in the object and image spaces and thus to produce an image free from keystone distortion due to the oblique distortion. Fig. 1 shows a symmetrical system for producing an image at unit magnification, comprising a projection lens A and two equal collimating lenses B<1>, B<2> having their principal foci at the nodal points A<1>, A<2> of the lens A. The lenses B<1>, B<2> are spherically corrected with respect to the nodal points A<1>, A<2> respectively, and fulfil the sine condition within the angle subtended by the lens A. The object and image planes D, C are equally inclined to the axis of the system and the image is undistorted. This system can be adapted for producing a focused image free from keystone distortion at other than unit magnification by using lenses B<1>, B<2> of different focal lengths and arranging the system so that the ratio of the tangent of the angle between the object plane and the axis to that of the angle between the image plane and the axis is equal to the magnification. The image so produced will suffer from rectangular distortion, and this defect can be remedied by modifying the system by tilting one or more of the lenses. The system shown in Fig. 2 comprises two collimating lenses F<1>, F<2> of different focal lengths having their axes coincident with that of the system, and a projection lens E which is tilted through such an angle that the object and image planes G, H are equally inclined to the axis. To correct for aberrations in this system, the lens E is preferably an anastigmat and may consist of a divergent component between two symmetrically arranged convergent components. Instead of tilting the lens E, the lenses F<1>, F<2> may be tilted through the same angle. As a further alternatlive all three lenses are equally tilted, in which case the lenses F<1>, F<2> are preferably anastigmats. The above arrangements are suitable for projection on to a screen &c. ; an arrangement in which the image can be directly viewed by an eyepiece is shown in Fig. 5. An image of the object D is produced by the system B', A, B<2> of Fig. 1 on a transparent surface C<2> on which graduations are marked. An image of the surface C<2> is produced on a surface P at right-angles to the axis of the system by a lens O, so that this image can be viewed by an eyepiece Q, the lens O being tilted to enable a focused image to be obtained at P. This image is distorted, but the image of the graduations is equally distorted so that the graduations can be used for measurements of the final image. This image may be corrected for keystone distortion by a field lens R. Specifications 498,377, 498,378, 498,382, 498,420, and 498,458 are referred to.