GB897361A - Improvements in or relating to optical systems - Google Patents

Abstract

897,361. Projection systems; cameras; colour photography. HARRIES TELEVISION RESEARCH Ltd. May 20, 1959 [May 29, 1958], No. 17182/58. Classes 97(1), 98(1) and 98(2). [Also in Groups XL(a) and XL(b)] An oblique optical system includes a device for correcting keystone distortion of the image formed, this device having a surface which varies in slope and which comprises a plurality of facets separated by lines of discontinuity of slope, the gradients of each facet being such that the paths of a bundle of rays from the object in the optical system which pass through that facet are modified so as to displace the points of arrival of the rays at the image surface of the optical system into such positions that keystone distortion in the image is substantially avoided. As shown in Fig. 3 for a television reception system including an electron gun 16 in a vacuum envelope 18, the light from a phosphor screen 20 is reflected by concave mirror 24 through clear glass 32 aperture stop 26 and meniscus 28 and thence through a distortion correcting device 30 to the viewing screen 22. Optical data are given for this system of which the correcting device is a light transmitting plate with one plane surface. The other surface of the plate is divided into sectors separated by radial lines of discontinuity of slope, each sector having zero slope along the path of an arc centred on the point 0<SP>1</SP> as shown in Fig. 2A. The sectors are symmetrical about a diameter YZ. The configuration of each radial sector of a plate designed to correct keystone distortion of the system of Fig. 3 and also for pincushion distortion is given in the Specification. The cathode ray tube may be of the kind described in Specification 897,359 this tube being provided with an insulating casing 46 which is cemented to the moulded glass window 32 and which houses the meniscus 28 and the distortion correcting plate 30, Fig. 4. In order to provide for uniformity of focusing in the image, the phosphorus screen is tilted with respect to the optical axis by an angle equal to the angle of obliquity of the optical axis with respect to the normal viewing screen divided by the magnification. The distortion corrector can be injection moulded from polymethyl methacrylate and may have facets in the form of concentric rings or straight strips, these facets sloping in orthogonal directions. A distortion corrector may be associated with each of the oblique projection systems of a three-colour cinematograph projection apparatus including one projector which is disposed with its axis perpendicularly to the screen and which is provided with the usual aspherical correcting device. Alternatively, the device may be used as a camera system in which the film is tilted with respect to the camera axis. The device may also be used in a three-colour television reception or transmitting system employing three or four image tubes.