Patent GB-1452070-A (GB)
Title: Projecting a tv screen image onto a larger screen
IPC: H04N5/74
Filed: 1973-10-16
Abstract: 1452070 Television receivers M AMBATS T D SHANNON and W J DE MARIA 16 Oct 1973 [20 Oct 1972] 6472/76 Divided out of 1452069 Heading H4F The image on the screen of a standard television receiver is projected on to a remote viewing screen by means of a lens assembly comprising five lens elements formed of a transparent thermoplastics material, the first element being concavo-convex, the second element double convex, the third double concave, the fourth double convex and the fifth double concave. The fifth element is substantially the same size as the image and to correct for the optical inversion introduced by the system switch means allows the polarity of the drive from the deflection amplifiers to be reversed. Additionally, the switch means may be arranged to cause an increase in the anode voltage of the tube in order to increase the intensity of the image. As shown (Figs. 1 and 2), the lens system is contained in a housing comprising a cylindrical front section 12 and a rectangular rear section 13 provided with a bracket 14 by means of which and a rectangular pin 15 the housing may be attached to a standard TV receiver. The switch means may be provided with a projecting actuator pin which is engaged when the housing is mounted in position. The system is constituted by five elements A to E, the element A to D being carried by a barrel 20 which is slidable in the front section 12 by means of a rack and pinion mechanism 21-23 for focusing purposes. The element E serves to "flatten" the curvature of the cathode-ray tube screen. However, if the latter is flat element E is unnecessary. Numerical values in respect of a mean focal length of 10 units are given for the various parameters indicated in Fig. 3, viz. R= radius of curvature; T = axial thickness; S = axial separation, together with numerical values for the index of refraction (for the D line of the spectrum) and for the Abbe dispersion number. Additionally a formula and numerical values are given to enable the radii of curvatures of the aspheric surfaces R 1 , R 6  and R 9  to be calculated, The aperture ratio of the assembly is 1 : 1À5 and the back focal length is substantially zero. The assembly is corrected for all optical errors over the entire field of view of Œ20 degrees. It is stated that flat Fresnel lenses may be employed. When "cropped" wide screen film is reproduced by the TV receiver, suitable lenses may be employed in the projection system to "expand the" image. The lenses are made from a plastics material such as transparent acrylic or polystyrene and may be formed by compression moulding for thick lenses and by injection moulding for thin lenses. The injection process may however be used for fabricating thick lenses by introducing a solid planar optically shaped plastics block 29, Fig. 5, of appropriate thickness into the mould 30, the block serving as a core and then injecting molten plastics under high pressure to encapsulate the core which is of identical plastics material. In this manner problems caused by the differential cooling of the centre and the outer zone with thick lenses are avoided. Instead of employing a core piece a solid acrylic plate may form one face of the mould into which molten plastics is injected.