Journal of the Society of Motion Picture and Television Engineers (1950-1954)

kinescope image will lie in a flat surface where it may be received and viewed on a conventional flat motion picture screen. As with other projection systems, the distance to the projection screen may be freely chosen, the system being adapted to a wide range of distances as described below. Projected picture size will be substantially proportional to distance from tube face to screen. It is also inversely proportional to optical system focal length. For all practical purposes, curvature of the kinescope screen is independent of projection distance providing optical system focal length does not change. The design of the ogee lens is dependent upon projection distance. Its ogee curve must be "tailored" to fit the particular magnification at which the optical system is intended to work. The purpose of the ogee lens is to compensate for certain inherent qualities in the imagery of the outlying portions of the spherical mirror, and the needed compensation changes with projection distance. The lenses supplied with the systems are carefully designed to suit the distance from projector to screen. The focal length of the optical system remains unchanged when lenses are changed to take care of a series of projection distances. In manufacture, the ogee lens is given its shape by a master surface. The exact methods used depend upon the material chosen for the lens. Sagging or "dropping" methods are used with glass, casting with thermosetting materials, and pressing with thermoplastic materials. Whatever the material, preparation of the master surface and the process of forming the lens are carried out with great care. The lenses are made by the best techniques that it has been possible to develop. At the present time the Ultra-Speed System uses only glass lenses, since they are the best obtainable. There is every reason to expect lenses to be made just as well from plastic materials, and it is anticipated that suitable techniques will be developed to accomplish this. The glass lenses are more expensive than plastic, but they are more resistant to scratching, and if made of suitable glass, will also serve as radiation filters for the system. It is not known how the quality of glass and plastic ogee lenses will eventually compare, but the prospect of lower cost makes plastic lenses very attractive. The ogee lenses must be applied economically in order to keep equipment cost at a minimum. It is readily appreciated that there is a practical limit to the number of lenses that can be designed, made and stocked in order to adapt a given ultra-speed system to the range of projection distances encountered in theaters. It would be absurd, for example, to make a separate ogee lens for each 1-ft increment of projection distance between 40 ft and 70 ft. Fortunately, there is a rather large practical range of projection distances at which a given ogee lens may be used by merely focusing the system accordingly. This range is observed to extend from about 10% more than ideal design distance to about 15% less. Additional flexibility results from changing kinescope raster dimensions electrically. An extensive range of picture sizes, as well as projection distances, is therefore available with a projection system equipped with a single ogee lens. Two different lenses will adapt the projection system to a large range of projection conditions as illustrated in Fig. 3. The amount of light received by the optical system from each unit of area on the kinescope screen depends on the angular diameter of the cone which the optical system effectively subtends at the faceplate. In the Ultra-Speed System this angle is approximately 90° for maximum efficiency consistent with best image quality. It is obvious that if this light is projected into a small picture a brighter screen will result than if it is projected into a large picture, because the total light available is approximately constant. At long throws, a given optical 428 November 1951 Journal of the SMPTE Vol. 57