Radio age research, manufacturing, communications, broadcasting, television (1941)

TELEVISION PROJECTION TUBES Design of 5-Inch Kinescope for Large-Screen Home Receiver Inooiued Long Study by RCA of Glass. Pfiosphors and Electronics. WHEN representatives of the press witnessed the first pub- lic demonstration of RCA's ad- vanced development model large screen television receiver on March 15, they gave unanimous praise to the size, brightness and clarity of the picture. But few of the observ- ers realized the number and com- plexity of the problems that had been met and solved in the long process of producing a practical instrument. The earliest television images were about 1 inch square and were poorly lighted. The latest model ha.'j a picture area 350 times larger with illumination that compares favorable with that of theatre movie screens and well above that of the average home movies. How that progress was made is a story of carefully planned research con- ducted at the RCA Laboratories, Princeton, and at RCA Victor Man- ufacturing plants. The problem of providing a tele- vision image large enough for com- fortable viewing by a group of people was on its way to solution before the war when RCA intro- duced a receiver with a 12-inch kinescope which provided a T^s by 9% inch image. While it is believed that this size picture will continue to satisfy many users, there will also be a definite demand in post- war days for a deluxe-type receiver having a much larger image. How Image Size is Increased Larger television images may be produced in two obvious ways. The kinescope tube diameter may be increased for direct viewing, or the image from the face of a smaller tube may be projected through a suitable optical system. Sealed-off glass tubes as large as 24 inches in diameter have been made but un- fortunately glass is not a good structural material for such large kinescopes. As the size is increased, the glass must be made very thick and the viewing screen must be made nearly spherical in order to withstand the atmospheric pres- sure. The technique of motion picture projection led investigators to the THREE LATE-MODEL KINESCOPES FOR TELEVISION. LEFT TO RIGHT: 5-INCH FLAT-FACE AND 3-INCH FLAT-FACE FOR REFRACTIVE SYSTEMS; 5-INCH CURVED- FACE FOR REFLECTIVE OPTICAL SYSTEMS. idea of projecting large television images from a kinescope. But here, another problem appeared. Few people appreciate the low efficiency of a projection lens in transmitting light. An f4.5 lens, for instance, is considered fast; an fl.5 lens is in the realm of super-speed. Yet the former transmits only about 1 200th, and the latter only l/25th, of the available light. Thus, even with the fl.5 lens, the image on a kinescope would have to be so bril- liant to begin with that four per- cent of it, when spread over a screen surface 30 times greater than the tube face, would still sup- ply a screen image of satisfactory brightness. How well these difficulties were overcome is evident in the fact that the RCA reflective optics system, the heart of the large-screen televi- sion set, has an efficiency four times that of previous fast projection lenses. Three Factors Govern Design Three basic factors govern the design of a satisfactory projection kinescope: the electron gun, which generates the electrons, controls them, accelerates them and focuses them into a beam; the fluorescent screen and the over-all cost of the unit. Since the brightness of fluor- escent screens increases rapidly with the voltage used in the tube, the 5-inch developmental projection type used in the RCA receiver op- erates at 27,000 volts. This com- pares to the 7,000 volts of the 12- inch kinescope. The presence of the high voltage introduces numerous other difficulties which have been overcome by continued research. For one thing, the surface of many elements within the tube must be highly polished to prevent the emis- sion of unwanted electrons. If not suppressed these faults would affect [RADIO AGE 15]