American television directory (1946)

ADVANCES IN LARGE SCREEN TELEVISION Progress along two lines — on direct-view and projec¬ tion types of home receivers — promises television au¬ diences steadily improving quality of visual reception. By D. F. SCHMIT _ Director of Engineering, RCA Victor Division, Radio Corporation of America I mages obtained on most prewar tele¬ vision receivers were about 1V2 by 9 inches. Because the size of the image determines the approximate number of persons who can satisfactorily view the screen of a receiver, a need was felt for receivers providing larger images, par¬ ticularly for use in commerce, industry, and educational institutions. Theatre television, of course, demands an even larger screen. Two methods of providing larger im¬ ages presented themselves. One was to build a receiver employing a larger cathode-ray tube; the other, to find a means of optically enlarging the image appearing on the face of the tube. Develop Projection Television Believing that many people will still prefer the direct-viewing type of set, especially for home use, we are develop¬ ing a cathode-ray tube which will pro¬ vide an image somewhat larger than those available before the war. Recog¬ nizing, however, that the size of a direct-viewing screen is limited by the size of cathode-ray tube it is practicable to manufacture, place in a cabinet and operate, we have also developed a re¬ flective optical system for projection of television images from the face of the tube onto larger screens. This system has been used success¬ fully to project television images onto a built-in home receiver screen 2iy3 by 16 inches — about the size of a full page of a newspaper, or five times the area of images obtained on a 12-inch prewar direct-viewing tube. And larger images can be projected by similar systems de¬ signed for use with screens remote from the receiver, such as would be used in schools, meeting rooms, and theaters. Home console models providing pro¬ jection-type television, FM (frequency modulation), and standard broadcast receiving facilities will cost initially about $450. Models equipped with directviewing picture tubes will be generally lower in price, and RCA-Victor will offer at least one table model priced at approximately $195. An obvious means of obtaining large images — to magnify them by placing a lens in front of the tube — has little practical value. The field is sharply lim¬ ited from which images can be viewed. The two basic problems of projection television are: (1) providing a cathoderay tube capable of producing very bright pictures with the necessary reso¬ lution, and (2) providing an optical system that will project to the screen the largest possible percentage of the light generated by the tube. These problems were vigorously attacked, and very satisfactory progress was made. Intensive work on electron guns, lu¬ minescent materials, glass envelopes, and other components resulted in the development of cathode-ray tubes de¬ signed to operate at a rated voltage of 27,000 volts — nearly four times the vol¬ tage used in prewar direct-viewing tubes — and capable of producing much brighter images. These tubes, also made substantially smaller and lighter, tend to compensate for the size and weight of the optical system, and may mean lower tube replacement costs. The problem of developing an effi¬ cient optical system at first appeared to be very formidable, due largely to the nature of the light source. For prac¬ tical purposes, the luminescent screen of a cathode-ray tube radiates light as a perfect diffusing surface. In pro INGENIOUS: Broken lines indicate the path of light beams in the Schmit optical system. jecting light from such a source onto a viewing screen, a conventional F:2 movie projection lens is capable of col¬ lecting, at most, only 6.25 percent of the light emitted by the tube. An optical system was sought which should be able to focus large fields — up to 50 degrees — with an efficiency of 20 to 40 percent. Plastic Lens Cuts Cost The answer was found in a reflective optical system consisting of a spherical front surface mirror and a weak aspherical correcting lens, the latter located at the center of curvature of the mir¬ ror. A handicap of this system, for use in a home receiver, was the high cost of the aspherical lens. This has been overcome by the development of ma¬ chines for making aspherical molds and RCA’s development of a process for molding aspherical lenses from plastics. The spherical mirror may be visual¬ ized as a shallow bowl, with its reflec¬ tive coating on the concave surface. The lens, molded of clear methyl methacry¬ late resin, is flat on one side, with the opposite surface rising slightly at the center and at the edges, but depressed in the intermediate area. Its function is to correct the spherical aberration of the mirror and bring the image to a sharp focus on the screen. The system is mounted with the picture end of the cathode-ray tube protruding through an opening in the center of the lens and facing the cen¬ ter of the mirror. Images appearing on the face of the tube are picked up by the mirror and reflected though the lens to the viewing screen. For home re¬ ceivers and other systems employing built-in screens, images are projected onto the back of the screen. The great light-gathering power of this type of optical system makes it possible to transfer to the viewing screen about six times as much light as could be delivered by a conventional F:2 lens, without loss of image quality. Other RCA developments contribut¬ ing to the quality of large-screen tele¬ vision reception include improvements in built-in viewing screens, and an automatic frequency control system which virtually eliminates picture dis¬ tortion resulting from automobile igni¬ tion and other noise impulses. Ill