International projectionist (Jan-Dec 1946)

A RESEARCHER VIEWS TELEVISION* By E. W. ENGSTROM RESEARCH DIRECTOR, RCA LABORATORIES, PRINCETON, N. J. PRIOR to the war. television had crossed the threshold leading out of the research laboratory. Television was then a practical reality with promise of becoming a significant factor in American life. As a result of the initial efforts to provide a regular program service, the participating public seemed anxious to see television service continue and expand. Television was ready — but the war intervened. We are now at the stage where plans are being made effective for post-war services. Television is a new service of major importance and should receive favorable attention and consideration. Progress during the war has sharpened the tools used in television. We know better how to build electron tubes and circuits. We know better how to handle the radio frequencies used for television broadcasting. We are more sure of the techniques which will provide satisfactory pictures in the home. To aid in visualizing the progress which has been made, let us review a few of the problems which were under consideration in the period immediately preceding Pearl Harbor. Pre-War Viewing Sets Pre-war television receivers provided an image which was viewed directly on the end of the Kinescope or cathode-ray tube. or. in some receivers, the tube image was reflected in a plane mirror for more convenient viewing. In all cases, however, the size of the tube imposed a practical limitation to the size of the picture that could be obtained. A typical home receiver employed a 12-inch Kinescope, which gave a fairly satisfactory picture size but made it necessary for the audience to sit quite close to the set if they wished to see all the picture detail. As a result of the experience with these pre-war receivers, in the areas then provided with television program service, there was evident a need for larger images on the viewing screen in the home. Enlargement of the image in the direct-viewing type of tube was considered impractical for general application since it would require a tube of unusable proportions. Solution seemed possible through some means of providing an image by projection enlargement. For a number of years before the war, t Teleciser, Summer, 1945, Edition. active work had been in progress to develop a small tube giving a very bright picture which could be projected through a suitable lens system on to a large viewing screen. These projection Kinescopes were based on the same principles as the direct-viewing tubes, but employed higher voltages, delivering beams of much greater power to the fluorescent screen. The screens, of course, had to be processed in such a way as to be stable under these operating conditions. By 1937 it was possible to demonstrate, on an 8 x 10-foot screen, a projected picture which could be viewed without undue fatigue in a well darkened room, although the picture brightness was not all that might be desired. The next big step forward was the substitution, in place of the conventional lens, of an optical system adapted from that used by Schmidt for astronomical purposes. Essentially, this system consists of a spherical mirror which collects the light from the projection Kinescope, and passes it through an aspherical lens element on to a viewing screen. The aperture of the mirror is located at the center of curvature and the aspherical lens element is placed at the aperture to correct the aberrations of the system. Such an optical system makes possible many times more efficient utilization of the light from the Kinescope and brings projection television well into the realm of the practical. One of the basic problems solved during the research on this optical system was that of economically manufacturing the aspherical correcting lenses. To grind these lenses was out of the Your RED CROSS must carry on question from a cost standpoint, because of their peculiar shape. A method was therefore evolved for molding them from one of the clear transparent plastics. This made the lenses a relatively inexpensive part of the projection optical system. Post-war television receivers having screens large enough to be viewed with ease in a living room of average size are thus made commercially possible. At the transmitting end of the pre-war television system, program experience indicated definite interest on the part of viewers for seeing events as they happened. Often this called for using television cameras under conditions of relatively poor light conditions. Many events and happenings could not be televised because the usual light levels were too low. This called for major increases in the sensitivity of the Iconoscope or camera tube. Research on this had progressed to the point where substantial sensitivity gains were in sight when war called a halt to television work. The progress made gives promise of a solution to this important phase of television broadcasting. The flexibility in programming which such an advance will permit is naturally of great importance. To be able to televise all scenes which may be seen directly will add immeasurably to the immediacy and spontaneity of television programs. Compared to the broadcast receiver of sound only, the pre-war television set was an expensive item. In the first place it was really two receivers — one for sound broadcasting and one for television— and the television unit involved an expensive viewing tube. These factors were further aggravated by the original lew quantity production. With the improvements that have been made in circuits and the design of circuit parts, and with mass production quantities, it is anticipated that post-war television receivers can retail at prices which will encourage rapid growth, resulting soon in a large television audience. Network Television If television and its auxiliary services are to expand rapidly, thereby providing a new American industry and a source of large-scale employment, we must have the means to carry programs from city to city over nationwide distributing networks. For years, forward-looking research and invention have been directed toward making it possible to provide JANUARY 1946 11