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Television Pickup for Transparencies
BY ROGER D. THOMPSON
ALLEN B. DuMoNT LABORATORIES, PASSAIC, NEW JERSEY
Summary — Scanning an opaque subject or photographic transparency with a moving spot of light found application in the early days of television. A modern version, using a short-persistence cathode-ray tube as the light source, can produce television pictures of excellent quality. Equipment restricted to pickup of transparencies can be of simple and reliable design. A motor-driven slide-changing mechanism accommodating as many as twenty-five 2 by 2-inch glass slides is described. Esthetic transitions possible include automatic picture fading preceding and following the slide change as well as unblanked changes to give the effect of instantaneous change-over. Artistic effects, particularly adapted to the flying-spot device, extend its flexibility.
SINCE THE EARLIEST ATTEMPTS to transmit pictorial information between remote points by electrical means, scanning subject material with a small spot of light and projecting the transmitted or reflected light upon the cathode of a phototube has received attention.1 The first television systems employing this principle utilized mechanical methods of scanning. With the advent in 1934 of electronic means for producing television pictures, the flying-spot method became less popular than the use of the image dissector, the iconoscope, and later the orthicon and image orthicon. The competitive position of the older scheme has been favored, however, by the recent development of cathode-ray tubes having very short-persistence screens. When an electron-multiplier-type phototube is utilized for translation, a simple pickup device is possible.2
Achieving a pleasing picture by the flying-spot principle in a direct pickup of live talent seems difficult when compared to present camera practice. The use of photographic transparencies is more convenient because the problems of depth of field and intensity of light are greatly simplified.
A practical adaptation suitable for televising 2 by 2-inch glass slides is outlined in Fig. 1. A television raster is formed on the face of the 10-inch cathode-ray tube, which is magnetically focused and
* Presented April 5, 1949, at the SMPE Convention in New York.
AUGUST, 1949 JOURNAL OF THE SMPE VOLUME 53 137