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1948-49 Theatre Catalog, 7th Edition, Page 543 (528)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 543
Page 543

1948-49 Theatre Catalog, 7th Edition, Page 543

Large Screen Television

An Analysis* of the Advantages and Disadvantages of the Existing Methods With Theatre Application

In its phenomenal development, television has already reached vast audiences in areas served by television broadcasting stations. Another potentiality of television for entertainment and education is its use in the theatre.

Two basic systems of large screen television are being evaluated through a series of practical tests. One is the direct Projection system by which high brilliance kinescope images are projecte ed through an efficient reflective optical system; the other an intermediate film system using standard motion picture

projection technique, after television images have been photographed on motion picture film and suitably processed.

Direct Proiection

The direct projection television system consists of three major elements; (1) the projection kinescope which is the source of the light image; (2) the optical system which projects the image onto the screen; (3) and the screen from which the final image is viewed. The three elements as shown in Fig. 1, must be designed into a coordinated system to provide the maximum overall efficiency. Let us examine each element of the system in order to understand the

FIGURE NUMBER 1. Simplified diagram of projector using reflective optics.


Theatre Television Products Engineer Radio Corporation 0] America, Camden. N. I.

problems involved as well as the possibilities and limitations of each element.

Proiec'l'ion Kinescope

The kinescope used in the direct system is similar to the direct viewing tube used in the conventional television receiver; except that projection kinescopes have a much greater light output due to higher voltage operation and for which they are specially designed. We see, in Fig. 2, the diagram of a typical projection kinescope; the electron gun emits a stream of electrons which are focussed by an electron lens controlled by the first anode voltage; then the electrons, accelerated by the high voltage potential, strike to the screen where they cause the phosphor coating of the tube face to emit light in accordance with the density of the electron beam. The controlling element of the tube is the control grid which is fed from the video signal. A deliection yoke surrounds the neck of the tube providing the scanning raster necessary to form a picture image. The phosphor screen is backed with an aluminum coating which increases the tube eiiiciency by; (a) reflecting the majority of the light produced in the phosphor through the .face

plate, (b) permits higher current densities without burning screen and (c) allows secondary emission to be carried off rapidly so that sticking or current limiting does not take place.

Since television is not adapted, at the present state of the art, to the use of supplementary light sources as in motion picture practice, the brightness of the image for projection depends upon the efficiency of the kinescope and the operating potentials. An average picture on a projection kinescope will draw a beam current of approximately one milliampere at a potential of eighty kilovolts. This is a power of only eighty watts and with a screen efliciency of five candle power per watt, provides a light output of 400 candle power. Future efforts will be concentrated on the development of tubes capable of handling greater beam currents and operating at higher potentials in an effort to increase the light output.

Optical System

The familiar refractive projection optics used in motion picture film projectors delivers approximately six per cent of the light from the arc light source to the screen, on the other hand the re *Puper presented at the National Electronics Conference. Chicago, Illinois. Nov. 51h. 1948.

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1948-49 Theatre Catalog, 7th Edition, Page 543