International projectionist (Jan-Dec 1940)

" «u>»j ■. ■. ■ r -;■•; .-, >" ..■■■■■...■■ n I c I! VOLUME XV SEPTEMBER 1940 NUMBER 9 The Caesium Gas-Filled Phototube A COMPLETE understanding of the functioning of all units in a sound-projection equipment means better operation, better performance, and a better show. A greater personal satisfaction and a bigger thrill are experienced by projectionists in operating this apparatus, if mysteries are given no place in the projection room. A knowledge only of how to operate a system does not suffice and should be supplemented by knowing '"what makes it tick." Ever since the first sound film reproducer was devised, much has been published regarding the main units which make up a complete system, such as soundheads, amplifiers, loudspeakers, and power supplies, because of the continual progressive design changes which have been made in the characteristics and external appearance of these units. However, there is one extremely interesting little device that has remained essentially the same throughout these years — that is the light sensitive cell, usually termed the phototube— the veritable heart of the system. Original development and design work on the caesium tube with gas amplification has proven so satisfactory for film sound reproduction that nothing superior to it has been developed in the intervening years. What goes on inside this device to convert the flickering light of the scanning system into an electric current? This is a subject on which By PAUL P. MELROY PHOTOPHONE SERVICE DIVISION RCA MANUFACTURING COMPANY, INC. volumes might be written, if all facts, including those relating to early important experiments, were collected for review. However, if this discussion is confined to the bare essentials necessary to describe its internal working, it should be both useful and interesting. A non-technical definition of a phototube would probably infer that it is a tube which changes light variations into electrical impulses. That would be correct for sound-film reproducer applications, and would constitute as general a statement as could be conceived for a comprehensive definition. However, a more basic definition would be to say that a phototube is a device in which electron emission is produced by the illumination of an electrode. The difference between these definitions is that the former considers the final effect, while the latter gets at the cause. • Types of P.-E. Devices The internal working of photo-electiic devices, in general, can be classified under three different groups: (1) photo-emission (2) photo-conduction and (3) photo-barrier or photo-voltaic — all depending for their action on radiant energy. Inasmuch as the soundpicture phototube comes under the first group in which electrons are emitted from the cathode and collected upon the anode, the different reaction of the other two groups will be but briefly mentioned. The photo-conduction second group depends upon the internal resistance of the device varying in accordance with illumination changes, such as the selenium cell; the photo-barrier or photo-voltaic third group generates an internal voltage proportional to the amount of light impressed on either an electrode or its electrolytic medium, such as in exposure meters. As light-sensitive devices depend on radiant energy for their operation, a clear understanding of the fundamentals and sources of radiant energy is essential. Energy associated with electromagnetic oscillations travelling through space is radiant energy. The frequencies of such oscillations extend over an almost unbelievable range: from the lowest frequency of alternating current up through the radio frequency spectrum, light spectrum, X-rays, gamma rays, to the cosmic rays with a frequency as high as ten thousand billion cycles per second. Of particular interest is the fact that the only part of this vast spectrum visible to the human eye is a tiny band of frequencies near 750 thousand billion cycles per second. This small band of visible light rays includes red, orange, yellow, green, blue, and violet colors in their respective frequency order and make up the useful range of radiant energy to which the more SEPTEMBER 1940