International projectionist (Oct 1931-Sept 1933)

July-August 1933 INTERNATIONAL PROJECTIONIST 19 ABC'S OF PHOTO-ELECTRICITY A. J. McMaster G-M LABORATORIES, INC. ALTHOUGH some of the electrical effects of light on certain substances have been known for nearly one hundred years, it is only within the past decade that substantial and significant progress in the engineering application of light sensitive devices has taken place. In order that we may find a proper place for photoelectricity in the modern scheme of engineering practice, let us consider for a moment some of the many forms of engineering in which the primary objective is the transformation of one form of energy to another. Thus, we have various forms of socalled electrical generators in which mechanical energy is converted to electrical energy. This process is readily reversible in transforming electrical to mechanical energy by means of motors, solenoids, etc. Again either mechanical or electrical energy may be changed to heat. Such processes are in some cases also reversible as evidenced by friction, electrical heating, the turbine, and thermo-electricity. Chemical energy may serve as a source of electrical power, and when we charge our automobile battery we again reverse the transformation process. One could name other examples of this universal effort to change one form of available energy to another, but sufficient mention has been made to establish the desirability of seeking engineering means for making all forms of energy interchangeable. But we have not as yet considered one of the most abundant forms of energy, namely, that of light. Abundant not only in nature, it is of course produced artificially on a tremendous scale. The utilization of solar energy goes on everywhere about us, but not in a direct and readily controllable manner. It is not the purpose of this paper to suggest a photo-electric method for the direct transformation of the sun's energy. This process, though possible, is yet too inefficient to be of the least economic value. However, we shall direct our attention briefly to the devices available for the utilization of light energy for measurement and control purposes. Types of Cells Photo-electricity has been defined as any electrical effect produced by the action of light. Three fundamental types of light sensitive phenomena will be considered briefly herein. The photo-emissive effect, though not the oldest, is perhaps the most common. The term "photo-electric cell" is generally used to designate a cell or tube in which electrons are emitted under the influence of electro magnetic radiation — usually visible, ultraviolet, or infra-red radiation. The photo conductive effect refers to devices in which light produces a change in electrical conductance. Such devices are known as photo-conductive cells of which the most common example is the selenium cell. The third classification is that of photo-voltaic cells of which there are the so-called electrolytic and electronic types. Such cells require no external source of voltage and have relatively low internal resistance. Photo-emissive cells have been made with many different types of sensitive surfaces. For use in the visible spectrum most cells have a surface of a pure or compounded alkali metal. Such surfaces must be prepared and main tained in a vacuum or an atmosphere of inert gas at low pressure. The cells generally take the form of a glass envelope within which the sensitive cathode is either deposited on the inner wall of the bulb or on a suitable plate structure. The anode is usually of considerably smaller area than the cathode and may take the form of a wire or ring which serves to collect the electron emission from the cathode. If the finished cell is highly evacuated, it is known as a vacuum cell, in which the photo-electrons are the only current carriers. If the cell is filled with an inert gas at low pressure it is called a gas-filled cell, in which the primary electrons ionize the gas, resulting in a total photo-electric current of from one to 50 times as great as that in a similar vacuum cell. Photo-Emissive Characteristics Some of the principle characteristics of photo-emissive cells are shown in Figures 1 to 8. In Fig. 1 the relative response of the various alkali metals to various wave-lengths of light is shown. It will be noted that all of the alkalis are sensitive in the visible spectrum which extends approximately from 4,000 to 8,000 Angstrom Units. The maxima of sensitivity are found farther toward the longer wave-lengths for increasing atomic weight of the alkali metals. Sodium and potassium are generally sensitized with hydrogen, which process shifts the maxima of these two alkalis slightly toward the red end. Tungsten incandescent lamps are generally used as light sources with photo-electric cells. The distribution of energy from a tungsten filament operated at 5000° K is shown in Fig. 4. Because this source emits most of its energy in the red and infrared, it has been desirable to produce COtO;! SENSITIVITY OF .„ ALtTALI METALS (SCILEFi) fi Li. |<o 6 //f : F'G.i ^3. / n\ *^ •4 //\\\^;e4 "^ ^° / V(\i(— ~<^'' s 2 -^^^^SnJI" COLOR SENSITIVITY OF POTAS ilUMSr SODIUM SUi FIDa CELLS iOLPIN) 4000 fOOO 600O 7O0O &00 0 WAVE LEuaTH-ANesmoMS lost by radiation into the open air overhead. Knowing the distribution characteristics and efficiency of a loudspeaker, and the sound pressure or acoustic power desired at any point in the sound field, it is possible to determine the electrical power required at the input of the loudspeaker. Calculations show that the power output or size of equipment varies directly with the area or number of seats to be covered. Such calculations show that for normal conditions of noise, the power out put (40 watts), of the Photophone PG-32 equipment is ample for covering the listening area in the Driv-In Theatre. The rugged construction of the dynamic cone driver in the directional baffle type of loudspeaker makes possible an unusually large power handling capacity and consequently a minimum number of loudspeakers. Three directional baffle loudspeakers are more than adequate for handling the output of the PG-32 equipment without distortion. photo-electric cells which have greater sensitivity for long wave-lengths of light. Progress has been made in this direction at Bell Telephone Laboratories as illustrated by the curves in Figure 2. The most generally used cell today is that known as the Cs-Cs^O-Ag, or more simply, the caesium-oxide tj'pe. The sensitive surface consists of a silver film or plate which is oxidized and upon which a carefuly controlled c]uan