Radio Broadcast (May 1928-Apr 1929)

TECHNICAL DATA ' SOUND MOTION I I PICTURES Further Data on Photo-Cell Characteristics BY CARL DREHER THE construction and general theory of operation of photo-electric cells has been discussed in a previous article (November, 1928, Radio Broadcast). The fundamental importance of the device makes it advisable to consider its properties further, however, and in more quantitative terms than formerly. Dr. Herbert E. Ives in his paper "The Alkali Metal Photo-electric Cell," Bell System Technical Journal, April, 1926, gives curves showing the behavior of cells used in picture transmission. The characteristics here reproduced as Fig. 1 show the voltagecurrent relationship for vacuum cells with a small centrally placed anode within a concentric spherical cathode. That is, the positively charged plate or collecting member is placed within, and about at the center, of a considerably larger sphere, the inside of which is coated with the emitting material, and the cell contains no gas. The anode receiving a positive charge with respect to the cathode, the current for a given illumination increases with the voltage between the electrodes, at first rapidly, then more slowly, until a saturation point is reached. The shape of the curve depends somewhat on the wavelength of the light falling on the cell ; at longer wavelengths the electrons given off by the cathode move more slowly and are collected more quickly by the anode. Now if a small quantity of gas is introduced into the cell, the curve changes remarkably, to the shape shown in Fig. 2. The wavelength of the incident light still plays a part, but now instead of saturating, so that increasing the voltage beyond a certain point no longer effects an increase in the space current, the cell has a critical potential at which the gas breaks down. With the addition of the gas (very little is introduced — a fraction of a millimeter of mercury being the pressure) the cell becomes more sensitive, and also somewhat more liable to variations in production and damage in use. The electrons liberated by the cathode collide with mole cules of gas, producing ionization phenomena which increase the current. In both vacuumand gas-type cells, if the voltage is held constant and the illumination is changed, a linear relationship between illumination and current This is the second of a series of articles dealing with sound motion pictures. Radio Broadcast was first and alone in its field to provide intelligent and authoritative articles on the engineering aspects of broadcasting and we are happy to be first now with authoritative articles on sound movies. The latter field is so close to broadcast engineering that it is proving of absorbing interest to almost everyone in radio. Pages in this magazine will regularly be devoted to this subject — The Editor. 0 50 100 -VOLTS ON CATHODE Fig. 2 -VOLTS ON POTASSIUM Fig. 3 is found, provided some structural precautions are taken. If the window of the cell, through which the light enters, is made too large, it is likely to become charged and to cause a curved illmnination-current characteristic. Fig. 3, also taken from Ives, shows the current-voltage relationship in microamperes and volts for a potassium cell used in picture transmission, the illumination being 100 meter candles from a tungsten lamp, with an aperture area in the cell of 1.5 sq. cm. The luminous flax reaching the cathode is given as 0.015 lumen. The meaning of these photometric terms may as well be explained before the discussion is continued. The Handbook of Chemistry and Physics, compiled by C. D. Hodgman and N. A. Lange, published by the Chemical Rubber Company at Cleveland, and now in its thirteenth edition, is useful in this connection. The principal definitions required are given on pages 10041005. Since light is radiated in all directions, we must first consider some solid geometry. The surface of a sphere of radius r is given by 4xr2. The unit of solid angle, called the steradian, is the angle which encloses a surface on a sphere equivalent to the square of the radius, that is, the total area divided by 4x. The Mai luminous flux from a light source is the total visible energy emitted in unit time. The unit, called the lumen, is the flux emitted in a unit solid angle as defined above, from a point source of one candle intensity. It follows that the total emission of one candle equals 4t lumens. Luminous intensity or candle power is the property of a source of emitting luminous flux; it is measured by the flux emitted per unit solid angle. The unit is the international candle, which is approximately the intensity of a standard English sperm candle (there are more constant and precise standards). Illumination on a surface is measured by the luminous flux incident on a unit of that area. The common units are the lux, one lumen per square meter, and the lumen per square foot. At unit distance from a point source of unit intensity the illumination is unity, hence such units of illumination as the meter candle (lux) and the foot candle (one lumen per square foot). In slightly less technical language, we may summarize the above by saying that a light source emits luminous flux, measured in lumens, that the intensity of the source in candle power is measured by the flux or flow in a unit solid angle, and that the illumination is a matter of flux per unit area, and depends on the intensity of the source and the distance from it of the surface. Applying these ideas to the cell cited by Dr. Ives, we note that the illumination given is 100 meter candles. This might come from a source having an intensity of 100 candles placed one meter from the cell. Such a source evidently emits 400x lumens over a sphere with a surface, at a distance of a meter, of 4x(l)2 square meters, or 4x square meters, or 40,000x square centimeters. We are also told that the area of the window of the cell is 1.5 square centimeters. Hence the flux in lumens reaching the interior of the cell will equal 1.5 400x 40000x = 0.015 lumens This checks the figure given in the paper. If the calculation is not clear, the reader is referred to Fig. 4, which is not taken from Dr. Ives' paper, but has been added to clarify the present discussion. If the units are consistent, the flux entering the cell may be obtained by multiplying the iflumination by the area of the window. Current-voltages characteristics for another type of photo-cell are shown in Fig. 5. For convenience this cell, which is also of the gasfilled type, will be referred to as Cell No. 2. The window in this case is larger (about 8 square centimeters as compared to 1.5 square 15 CM dowof photo-cell '^x\(FluX= 0.015 *0 *<%\ lumens) Light Source -dOO Candles) \ Fig. 4 • february, 1929 page 244 •