Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, April, 1930 Page IS Photocell Applications in Projection Work Operating Circuits of Cells of Two General Classes By C. H. W. Nason* ALMOST a decade ago, fresh from school, the writer was invited to witness the operation of a device for sorting tobacco leaves according to eight gradations of color. The apparatus was accepted as a cog in the mechanism of production of one of the great tobacco companies and, according to advices, is still in operation. Students of ten years ago were well aware of Hallwaeh's effect and of the work of Hertz in the photoelectric field. Already deForest and Case had utilized the photoelectric cell in connection with the talking moving picture and I believe it was Jack Hammond whose "electric dog" followed a pocket flashlight faithfully about by virtue of the . >20 H rJwIO Photocell in connection with a-c. amplifier for photoelectric measurement. -3L fig.3 upon the surface of certain metals an electron emission occurs. The basic measurement established is that the number of electrons given off varies in linear relationship with the intensity of the incident light, provided the | | <* .../ V — p OTA SSI JM Cfi ES! UM c£" v^^ sc DIUM "j '/ -~_ "X \ — i^ — --_ ■•*, \ N ~*~ -~. ^*C '« s»% ""- - -^ Color sensitivity of photoelectric cells. ANGSTROMS VISIBLE LIGHT RANGE | I INFRA I I VIOLET FIG. -I RED \ RED selenium cells his master had given him for eyes. Catalogs of companies dealing in "wireless" goods listed the element selenium in stick form and one organization in Cleveland gave detailed instructions regarding the preparation of selenium cells. All these facts are common knowledge to experimenters of a few years standing. It is the photoelectric cell of today which claims our attention. It is known that when light falls * Jenkins Television Corp. wavelength of the light remains constant. The wavelength of light is measured in meters, as are radio of one of the alkali metals, applied uDon the inner surface of the glass envelope and constituting the cathode of the cell. The anode is an electrode interposed between the photo-sensitive material and the light source, usually in the form of a wire loop. Potassium, sodium or caesium are those most usually employed, the three differing widely in their color sensitivity as Is shown by the family of curves in the figure already referred to. Where extreme sensitivity to "blue" or to the ultra violet is required, the envelopes must be constructed of fused quartz. Otherwise, a good grade of commercial lime is satisfactory. Two General Classes of Cells The photoelectric cells in general use today fall into two general classifications other than as to the active material employed. Those evacuated to the degree of one or two microns, and the gas-filled cells which contain some inert gas at a pressure of from 15 to 200 microns. The latter have Photocell and amplifier as employed in sound reproduction and in television. Photocell connection for relay operation. waves, but the wavelengths involved are so short as to render the meter a cumbersome reference point and the Angstrom is taken as the unit of measurement. The Angstrom is taken as A=cwxl0-8. The curves in Fig. 1, which are those of several photocells with the relative response or sensitivity plotted against the wavelength of the incident light, demonstrate quite clearly the range of visible and invisible light in Angstroms. Photoelectric cells are constructed with the active medium, the hydride a considerably higher output than the extremely hard type but suffer to some extent from hysteresis and from nonlinearity of their sensitivity characteristic. The principle of the gas-filled cell with its attendant higher output lies in the ionization of the gas by the electrons emitted under incident light. The ionization current is added to the current through the electron stream. The remarks concerning hysteresis and non-linearity must not be taken as reflections upon the excellence of the