Photoelectric Cells

Author: E.E. Kimberly

The photoelectric cell, or phototube, is a diode in which the cathode is usually of half-cylindrical shape and covered inside with a material such as lithium, potassium, rubidium, sodium, or cesium which is capable of emitting electrons when it is struck by light. The anode is a small metal rod located at the axis of the cathode, and is made slender so as to cut off as little light as possible, When light strikes the emitting material, electrons are emitted as from a hot cathode but in relatively much smaller numbers. The resulting current is very small, and the plate voltage must be high to force that current through a resistance large enough to produce an IR drop that is usable as output voltage to the grid circuit of an amplifier. Fig. 27-44 shows a simple circuit for a phototube.

Fig. 27-44. Simple Photoelectric-Cell Circuit

Fig. 27-45. Sample Characteristics of Vacuum Phototube

The phototube is not equally responsive to all colors or wavelengths of light. Lithium is most responsive to the blue region of the spectrum, while cesium is most responsive to the red region. By combining different metals the spectral response may be extended over a wide range. Phototubes are made in both vacuum and gas-filled types. The characteristics of the vacuum type are shown in Fig. 27-45. The vacuum phototube has almost linear response to variations of light intensity if the color quality of the light remains unchanged. It is therefore adaptable where reproducible results are necessary. The load line may be used on phototube characteristics the same as on those of a triode.