Quantum efficiency of a light sensitive device is defined as the number of electrons released per unit incident quantum. Values of efficiencies are obtained from spectral response curves plotted in absolute values. The necessity of absolute values requires the use of a calibrated thermopile. The calibration of the thermopile and the method for obtaining spectral response curves are described. Curves are plotted for the visible region giving the maximum theoretical yield and also the yield and quantum efficiency for two types of Photox, two types of selenium photovoltaic units, a cesium oxide phototube and the thermopile. The maximum yield for photovoltaic units is found to range from 0.045 to 0.125 ampere per watt with a quantum efficiency range of from 9 to 28 percent. The maximum visible yield of the cesium oxide surface is 0.002 ampere per watt and a quantum efficiency of 0.4 percent, while the thermopile yield is 0.022 ampere per watt with an efficiency of 5 percent.

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