814 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



PHOTON EFFICIENCY 



It is of interest to examine the spectral photon efficiency of a cell made 

 by bombardment. As an example, there may be taken the 3-kv cell 

 whose spectral response is shown in Fig. 8. When illuminated by a tung- 

 sten light of 2848°K color temperature at 10-foot candles, a sensitivity 

 of 3090 microamps per lumen was secured. Allowing for a surface re- 

 flectance loss of 0.385, this value becomes 5020 for the radiation actually 

 absorbed. From these data the sensitivity in microamps per microwatt 

 at the peak 0.725 u, calculates to be 0.388 and the photon efficiency, i.e., 

 the electrons per photon, 0.66. Fig. 12 gives the efficiency through the 

 spectrum. Note that the efficiency rises some on the short wave side 

 shifting the peak of the equi-energy curve (Fig. 8) over to 0.625 n. This 

 increase is evident from the fact that if the equi-energy curve decreased 

 linearly from the peak at 0.725 ju to zero at /x, the photon efficiency 

 would remain constant and ec^ual to that at 0.725 m- For the purpose of 

 the above calculation, the curve in Fig. 8 has been taken as going to zero 

 at about 0.40 iu, a fact experimentally checked. If unity is considered to 

 be the maximum possible efficiency at any wavelength, 72 per cent of it 

 is attained at 0.625 m and nearly half of the spectral range is 50 per cent 

 or higher. 



0.6 0.7 0.8 0.9 1.0 



WAVELENGTH IN MICRONS 



Fig. 12 — Spectral photon efficiency of the 3-kv cell of Fig. 8. 



