GENERATION, CONTROL, AND MEASUREMENT 



237 



1200-m/i limit of the S-1 photoemission cell to 3000 m/z for the uncooled 

 lead sulfide cell and 5500 m/x for the Hquid-air-cooled lead telluride cell. 

 Up to their respective limits these cells have a much higher sensitivity, 

 as limited by internal noise, than the best of the thermal detectors and 

 are much faster. A lead sulfide film, 0.1-1 fx in thickness, is deposited 

 either by evaporation in vacuo on the inner surface of a glass envelope 

 or by chemical precipitation onto a glass plate. The chemically precipi- 

 tated cells (Ektron) do not require evacuation and may be used with the 

 film exposed to the air or partially protected by a film of transparent 



100 - 



o 



Q- 



80 



60 - 





40 - 



20 - 



300 



1000 



5000 



FiG. 3-35. 

 cells. 



2000 3000 4000 



WAVE LENGTH, m^ 



Spectral sensitivity of the lead sulfide and lead telluride photoconductive 



plastic. The spectral characteristic^ vary considerably with the method 

 of preparation; typical curves are given in Fig. 3-35. 



The dark resistance for square films is usually in the range from 0.1 to 

 several megohms. The cells are used in a two-arm bridge similar to that 

 given for the thermistor bolometer in Fig. 3-25. The time constant is of 

 the order of 100 ^sec, so that flux-modulation frequencies of 60-1000 cps 

 can be used conveniently. Photoconductive cells are seldom used with 

 unmodulated flux because of slow fluctuations in resistance due to tem- 

 perature and fatigue. It is much more difficult to compensate for these 

 changes by the use of a pair of matched cells than with bolometer 

 elements. 



The lead telluride cell has such a long wave-length limit that it is 

 necessary to operate it at the liquid-air temperature of — 190°C to reduce 

 thermal-radiation noise. However, cooling creates difficult practical 

 problems of moisture precipitation on cold windows and the restricting 

 of the thermal flux that enters the window. 



Photovoltaic Cell. The photovoltaic cell has a low internal resistance 

 and generates suflacient voltage to operate microammeters directly. It 



