GENERATION, CONTROL, AND MEASUREMENT 



239 



layer on an iron plate (see Fig. 3-36). The layer is then annealed to 

 return it to its crystalline state. A semitransparent layer of gold or other 

 refractory metal is evaporated, sputtered, or sprayed onto the selenium. 

 Connection is usually made to the top film electrode by a metal ring. 



The spectral-sensitivity curve of the selenium cell (Fig. 3-37) is similar 

 to the day or photopic response of the human eye. With a green glass 

 filter, the over-all response can be made to match very closely the lumi- 

 nosity curve of the International Committee on Illumination for the 

 Standard Observer. The Viscor 

 Weston Photronic cell is an example 

 of a filter-corrected photometer de- 

 tector (Weston Electrical Instru- 

 ment Corporation; Zworykin and 

 Ramberg, 1949). The filter reduces 

 the over-all sensitivity by about 

 40 per cent. Filter-corrected sele- 



3000 



Ri 



Fig. 3-38. Equivalent, circuit of the 

 photovoltaic barrier-layer cell. 



500 1000 1500 2000 



IRRADIANCE, ft-c' 



Fig. 3-39. Irradiance-current response 

 curves for the Weston Photronic cell at 

 various values of load resistance. {From 

 Weston Electrical Instrument Corporation.) 



nium cells are extensively used in such portable photometric instruments 

 as light meters calibrated directly in the foot-candle or lux. These instru- 

 ments require periodic calibration, because the best cells slowly deterio- 

 rate with time and especially upon exposure to intense sunlight. 



The equivalent circuit of a barrier-layer cell is given in Fig. 3-38. It is 

 equivalent to an emf eo shunted by a capacity C and a variable resistor Ri 

 which are in series with a second resistor Rs. The shunting resistor Ri is 

 the resistance of the semiconductor film which decreases with the inten- 

 sity of the radiant flux. Consequently the external open-circuit voltage 

 is not a linear function of intensity, and the voltage increases less rapidly 

 than the irradiance (Fig. 3-39). It will be noted from the nature of the 

 equivalent internal circuit that, as the load resistance, which may be a 

 microammeter, decreases, the effect of Ri becomes correspondingly less. 

 At a very low resistance of a few ohms, the response is quite linear, since 

 the shunting effect of Ri is negligible. Unfortunately this is a condition 

 of inefficient power transfer, because the load resistance is low compared 



