LIGHT SOURCES AND DETECTORS 



window and enclosed in a metal or bakelite housing, are also manufactured 

 (Megatron Ltd.), which enable them to be used in the presence of moisture 

 or corrosive vapours. 



When light falls on the cell the upper surface becomes electrically negative 

 with respect to the base plate. If an external load is connected across the 

 electrodes a current will flow; thus the cell may be connected directly to a 

 suitable galvanometer or meter and the resulting current measured. 



Transparent 



metallic 



film 



Sprayed 



metal 



electrode 



Selenium 



compound 



layer 



Figure 28.35 Cross-section through a selenium barrier-layer photocell 



A convenient equivalent circuit is shown in Figure 28.36. A current 

 generator /,, is shunted by a capacitor C and an internal resistance /?, in 

 series with a resistance R^. If the external load resistance is R, the current 

 through it becomes : 



R, + Rs + R 



Rg is mainly the value of the thin collecting electrode and is about 50 

 ohms in most cells. C varies from 0-1 to 0-5 /uF; depending upon the cell 



(Load resistance) 



Figure 28.36 Equivalent circuit of a barrier-layer cell 



area. This capacity limits the frequency response of the cell, i^ and R^ vary 

 with the intensity of the incident illumination. The relationship between 

 illumination and 7?^- for a 45 mm diameter Weston cell is shown in Figure 

 28.37. It can be seen that the equivalent internal resistance R^ varies both 

 with the external load R and with the illumination. The internal resistance 

 rises to a high value in darkness and this permits a number of cells to be 

 connected in parallel without mutual short circuiting when they are indepen- 

 dently illuminated. 



The maximum potential generated with high illumination is about 0-4 V 

 and it varies non-linearly with light intensity. The potential does not however 

 vary much between cells of different area. The low voltages and relatively 

 low impedances associated with barrier-layer cells render them unsuitable for 

 use with valve amplifiers. If further power amplification is required transis- 

 tors may conveniently be used (e.g. page 697). 



368 



