LIGHT SOURCES AND DETECTORS 



image on the cell, then cells with a larger photosensitive area are available. 

 For example the 10 X 10 mm cell will pass 10 mA in an illumination level of 

 100 Ix, with a dark current of 6 ixK when working on 6 V, giving a sensi- 

 tivity of 1 A/lm. 



20 40 



P.D. on cell 



60 



Figure 28.48 Characteristic curves for a Schwartz cadmium-selenide cell 



The time constant of Schwartz cells varies with the size of the sensitive 

 area and is of the order of 1 to 10 msec. No definite information is available 

 on fatiguing tendencies but it may be significant that the manufacturers 

 advise that these cells are intended for monitoring purposes only, and not for 

 the quantitative measurement of hght intensities. 



The General Electric Company Ltd. are developing cadmium-sulphide 

 photoconductive cells of two types — a miniature single-crystal cell and a 

 large-area powder-layer cell. Various spectral sensitivities can be obtained 

 by varying the production technique. Peaks lying between 500 and 700 m// 

 have been produced. The sensitivity to tungsten light is about 1 A/lm. 

 Mullard are developing similar powder-layer cells which at low illumination 

 may be used to operate relays directly. 



Germanium diffusion-junction photocells 



Standard Telephones and Cables, Ltd., are now manufacturing germanium 

 diffusion-junction photodetectors which have the great advantage of small 

 size and large current or voltage output. The P50A has an average effective 

 sensitivity area of 1-5 by 0-1 mm. The unit is enclosed in a small sealed 

 tube 5-6 mm in diameter and 8-8 mm long. 



378 



