LIGHT SOURCES AND DETECTORS 



transistor circuits. The power available from the sun's radiation at ground 

 level is about 1 kW/m^ and the popular name for the barrier-layer cell is 

 becoming 'the sun battery'. 



120 



no 



> 



a> 



100 



90 



80 



70 



'> 



5) 60 



50 



40 

 30 

 20 

 10 



1 1 1 1 1 1 1 \ 1 1— 



• Bradley' Luxtron'(no cover) 



o Vickers self-gererating photocell (lacquer- coated) 



n General Electric (glass cover) 



X Weston 'Photronic' Type 3(glass cover) 



4 Weston 'Photronic' Type 1 (glass cover) 



2.000 3.000 4.000 



5.000 6,000 7,000 

 Wavelength 



8.000 



9.000 

 A 



Figure 28.41 Spectral response of various selenium barrier-layer cells of 



American manufacture 



Photo conductive cells 



Several substances exhibit the property of altering their electrical resistance 

 when radiant energy falls upon them, and this enables them to be used as 

 photodetectors. Interest in the detection of infra-red radiation during World 

 War II resulted in the development of a number of photocells in this class. 

 As light from a tungsten source contains much of its radiant energy in the 

 infra-red, these detectors work well in conjunction with a tungsten lamp. 

 The lead sulphide cell is the best known. 



372 



