238 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



visible light, equals 1 lumen. Sometimes (but rarely) the Interna- 

 tional Candle is used as a standard source. The International Candle 

 furnishes a radiation of 1.05 x 10~ 4 watt/cm 2 at a distance of 1 m. 

 For a maximum output, the peak of the photocathode response 

 characteristic should be matched to that of the light source. The 



100 



80 



■t 60 



40 



20 



100 



900 



Fig. (5-1)5. Relative sensitivity of different 

 commercial photocathodes (from RCA Tube 

 Manual; by permission). 



photocathode is not always equally sensitive over its entire surface 

 but may have patches of higher or lower sensitivity. Uniform illumi- 

 nation of the cathode is recommended, therefore. 



Undesirable output signals in photoemissive tubes are caused by 

 dark current, i.e., a current passing through the tube even in the 

 absence of light, as well as by shot effect and by Johnson effect. 

 Sources of undesirable signals of minor importance can be flicker 

 effect and wall charges. 



The dark current can have four causes: (1) thermionic emission 

 from the cathode and, in photomultipliers, from the dynodes, (2) 

 leakage current over the glass wall, (3) current caused by the impact 

 of positive ions upon the cathode (or, in photomultipliers. the 

 dynodes), and (4) field emission. 



