Sec. 5-1] RADIATION TRANSDUCERS 235 



wavelength from zero at the threshold wavelength, in general, in a 

 monotonic function. A normal photoelectric effect is demonstrated 

 by Fig. (5-1)2, curve a. Some metals, e.g., alkali metals in an atmos- 

 phere containing oxygen or hydrogen, show a considerably increased 

 yield in a narrow wavelength range. A selective photoelectric effect 

 is shown by Fig. (5-1)2, curve b. The selective photoeffect has its 

 origin in a thin surface layer. 



The quantum yield on pure metal surfaces is of the order of 0. 1 per 

 cent, i.e., one electron emitted per thousand absorbed quanta. Com- 

 mercial photoelectric devices have a quantum yield of the order of 5 

 per cent. Quantum yields as high as 30 per cent have been observed 

 and used in photoemissive devices. 



The photoelectric effect is not limited to metals but occurs also on 

 insulators and semiconductors. 



The photoelectric current / is proportional to the illumination O 

 of the cathode 



where S is the sensitivity, a magnitude depending upon the composi- 

 tion of the cathode surface and, to a lesser extent, upon the tube 

 construction. <I> is the incident radiation flux, expressed either in 

 watts or in lumens. Correspondingly, the sensitivity is expressed 

 either in amperes per watt ("radiant sensitivity") or in amperes per 

 lumen ("luminous sensitivity"). 



The sensitivity of photoemissive cathodes varies strongly with the 

 wavelength of the incident radiation. The response characteristic 

 I — fW depends upon the composition and the processing of the 

 cathode. A considerable amount of work has been done to produce 

 photocathodes of high sensitivity over the entire spectrum or in 

 certain ranges of the spectrum. 



For references, see V. K. Zworykin and E. G. Ramberg, "Photoelectricity 

 and Its Application," John Wiley & Sons, Inc., New York, 1949; P. Gorlich, 

 "Die lichtelektrischen Zellen," Akademische Verlagsgesellschaft Geest and 

 Portig K.-G., Leipzig, 1951; A. Sommer, "Photoelectric Tubes," Methuen & 

 Co., Ltd., London, and John Wiley & Sons, Inc., New York, 1951; for review 

 papers, see M. Ploke, Arch. tech. Messen, J 391-5, November, 1953, and J 391-6, 

 January, 1954. 



Three types of photoemissive cathodes are being used: 

 1. Ag-O-M type (M stands for an alkali metal). The cathode con- 

 sists essentially of a silver layer that is oxidized and covered with a 

 layer of an alkali metal (e.g., cesium). The cathode has one maximum 

 sensitivity in the region around 800 m// and a second maximum in the 



