VISIBLE AND NEAR-VISIBLE RADIATION 195 



minimize this weakness. The short-wave-length Umit of sensitivity- 

 arises from the absorption characteristics of the windows. If a quartz 

 window is used, the sensitivity rises again in going from 3000 to 2400 A. 

 The long-wave-length limit occurs somewhere between 11,000 and 12,000 

 A. Number 5 shows the typical sensitivity of an alkali metal cell, 

 employing a sodium surface. In all such cells, the sensitivity varies 

 widely, depending upon the methods of manufacture. Individual cells 

 of the same type also vary over a considerable range. If such a photocell 

 is filled with gas instead of evacuated, the primary electrons produce 

 secondary electrons in the gas and a correspondingly higher sensitivity 

 results, the increase generally varying from four to ten times. The 

 dotted curves indicate the sensitivity of corresponding gas-filled cells. 

 Another type of photoelectric cell deserves particular mention, namely, 

 the photovoltaic cell. That a cuprous oxide rectifier cell (CU2O on Cu) 

 shows a response to light was observed by Grondahl in 1927 (33). Since 

 then, such cells have been widely developed experimentally as light 

 detectors (10). They consist of two or three thin layers in close contact, 

 and in common practice they are not evacuated. Another example is 

 iron on selenium on lead, the iron being so thin as to be practically 

 transparent. 



The sensitivity curve of a commercial photovoltaic cell (Weston 

 photronic) is indicated on a reduced scale by the dash-dot curve, No. 6, 

 Fig. 31. The sensitivity values for this cell must be multiplied by 10. 

 The cell of copper oxide on copper shows a longer wave-length maximum 

 and extends deeper into the infra-red. 



The type of cell indicated by curve 6 has received wide application for 

 three reasons: (a) As it generates electromotive force in the presence of 

 light, it need only be connected with a galvanometer in order to secure a 

 deflection. (6) Such cells produce a current higher by an order of magni- 

 tude than other types of photoelectric cells, and consequently require a 

 less sensitive current-measuring device, (c) It exhibits a sensitivity 

 curve very similar to the visibility curve of the eye, and hence has a wide 

 applicability for illumination purposes. 



If it is possible to prepare special cells or to secure them, a wide range 

 of sensitivity characteristics is available. Figure 32 is representative of 

 the general possibilities. Section (1) of this figure (39) shows the relative- 

 sensitivity characteristics of the different alkali metals, exhibiting maxi- 

 mum sensitivity varying from 2800 to 5500 A, with long-wave-length 



o _^ 



thresholds varying from 5000 A into the infra-red. For photocells whose 

 long-wave-length thresholds occur in the blue and ultra-violet, a number 

 of possibilities are indicated in (2) and (3) of this figure (data through 

 courtesy of H. C. Rentschler, Westinghouse Electric Co.). The short- 

 wave-length limits of these cells, as indicated, arise chiefly from the win- 



