LIGHT SOURCES AND DETECTORS 



It follows that, providing the anode voltage is kept above this saturation 

 voltage, the anode current will be proportional to the light flux falling upon 

 the cathode. The voltage applied to the anode of a vacuum cell should not 

 normally exceed 100 V nor should the cathode current exceed 20 //A for the 

 type 20CV cell. The dark current for a cell of this type is less than 0-05 fiK. 

 Many users will not wish to detect or measure light intensity by measuring 



< 

 15 



10 



25 



50 



75 



100 



volts 



Figure 28.27 Performance data of the Mallard 20CV photocell. Anode 

 current plotted against anode voltage with total illumination as parameter 



current of the order of 10 //A and would prefer a voltage output. This is 

 readily done by applying the anode voltage through a high resistance so that 

 any increase in anode current will produce a voltage drop across the resis- 

 tance. If a linear response is required the resistance should be chosen so 

 that the anode voltage remains above the saturation voltage. In this way a 

 voltage swing of some 75 may be achieved over the working illumination 

 range. In practice it is difficult to use anode loads higher than 10 megohms. 

 Reference to Figure 28.27 will assist in selecting a suitable anode load. The 

 20CV has a sensitivity of 25 //A/lm when the whole cathode (projected area 

 6-7 cm^) is illuminated by a lamp of C.T. 2,700°K and tested with a series 

 resistance of 1 megohm. 



The characteristics of the gas filled photocell 20CG is shown in Figure 

 28.28. Under favourable conditions the sensitivity of this cell is 150 /^A/lm 

 or about 5 times that of its vacuum counterpart. It should be noted that the 

 maximum current (5 /iA) which may be drawn from the cathode is consider- 

 ably reduced in order to prevent damage to the cathode by ionization of the 

 gas filling. The anode voltage must not exceed 90 V as complete ionization 

 or 'gas glow' may occur with resulting damage to the cathode caused by the 

 heavy current. This may occur even in the absence of light. The sensitivity 

 of the cell varies with the anode voltage, being greatest at the point just 



360 



