PHOTOELECTRIC DETECTORS 



Lead-sulphide photoconductive cell 



The lead-sulphide cell consists of a layer of specially prepared lead sulphide 

 a few microns thick deposited on glass and sealed in a protective mount. 



The Mullard 61SV is a typical example of an uncooled lead cell having a 

 high sensitivity at room temperatures. The sensitive area is 6 by 6 mm, and 

 the device has a resistance of about 6 megohms in darkness. The spectral 

 response curve is shown in Figure 28.42. It can be seen to have a peak at 

 2-5 [I with a range from 0-3 to 3-5 (.i. 



100 



30 



10 



in 



Q. 

 tA 



q: 



03 



01 



Figure 28.42a Spectral response curve of the Mullard 61 SV lead-sulphide 



photoconductive cell 



The cell may be used connected to a direct voltage source and the change 

 in resistance detected by placing a meter in series; or a more satisfactory 

 arrangement might be to place the cell in one arm of a Wheatstone bridge 

 and then back off the standing current which would otherwise flow through 

 the meter when the cell was in darkness. A chopped light source may be 

 used and the alternating portion of the signal may then be fed by means of 

 a coupling capacitor to an a.c. amplifier. 



In Figure 28.42 is shown the output of a 61 SV under various loads, 

 voltages, and light intensities. If tested with a lamp of C.T. 2,700°K, the 

 sensitivity is found to be about 3 ma/lm. Due to the good infra-red perfor- 

 mance of these cells, a source at a temperature of only 100 to 200°C can be 

 detected. Without any optical system such a cell could detect the presence 

 of a 350°C soldering-iron at a distance of 100 yards. The signal conversion 

 factor with respect to 200°C is shown in Figure 28.43. 



If the 61SV is cooled, an increase in signal-to-noise ratio can be obtained, 

 as shown in Figure 28.43. The British Thomson-Houston Co., Ltd. make a 



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