LIGHT SOURCES AND DETECTORS 



poor compared with large-area photodetectors such as the selenium photo- 

 emissive cell. However, a good signal-to-noise ratio can be obtained with 

 focused light beams. The noise produced by the cell is little greater than that 

 to be expected from Johnson noise alone. The sensitivity of the cell to 

 oblique illumination is shown in Figure 28.51. 



"■ — " Clockwise rotation Anti-clockwise rotation — ^ 



Figure 28.51 Relative output of P50A at various angles of light incidence 



100 



80 



Oj 60 



c 

 o 



Q. 

 (/) 

 (b 



"^ AO 



20 



0-5 



1-0 



1-5 2-0 



A 



2-5 



Y- 



Figure 28.52 Spectral response curve of the Mullard type OCP71 



phototransistor 



One of the main disadvantages of this type of detector is the variation of 

 the characteristics with ambient temperature. This should be borne in mind 

 when considering any application. 



380 



