LIGHT SOURCES AND DETECTORS 



The relationship between current flow and illumination for diff'erent 

 values of external resistance is shown in Figure 28.38. It can be seen that if 

 a low resistance measuring instrument is used a fairly linear relationship 

 exists between current and illumination intensity (Weston, 45 mm diameter 

 cell). Individual cells, even from the same manufacturer, vary widely, 

 however, in output. Short-circuit current sensitivities vary from 100 to 

 600 /LiAllm. 



The selenium barrier-layer cell generates a remarkable amount of power. 

 A Weston 45 mm diameter cell with an external load of 350 ohms and an 

 illumination level of 200 foot-candles would deliver 100 /^- watts to the load 

 (Figure 28.39). Their efficiency as energy converters when working under 

 optimum conditions is about 2-5 per cent, therefore their quantum efficiency 

 must be quite high. 



By selecting an appropriate load the effect of ambient temperature changes 

 can be minimized as demonstrated in Figure 28.40. 



These cells suffer from fatigue effects. At worst, the output may increase 

 or decrease by about 4 per cent depending as usual upon the individual cell. 

 When high accuracy is required, the current output at a given illumination 

 should be plotted against time for an individual cell and the time to reach 

 equilibrium noted. Further readings should only be taken after this initial 

 time has elapsed. The amount of fatigue which occurs depends not only 

 upon the intensity of the light, but also upon the colour, being more marked 

 with light from the red end of the spectrum. The long-term stability of 

 barrier-layer ceffs is good even after many years of continuous use provided 

 they are protected from very high illumination levels and from heat. Use of 

 a heat filter is sometimes advisable if long exposure to tungsten light sources 

 is required. 



Due to the high internal capacity of these cells their frequency response is 

 poor. At 3,000 c/s the response is down by about 5 per cent and at 10,000 c/s 

 it is down by approximately 80 per cent. Small area cells have a better 

 frequency response than large. 



Typical spectral sensitivity response curves for a number of selenium 

 barrier-layer cells are shown in Figure 28.41. The response in the near 

 ultraviolet region is fairly good and is sometimes useful. For example, the 

 output tends to be high when a cell is exposed to violet-rich skylight out of doors, 

 when compared with tungsten illumination of equal intensity indoors. This 

 is convenient for a photographer using panchromatic film, which has a 

 similar violet sensitivity ; but it is inconvenient when measuring the equivalent 

 visual intensity of the light. If an Ilford filter type 827 is placed before it, its 

 response curve approximates to that of the eye. To obtain an even closer 

 fit, more complex filters are required (Preston^*). Megatron Ltd. manufac- 

 ture a selenium cell which is sensitive out to about 900 m^ in the near infra- 

 red. It is a cell of high sensitivity, especiaUy when used with a tungsten light 

 source. 



The selenium cell is particularly valuable as a portable photometer when 

 used in conjunction with a suitable meter. It may be conveniently used in 

 the laboratory for approximate photometry as its spectral response curve is 

 near to that of the eye, especially if suitable colour filters are added. It has 

 also been used as a portable generator of electrical energy to supply power to 



370 



