LIGHT SOURCES AND DETECTORS 



when operating on an a.c. supply. On a d.c. supply there is only one bright 

 area — at the cathode. 

 Figure 28.12 shows how a change in power supply voltage affects the 



350 



300 



Art 



I 



250 



200 



75 o) 



^o| 



60 E 

 55 -" 



90 



95 



100 105 



110 



Mains voltage % 



Figure 28.12 Effect of variation of mains voltage on characteristics of 250 W 

 type ME lamp. Lamp operating from B.T.H. MRA 246 choke on a.c. supply 



at 50 c/s 



3,000 



5,000 



7,000 



9,000 



11,000 13,000^ 



Figure 28.13 Energy distribution curve for 250 W type ME high-pressure 



mercury vapour lamp 



lamp characteristics. A rise of 10 per cent in voltage increases the light 

 output by 20 per cent, compared with 36 per cent for a 250 W tungsten 

 projector lamp. The life of the compact source lamp is only slightly affected 

 by supply voltage changes and there is also very little change in the spectral 

 distribution of the light output with change of supply voltage. 



The spectral energy distribution curve for a 250 W ME lamp is shown 

 in Figure 28.13. At a supply frequency of 50 cycles the modulation of light 

 output causes it to fluctuate about 60 per cent above and below the mean 

 level. Lamps of this type may be obtained with a variety of ratings up to 



346 



