LIGHT SOURCES 



a well-regulated 



Secondly, as light output varies approximately as V^'^ 

 constant voltage is required if constant light output is desired. For low- 

 voltage lamps lead accumulators are a convenient constant voltage supply if 

 used on their 'voltage plateau', or if they are charged at a rate slightly less 



160 



in 



3 140 

 o 



i^120 



£100 

 ^-80 

 i 60 

 ^ AO 

 20 



too 105 110 115 120 125 130 

 Mains voltage 



Figure 28.8 Variation of characteristics of tungsten filament lamp with 



mains voltage 



than the discharge rate with the lamp running. Alternatively, the lamp may 

 be supplied from the a.c. mains through a constant-voltage transformer of 

 the saturated-core type, provided the 100 cycle ripple in the light output is 

 not a disadvantage. The amount of ripple in the hght output on a 50 cycle 

 supply depends upon the thermal capacity of the filament. Approximate 

 figures for a 200V, lOOW service lamp would be 3 per cent, and for a 6V, 

 36W automobile headlamp, 1 per cent. 



If a constant output is required from a lamp it is advisable to solder the 

 supply leads directly on to the lamp base ; there are few lamp-holders which 

 can be relied upon invariably to produce a low resistance coupling. Leads 

 to supply a meter to check the supply voltage should also be attached to the 

 base; in this manner the meter reading is rendered independent of resistance 

 changes in the supply leads. Failing this, it is better to control the lamp by 

 placing an ammeter in series. However, even if the supply voltage to the 

 lamp can be held constant a gradual decrease in light output will occur due 

 to evaporation from the filament. This change is most rapid with a new 

 lamp and some hours of aging should be given before setting in the equip- 

 ment. The ideal way of ensuring a constant light output is to monitor it with 

 a photoelectric cell and adjust the supply current either manually or auto- 

 matically. Even with this technique, however, it will not be possible to hold the 

 colour temperature in addition to the light output constant as the lamp ages. 



Most modern tungsten lamps are filled with a mixture of 80 to 90 per cent 

 argon and 10 to 20 per cent nitrogen to diminish the evaporation of tungsten 

 and thus permit a higher operating temperature with a resulting improve- 

 ment in light output per W. Argon is used on account of its low heat conduc- 

 tivity to minimize heat loss. The rarer and more expensive gas krypton is 



23 341 



