LIGHT SOURCES AND DETECTORS 



to the surface area of the filament and this increases linearly with filament 

 diameter; but the cross-sectional area increases as the square of the diameter 

 and thus increases more rapidly than the rate of evaporation. Thus low 

 voltage lamps are more efficient than high voltage lamps of equal wattage, 

 as they can operate at a higher temperature without increased evaporation. 



E 



100 



o 



c 



o 



c 



E 



50 



10 



Phofoflood 

 2-15 h 

 Automobile 

 lamps 



Gas-filled 



general service 



lamps 1,000 h. 



Vacuum 

 lamps 



,r Carbon 

 ■/ lamps 



II 



I I I I I I I 



I I I I I I 



I I I I I M . 



2.000 2,A00 2300 



Operating temp. 



3.200 



3.600 



Figure 28.7 Working characteristics of various types of tungsten lamps: 

 effect of filament temperature on efficiency and brightness {after Bourne'^) 



The life of a lamp varies approximately as the reciprocal of the fourteenth 

 power of the voltage. Thus the life of a general service lamp would fall by 

 some 13 per cent if its supply voltage were raised by 1 per cent. The effect of 

 altering the supply voltage to such a lamp is shown in Figure 28.8. Approxi- 

 mate figures might be that for an increase of 1 per cent in voltage there 

 would occur an increase of 0-5 per cent in current, 1-5 per cent in wattage, 

 3-6 per cent in light output and 2 per cent in light output per W. 



Several useful points arise from these figures. First, if a bright compact 

 light source is required it might be convenient to use a relatively cheap lamp 

 such as a 6V automobile headlamp and over- run it by 10 to 20 per cent of its 

 rated voltage. This would give the brightest source available from tungsten 

 (5,000 cd/cm") but with the disadvantage of frequent lamp changes. Lamp 

 life could be slightly increased by bringing the filament slowly to its maximum 

 temperature, perhaps by placing a suitable 'Brimistor'* in series. 



* A resistor possessing a large negative temperature coefficient. Its resistance is 

 initially high, but falls rapidly as the Brimistor warms up. ED. 



340 



