LIGHT SOURCES 



have a colour temperature of about 2,800°K and an average life of 800 

 hours. Their main use is for spothghting. 



Class E lamps — These are 500 W epidiascope lamps with a grid filament 

 and a life of 100 hours. 



Class F lamps — These are a most useful group of lamps for laboratory 

 work. They are low voltage type for 6 or 12 V supplies with a heavy small 

 filament, designed for microprojection work, etc., where a small uniform 

 source is required. The soHd ribbon filament type is particularly versatile 

 (6 V, 18 A, 108 W, 50 hours life) and can be obtained with a vertical or 

 horizontal filament. Because of the solid tungsten filament it is possible to 

 avoid the uneven lighting which usually accompanies a coiled filament. The 

 usual size of the filament is 20 ^ thick, and about 2 mm wide X 20 mm long. 

 Philips Electrical Ltd. manufacture special versions of these lamps with 

 optically ground glass or quartz front windows and reflection-free rear bulbs 

 for precise photometric work. 



A number of other small coiled-filament lamps down to 8 W are covered 

 by this class. If over-run by 10 per cent of their rated voltage they form 

 useful small bright sources. 



Class G lamps — These are called Exciter lamps and are intended for sound 

 film reproduction. They are manufactured to a high degree of physical 

 accuracy to ensure interchangeability and to avoid the need for refocusing 

 on refitting lamps. They are made in a n' mber of low voltage ratings and 

 wattages. The 10 V, 7-5 A, type are very convenient for over-running, as 

 the evaporated tungsten deposits mainly at the top of the cylindrical bulb, 

 leaving the filament area clear. Due to the heavy filament, the ripple or 

 'modulation' in the light output is small on a.c. supphes. 



Automobile lamps — This type of lamp is too well known to require 

 description. Automobile lamps can be obtained up to a rating of 48 W, with 

 both transverse and axial filaments. The double filament types, each of 

 24 W, can be useful. 



Unusual lamp types — It is not well known that small lamps of the flash 

 lamp type can be obtained with a rating of only 6 V at either 40 or 60 mA 

 (0-24 and 0-36 W). They are useful as indicators or fuses. 



Thorn Electrical Industries make lamps with the very small dimensions of 

 6-3 mm bulb diameter and 14-6 mm overall length in the following ratings: 

 28 V, 0-04 A; 28 V, 0-08 A; 12 V, 0-1 A; 6 V, 0-1 A; 1-5 V, 0-75 A. 



Miners' lamps, either argon or krypton filled, are available in a variety of 

 sizes and ratings with both single and double filaments. These are valuable in 

 portable equipment to save battery weight. 



The largest low-voltage lamp available is an aircraft landing light at 26 V, 

 350 W. 



For floodlighting a screen evenly to a high luminance reflector spotlights 

 are useful. These lamps are parabolic in shape and have an internal coat 

 of aluminium which forms an efficient internal reflector. Three types are 

 available in the 150 W range with a polar distribution of output as shown 

 in Figure 28.9. A 75 W version of the grid filament spotlight is also available 

 with a peak candlepower at 0° of 1,250. A 275 W photoflood spotHght 

 with internal reflector is made for photographic purposes running at a 

 C.T. of 3,200°K and having a life of a few hours. 



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