LIGHT SOURCES AND DETECTORS 



sometimes used further to reduce heat loss and evaporation, with resulting 

 gain in efficiency. Krypton is usually used in miners' lamps. These form 

 useful light sources in portable equipment where heat and the weight of 

 batteries may be limiting factors. 



In a gas-filled lamp there is a layer of relatively stagnant gas around the 

 filament. The thickness of this sheath is independent of the diameter of the 

 filament. The effective cooling surface therefore decreases relatively as the 

 diameter of the filament increases. The effective diameter of the filament 

 may be increased by winding it into a coil and thus reduce heat loss. This 

 process may be carried further with lamps having thin filaments, such as a 

 200 V, 100 W service lamp, by forming a coiled coil. Efficiency may be 

 increased by 20 per cent with this technique. 



Ultimately all tungsten lamps will fail due to fracture of their filament. 

 Evaporation of the filament tends to be slightly uneven, so that some portion 

 of the tungsten wire becomes thin, its resistance increases, and thus its 

 temperature is raised locally to the mehing point. Lamps running at a high 

 colour temperature are particularly liable to fracture if subjected to vibration 

 when alight. Automobile lamps and traction lamps are specially designed to 

 withstand vibration by appropriate arrangement of the filament and its 

 supports. Lamp failure due to filament fracture may be eliminated by 

 running at a lower temperature and by switching on the power gradually. 

 Higher voltage lamps with long filaments occasionally short-circuit on 

 fracture due to the filament falling across one of the supports. To minimize 

 the danger of a main fuse blowing under these circumstances, some 

 manufacturers incorporate a local fuse in the base of the lamp. 



Types of tungsten filament lamps 



The range of lamps available today is very large indeed and it is usually 

 necessary to consult the manufacturers' catalogues to ensure that the required 

 type is in production. It will only be possible here to mention a few of the 

 less familiar lamps which can be useful in the laboratory. 



Class Al projection lamps — This class of lamp uses a vertical grid filament 

 to give a high light output in a direction normal to the plane of the filament. 

 Projection lamps must be used vertically to prevent local overheating of the 

 glass and 'blow out'. The C.T. is usually about 3,000°K and the life about 

 50 hours. They may be obtained in power ratings up to 1,000 W; the 

 higher wattages require forced cooling. 



These lamps are ideal for lantern and film projectors as an image of the 

 filament grid may be focused in the plane of the slide or projection lens to 

 fill it evenly with light (see Martin^, for optical details). The spaces separat- 

 ing the filaments in the grid can be brightened by forming an image of the 

 filaments there by means of a concave mirror placed behind the lamp. This 

 can almost double the light output in a forward direction. The heat output 

 from these lamps can be very high and glass infra-red heat-filters may be 

 required for some purposes. See Corning glass catalogue for details. 



Class Bl floodlighting lamps — These lamps have a concentrated bunch 

 filament with parallel coils arranged around the circumference of a cylinder 

 and are for use with a parabolic reflector. They are mounted in round 

 bulbs and may be used in any position except cap uppermost. They usually 



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