LIGHT SOURCES 



lamps, the visual efficiency varies from 10 to 20 per cent, depending upon 

 the filament temperature. 



The actual temperature of a radiator is difficult to determine and it is 

 more usual to measure its colour temperature (C.T.). This could be defined 



Wavelength 



X 



Figure 28.6 Spectral radiation of tungsten filament at various temperatures 



as the temperature of a full radiator which would emit radiation of substan- 

 tially the same spectral distribution in the visible region as the radiation 

 from the light source, and which would have the same colour. Table 2 gives 

 a list of colour temperatures of common sources. 



LIGHT SOURCES 



Characteristics of tungsten lamps 



The diversity of form and size of the modern tungsten lamp makes it an 



indispensable light source in many experimental techniques. It is often 



possible to use a given lamp under conditions other than that intended by 



the designer, provided the basic characteristics of tungsten lamps are 



appreciated. 



A modern tungsten lamp running at the high C.T. of about 3,400°K has a 

 luminous efficiency of about 30 Im/W with a filament luminance of about 

 5,000 cds/cm^. Its useful life under these conditions, however, would be 

 short as it has to operate near the melting point of tungsten (3,650°K) where 

 rapid evaporation of the tungsten occurs. General service lamps operate at 

 lower colour temperatures — between 2,500 and 3,000°K — with a correspond- 

 ing lower luminous efficiency, luminance (1,000 to 2,000 cd/cm^), and longer 

 life (usually an average of 1,000 hours). The working characteristics of the 

 various ranges of lamps are shown in Figure 28.7. 



The useful life of a lamp is usually determined by the blackening of the 

 bulb due to deposits of evaporated tungsten which gradually reduce the 

 light output. This applies especially when a concave mirror is used behind 

 a lamp, as light from the reflector system has to pass through the glass 

 bulb three times. The blackening also increases the absorption of light by 

 the glass to such an extent that the concomitant temperature rise leads to 

 softening, and a 'blow out'. 



For a given operating temperature, the greater the diameter of the filament 

 the longer the life of the lamp. The amount of vaporization is proportional 



339 



