GENERATION, CONTROL, AND MEASUREMENT 



159 



life. Any factor that retards filament evaporation makes it possible to 

 use a higher temperature and thus to secure higher radiation efficiency. 

 The evaporation of tungsten filaments is greatly retarded by filling the 

 envelope with an inert gas and by coiling the filament. For the larger 

 lamps an inert-gas mixture of about 0.8 atm of 80 to 90 per cent argon 

 and 10 to 20 per cent nitrogen is used. Although the gas mixture 

 increases thermal losses, the layer of stagnant gas around the filament 



100 



90 



80 - 



>- 



z 70 



UJ 



z 

 < 



a. 



60 



50 



> 40 

 < 



S 30 



20 - 



10 



400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 



WAVE LENGTH, m^ 



Fig. 3-7. Spectral emission of various types of tungsten-filament lamps. 

 Forsythe and Adams, 1945.) 



{Data from 



retards the rate of evaporation of the tungsten. The filament of a gas- 

 filled lamp can be operated for the same useful life at a higher temper- 

 ature (2800°-3000°K) than the vacuum lamp (2500°K), and the resulting 

 increase in luminous efficiency more than compensates for the increased 

 thermal losses. 



The proportionate rate of evaporation from a fine wire is greater than 

 that from a large wire because of the more favorable volume/surface 

 ratio of the latter. Therefore, for the same life, low-voltage lamps can 

 be operated at a higher temperature than high-voltage lamps. The opti- 

 mum efficiency is at about 12 v for standard lamps since, at this voltage, 

 large-diameter low-resistance filaments are required. Coiling also tends 

 to retard evaporation, and the coiled-coil design used in low-wattage, 

 high-voltage lamps, in which the wire is first coiled on a small mandrel 

 and recoiled on a larger mandrel, involves an extension of this principle. 



The ribbon-filament lamp is a source of very uniform intensity over 

 the center portion of the ribbon. However, it cannot be maintained at 



