158 



RADIATION BIOLOGY 



Because of the low emissivity beyond 300 m/z and the inherently weak 

 ultraviolet emission of thermal radiators, the tungsten-filament lamp is 

 an inefficient ultraviolet source. Since any cavity in which the radiant 

 energy undergoes multiple internal reflections approaches a Planckian 

 radiator in emissivity, a coiled filament is intermediate in spectral emis- 

 sivity between that of a straight tungsten wire and a complete radiator. 



Table 3-10. Radiation Properties of Tungsten 

 (Data adapted from Jones and Langmuir, 1927a, and Forsythe and 



Worthing, 1925.) 



" Melting point of tungsten. 



The spectral-energy-distribution curves of Fig. 3-7 cover the range of 

 filament temperatures commonly encountered in commercial lamps. The 

 wave length of maximum spectral emittance shifts from about 1000 m^t 

 at 2600°K to 800 m^ at 3300°K. Also, with increasing temperature, the 

 near ultraviolet increases much more rapidly than the infrared. 



Lamp Life. The visible flux of an incandescent lamp decreases almost 

 hnearly with time until ultimately the filament burns out. The gradual 

 decrease in output is due to evaporation of the filament, which results in 

 decreased filament cross section, and to blackening of the bulb or enve- 

 lope. The rate of evaporation increases rapidly with temperature, as do 

 also the color temperature (whiteness of the fight) and luminous efficiency. 

 The lamp design is therefore always a compromise between efficiency and 



