VISIBLE AND NEAR-VISIBLE RADIATION 



183 



tungsten-filament lamp, 3200°K., yields only one-third of this value. 

 However, even in the case of the sun, less than 50 per cent of the energy 

 lies in the visible. Furthermore, since the blue and red are much less 

 effective than the yellow, it is not surprising to find the total (103 



Table 15. — Temperature, Color Temperature, Brightness, and Efficiency of 



Various Incandescent Lamps* 



*From Forsythe and Watson (29). 



lumens/watt) much less than the maximum luminous efficiency of 624 



o 



lumens/watt at 5560 A. 



It may be well now to consider some of the difficulties encountered in 

 using tungsten lamps. Tungsten-filament sources are subject to varia- 

 tion resulting not only from the difficulty of maintaining a constant 

 current through the lamp, but also resulting from deterioration. Deteri- 



