IGO 



RADIATION BIOLOGY 



SO high a temperature as the coiled-wire filaments for the same Hfe. Since 

 the ribbon must be thin to have sufficient resistance even for low voltages, 

 holes quickly form at high temperatures. Therefore the ribbon filament 

 is not usually employed where the highest intensity and color temperature 

 are required. 



Electrical Characteristics. The electrical characteristics of tungsten- 

 filament lamps have been extensively covered in several reports (Bourne, 

 1948; Forsythe and Adams, 1936; Forsythe and Watson,* 1932, 1934; 

 Jones and Langmuir, 1927a, b,c; Weitz, 1950). The voltage rating of an 



180 



400 



300 



200 



100 



iij 

 q: 



UJ 

 Q. 



80 100 120 



PER CENT NORMAL VOLTS 



Fig. 3-8. Effect of voltage variation on the characteristics of tungsten-filament 

 lamps. (From Weitz, 1950.) 



incandescent lamp is selected to give certain radiation characteristics and 

 lamp life, both of which change rapidly when the voltage is varied. For 

 the average general-service type of lamp (Tc = 2800°"3000°K) an increase 

 of 1 per cent in voltage results in an increase of approximately 0.5 per cent 

 in current, 1.5 per cent in wattage, 3.5 per cent in visible flux, and 2 per 

 cent in luminous efficiency, but the life is decreased 13 per cent. If the 

 voltage is increased 20 per cent, the luminous output is increased 85 per 

 cent, but the life is reduced 90 per cent. These characteristics are pre- 

 sented graphically in Fig. 3-8. An increase in the temperature of a ther- 

 mal radiator results in a shift of X„ toward the shorter wave lengths. 

 Consequently an increase in temperature causes the spectral energy to 

 increase more rapidly in the shorter wave lengths than in the longer wave 

 lengths. Using the equation given by Judd (1950) for a 500-w projection 

 lamp operating at a color temperature of 2800°K, 



rfF ^ 200^7; 

 ^"^ V Te - 620' 



(3-18) 



