282 



HANDBOOK OF PHOTOGRAPHY 



voltage V in terms of its rated life Lo when operated at its rated voltage Vq. 

 equation for the life of the lamp is 





y^-i 



^(r.) 



This 



(8) 



For the large user of illuminating incandescent lamps, power consumption and the 

 cost of power may be an import-ant economic factor. If the cost of power as well 

 as the cost of the lamp plus its installation charge is known, the most economical 

 operating voltage can be determined. Let c be the cost of one lamp plus its installation 

 charge, in cents, and let h be the cost of power in cents per kilowatt-hour. Then' the" 

 most economical operating voltage Ve for a lamp of rated power Po and rated voltage 

 W is 



1 

 "6Po(B2 



"{' 



M 



c{B, - B,) 



(9) 



where the values of Bn are those given in Table III. In general, the most economical 

 operation will usually be found to be obtained at a voltage sUghtly above rather than 

 at or below the rated voltage. Of course, if the lamp is operated at some voltage other 

 than its rated voltage, its life, luminous output, and other characteristics will be 

 affected in accordance with the equations given above from the paper by Barrow 

 and MeA^er. 



Photoflood Lamps. — Incandescent lamps may be operated at considerably higher 

 than their normal voltage for 1000-hr. operation. The main effects of this overvoltage 

 operation are: (1) considerably greater light output is obtained, (2) together with a 

 shift in spectral distribution toward the blue end of the visible spectrum (both factors 

 of which are advantageous from the photographic standpoint), (3) the normal life 

 of the lamp is greatly reduced, and (4) the temperature of the filament and the con- 

 taining glass bulb is raised, sometimes to such an extent that the use of such lamps may 

 require unusual methods of handling, ventilation, and fire protection. The photoflood 

 lamps commercially available for photographic purposes are of this overvoltage type. 

 If used in such a manner as to have a life of 1000 hr., their rated voltage would be 64 

 instead of 110 volts. The filaments of such bulbs must be heavier than the filaments 

 for bulbs operating at normal temperatures in order to insure a life of even a few hours. 



The reason why photoflood lamps are more efficient for photographic purposes 

 than ordinary electric lamps of the same power rating is because thej' are operated at 

 considerably higher temperature. Operation at temperatures of about 3400 to 

 3500°K. instead of the usual 2700 to 3000°K. for general service increases the efficiency, 

 measured in lumens per watt. At the same time it produces relatively more blue than 

 red radiation (compared with lamps for general service lighting), both of which effects 



' Ibid., p. 165-166. 



