278 HANDBOOK OF PHOTOGRAPHY 



Light sources for practical photographic work may, accordingly, be classified as 

 follows : 



1. Incandescent sources (electric lamps and flames) 



2. Arcs 



3. Gaseous-discharge devices 



4. Transient light sources 



Incandescent Electric Lamps. — At the present time, incandescent electric lamps are 

 probably the most important and most frequentty used of all sources of artificial light. 

 This is largely due to their convenience and cleanliness in operation, the production 

 of a continuous spectrum of desirable energy distribution, their stability and relative 

 constancy in use, the convenient methods of control of modeling and lighting which 

 they permit, and, finally, the reasonable cost of installation, maintenance, and opera- 

 tion. Compared to gaseous-discharge lamps, incandescent electric lamps are ineffi- 

 cient producers of light, but for most purposes this relative inefficiency is overbalanced 

 bj' the desirable spectral characteristics, the low cost, and especially the convenience 

 of the incandescent lamps. 



The radiant energy emitted from an incandescent source (whether incandescent 

 lamp or flame) produces a continuous spectrum. The total radiant flux, as well as the 

 spectral-energy distribution, is dependent upon the temperature of the incandescent 

 body. Consequentl^y, the temperature of incandescent bodies has an appreciable 

 photographic effect, and for precise sensitometric or photometric work, must be main- 

 tained within narrow limits. In the case of incandescent electric lamps, the temper- 

 ature may be most conveniently determined by controlling the voltage across the 

 lamp terminals or the current flowing through the filament. 



Types of Incandescent Lamps. — Incandescent lamps may be classified according 

 to the type of material out of which the filament is constructed, such as carbon, tan- 

 talum, or tungsten. According to this classification, tungsten-filament lamps are by 

 far in greater general use than any of the others. According to their evacuation, 

 incandescent lamps may be classified as vacuum lamps or gas filled. In the vacuum 

 lamps, the air and absorbed gases have been removed, and a high degree of vacuum 

 is required, whereas in the gas-filled lamps a small amount of inert gas is admitted 

 after the lamp has been thoroughly evacuated. The inert gas, which is usually 

 nitrogen, is introduced to reduce evaporation from the filament. Reducing evapora- 

 tion lengthens the life of the lamp and reduces the blackening of the bulb. Small 

 lamps are still of the vacuum type, but practicallj^ all 110-volt lamps of 50 watts 

 or high-power rating are now gas filled.^ 



Characteristics of Incandescent Lamps. — The resistance characteristics of carbon- 

 filament and metallic-filament lamps differ markedly. As the voltage is increased, the 

 resistance of the carbon-filament lamps, which was initially high, decreases; for the 

 metallic-filament lamps, the initial resistance is low and increases as the voltage is 

 raised. 



The life of most incandescent lamps is rated at 1000 hr.. although for particular 

 purposes the life may be either increased or decreased. The life of the lamp depends 

 upon the diameter of the filament and the temperature at Avhich it is operated. Slight 

 increases in operating voltage above the normal value result in substantial increases 

 in the operating temperature and considerable reduction in life. On the other hand, 

 reduction of the operating voltage below the normal value increases the life but reduces 

 the operating temperature and the luminous output. The spectral-energy distribution 

 is also changed as the voltage is varied, more of the energy being in the red when the 

 voltage is reduced and tending toward the blue as the voltage is increased. 



1 A good summary of the characteristics of incandescent electric lamps is given in the "Standard 

 Handbook for Electrical Engineers," McGraw. 



