PHOTOGRAPHIC LIGHT SOURCES 



283 



are advantageous from the photographic standpoint. These advantages are obtained 

 only at a considerable loss of life, however. 



Because of the short life of photoflood lamps, it is economical to operate them at 

 their full rated luminous output only during the time exposures are made, and to make 

 focusing and other camera and light adjustments with the lights operated at reduced 

 voltage. Three methods of accomplishing this result are shown in Fig. 16. At (A) 

 a resistance R is placed in series with the lamp, and a switch is connected across R. 

 While making preliminary adjustments, the switch is opened, which reduces the 

 current flowing through the lamp, and consequently requires that it operate at lower 

 temperature and at lower light output. When the switch is closed, the lamp operates 

 at its full luminous output. It is convenient to have the lamp operate at one-half of 



Siv/f-ch 



wwww^ 



Res/sfor-R 



H 0-120 volts 

 k 



Lamp 



110-120 volts 



s 



Lamp 



Lamp 



1 1 0-120 volts 

 Alternating current 



Autotrans former 



Lamp 



Fig. 16.- — -Three circuit diagrams illustrating methods which may be used to reduce 

 the voltage across photoflood lamps when not required for making exposure, thereby 

 increasing the hfe of these lamps. A and B may be used with a.-c. or d.-c. circuits, but C 

 can only be employed with a.-c. circuits. 



its normal operating voltage, and, in order that this may be accomplished, it is neces- 

 sary that R be of the proper resistance and have the proper power rating to carry the 

 current taken by the lamp when operated at reduced output. 



Because the resistance of incandescent lamps is not constant but varies with the 

 type of filament and its temperature, it is not possible to use the simple application 

 of Ohm's law for computing the resistance R. The manner in which Ohm's law may 

 be applied to this and similar problems, in which varying resistances are involved, is 

 outside of the scope of this book but may be found in any good text on electrical engi- 

 neering. We prefer merely to give the results; Table VI shows the resistance and 

 power rating which will be required for one of each of the three commonly available 

 photoflood lamps. If n lamps are used in parallel instead of only one, the resistance 

 given in the table should be divided by n and the power rating should be multiplied 

 by w, in order that the resistor may meet the electrical requirements of the circuit. 



