PHOTOGRAPHIC LIGHT SOURCES 



279 



While in general a life of 1000 hr. may usually be expected for well-made American 

 lamps intended for general illumination, ^ the lamps for motion-picture production 

 service have a rating of from 15 to 500 hr.; lamps for projection and stereopticon 

 service are rated at either 25 or 50 hr., and those for spotlight or floodlight service are 

 rated at 80 hr. or, more usually, at 200 hr. of life in the 110- to 120-volt sizes. Photo- 

 flood lamps for photographic service are operated at much higher temperatures than 

 lamps intended for general illumination and have a rated life of from 2 to 15 hr. 



150 



140 



130 



120 



it] 110 



I 100 



90 



^ 80 



70 



fe 60 



50 



/F E/ /P 



70 



130 



140 



3 90 \00 110 120 

 Per Cent of Normal Voltage 



Fig. 15. — Operating characteristics of incandescent lamps in terms of per cent of normal 

 operating voltage. The curves are designated as follows: /, current consumption; P, 

 power consumption; E, efficiency; and F, light output. 



The essential characteristics of operation of incandescent lamps are: (1) the 

 efficiency or lumens per watt, (2) the light output in lumens, (3) the power consump- 

 tion in watts, and (4) the current consumption in amperes; all of which are functions 

 of the voltage at which the lamp is operated. The average characteristics and proper 

 voltage of operation may be obtained from the manufacturer, but these characteristics 

 will change as the operating voltage is varied. The manner in which the character- 

 istics of incandescent lamps depend upon the operating voltage has been studied by 

 Barrow and Meyer^ at the National Bureau of Standards, and the following data are 

 summarized from their paper. 



1 Cheap incandescent lamps of Japanese manufacture which have been on the American market are 

 frequently found to have short life, relatively low luminous output, and higher power consumption than 

 the equivalent American product. While it may be difficult to determine definitely the cause for these 

 characteristics, it is not unlikely that these result from inadequate exhaust procedure. 



^ Barrow and Meyer, Characteristic Equations of Vacuum and Gas-filled Tungsten Filament 

 Lamps, Research Paper 502, Bur. Standards J. Research, vol. 9 (1932). 



