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HANDBOOK OF PHOTOGRAPHY 



Let E be the efficiency, F the light output, P the power consumption, and I the 

 current consumption for any voltage V at which the lamps are operated. Also, let 

 Eg, Fq, Po, and /o be symbols representing the same concepts when the lamp is operated 

 at the recommended voltage Fo, which is assumed to be the normal voltage. The 

 characteristics of lamps operated at a voltage V, in terms of the known characteristics 

 when the lamp is operated at normal, rated voltage Fo are given by the following 

 equations : 



+ c 



+ B 



log Q = A.(iog (r^y 



O-(^o)) 



+ B3(l0gQ) 



+ 5.(log (^)) 



(4) 

 (5) 

 (6) 



(7) 



The values of the coefficients An, Bn, and Ci differ for various sizes of lamps as may be 

 seen from Table III. These operating characteristics are shown graphically in Fig. 15 

 for voltage ranges which are likely to be encountered in practice. 



Table III. — Coefficients for Determining Characteristics op Operation 



OF Tungsten Lamps 



Simpler forms of the above equations which may be used between 77 and 130 per 

 cent of the normal rated voltage Fo with an error not exceeding approximately 5 per 

 cent have been derived by Moon,^ who also gives the life of a lamp L operated at 



' Moon, Parry, "Scientific Basis of Illumination Engineering," McGraw-Hill. 



