ILLUMINATION AND PlInTuMKTHY \ \ 1 



the radii of polar coordinate paper, a curve similar to that shown 

 in Fig. 396 is obtained. As would be expected, an incandescent 

 lamp in tin- ordinary position emits but little light in the up- 

 ward vertical direction, owing to the presence of the socket and 

 But little light is thrown vertically downward because of 

 th- small length of filament exposed in that direction. 



The mean spherical candlepower of the lamp is obtained by the 

 Rousseau diagram as follow-: 



A circle mnm' of some convenient diameter is drawn about 

 the polar candlepower diagram, as shown in Fig. 396. The 

 ends of the various radii are projected on a straight line </'//. 

 Perpendiculars are dropped from these projection points to ab 

 which is parallel to a'b' '. The distances od, of, etc., are laid off 

 from ab on these perpendiculars, as shown at o'd', o'f. The 

 average height of the curve gf'e' to scale gives the mean spherical 

 candlepower of the lamp. This average height may be obtained 

 by measuring the area abge'a and dividing by the base ab, the 

 proper scale being used, or by taking the average of several 

 equally-spaced perpendiculars. 



The output of the lamp in lumens is obtained by multiplying 

 the mean spherical candlepower by 4ir (12.57). 



The spherical reduction factor of a lamp is the ratio of the mean 

 spherical candlepower to the mean horizontal candlepower. It 

 is usually less than 1.0. This factor depends on the sliajx* and 

 -it ion of the filament and also on the reflector, if one be 

 used. For a given type of lamp, the spherical reduction factor is 

 nearly a constant quantity- Knowing this factor for a L 

 type of lamp, tin- mean spherical candlepower and the luminous 

 output of a lamp can be readily calculated after making one 

 measurement of the mean horizontal candlepower. Below are 

 the spherical reduction factors of a few standard lamps. 



Sun. HI* \i REDUCTION FACTORS* 



Treated carl. BUmenl <> 



lon (GEM i VM| 

 tun 

 hi, 60 watts . 



Gas-fillc.l 



Bbndboofc 1 1. 



