PHOTOMETRY AND ELECTRIC LIGHTING. 



317 



candles) but also the total light flux in spherical-hefners (or 

 spherical-candles). * 



In general, however, light is emitted by a lamp unequally in 

 different directions and it is necessary to distinguish between con- 

 ical intensity and total light flux. The total light flux in spher- 

 ical-hefners emitted by a lamp is determined by measuring the 

 conical intensity in hefners in every direction and taking the aver- 

 age, which gives the light flux in spherical-hefners. If this aver- 

 age is to be calculated in the ordinary 

 way by adding and dividing, the direc- 

 tions in which the separate readings are 

 taken must be distributed uniformly over 

 the surface of a sphere with its center at 

 the lamp. This sphere is called the 

 reference sphere for brevity. If the 

 readings are not so distributed, each read- 

 ing must be multiplied by the spherical 

 area which is to be properly assigned to 

 it, and the sum of such products must 

 be divided by the total area of the refer- 

 ence sphere to give the correct average. 



When a lamp can be rotated at a speed 

 of three or four revolutions per second Q 

 about its axis of approximate symmetry, 

 the total light flux may be determined accurately by taking read- 

 ings of the conical intensity in different directions in one plane only, 

 namely, a plane which includes the axis of rotation. Thus the 

 lamp, L, Fig. 178, is rotated about the vertical axis, PQ, and the 



* There is a widespread tendency among electrical engineers to confuse the unit of 

 conical intensity, the candle, with the unit of light flux, the spherical-candle. This 

 is due to the fact that, in the absence of exact data concerning the distribution of light 

 around a given lamp, the irregularities of distribution are ignored and the lamp is 

 thought of as giving out light uniformly in all directions. That is to say, electrical en- 

 gineers have been inclined to ignore that property of a lamp (the irregularity of dis- 

 tribution of the light of the lamp) which makes the distinction between the candle and 

 the spherical-candle important. 



Fig. 178. 



