416 ALTERNATING CURRENTS 



A luminous source which has a luminous intensity of one 

 candlepower in every direction has a mean spherical candlepower 

 equal to 1.0 and emits 4ir lumens. Therefore, the number of 

 lumens emitted by a light source is equal to 4?r times the mean 

 spherical candlepower. 



Example. An incandescent lamp has a mean spherical candlepower of 

 20. How many lumens does it emit? 

 The total light flux 



F = 47r 20 = 251.4 lumens. Ans. 



182. Illumination. Illumination is the amount of light flux 

 or the number of lumens falling on a unit area. This cor- 

 responds to flux density in magnetism. It will be remembered 

 that flux density is defined as the number of magnetic lines 

 passing normally through a unit area. (See Vol. I, Page 7, 

 Par. 13.) The unit of illumination is the foot-candle and 

 corresponds to one lumen per square foot, the square foot being 

 taken normal to the direction of the light flux. It is denoted by 



F 

 the symbol E, where E = -r- A is the area of the surface taken 



A. 



normal to the direction of the light flux. For example, in Fig. 

 377, one lumen is included by the solid angle B. If the sphere 

 be thought of as hollow and having a radius of 1 ft., a square 

 foot on its surface intercepts one lumen and the light flux is 

 perpendicular to the surface at every point. Therefore, as the 

 illumination is assumed to be equal in all directions, the illumi- 

 nation at every point on the inside wall of this sphere is one 

 foot-candle. Such uniform distribution of light seldom occurs in 

 practice. 



A sphere having a radius of 2 ft. has four times as great a 

 surface area as a sphere having a radius of 1 ft. With a fixed 

 luminous source at the center, both spheres intercept the same 

 total light flux. The light intensity at the surface of the 2-ft. 

 sphere is one-fourth the light intensity at the surface of the 1-ft. 

 sphere. Therefore, to obtain the illumination in foot-candles, on a 

 surface which is normal to the direction of the light flux, divide the 

 candlepower of the light source by the square of the distance in 

 feet from the light source to the surface illuminated. 



