ILU'MINATION AND PHOTOMETRY 415 



tho horizontal plane or one candlepower. (A candle of this 

 type is never met with in practice but is given here merely for 

 purposes of illustration. The ordinary standard candle emits an 

 intensity of one candlepower in the horizontal plane only, the 

 intensity in other directions being much less than one candle- 

 power.) Lot B be a unit solid angle at the center subtended 

 by an area of 1 sq. ft. on the surface of the sphere. A certain 

 amount of light flux will be confined by this unit solid angle and 

 as light flux is emitted radially in straight li.ies, no flux enters or 

 leaves the solid angle through its sides. 



The light confined by this unit solid angle and coming from 

 such a standard candle is the unit of light flux and is called the 

 I nun n. The number of lumens is denoted by F. 



As there are 4?r units of solid angle at the center of a sphere, it 

 is evident that each standard candle would emit 4ir lumens if its 

 light intensity were the same in every direction and equal to the 

 horizontal intensity. The difference between candlepower and 

 luminous flux should be clearly understood. The candlepower 

 is intensity of light emission and may vary in different directions. 

 ( )n the other hand, luminous flux represents the fofoZ light emitted 

 in any given region. 



In the past, incandescent lamps have been rated on their 

 mean horizontal candlepower, because in the carbon lamps, which 

 were then the only type in general use, the shape of the filament 

 >'.nd it- distribution in the bulb were practically the same in 

 all lamps. Therefore, all lamps had light dist.ibution curves 

 of the same general form. That is, the ratio of mean spherical 

 candlepower to mean horizontal candlepower was practically 

 constant in the lamps then in use. 



With the advent of new types of lamps, the disposition of the 

 filaments became quite different in the various lamps and the 

 mean In.ri/ontal candlepower was no longer a measure .>!' the 

 total light output of a lamp. The iWling at the present time 

 is that a lam}) should be rated according to the total light flux 

 which it emits, or in other words, a lamp should be rated MI 

 lumens and not in mean horizontal candl* 



The in, ,-;,! randlepower, which i- the averai- 



candlepower emitted in all <h . is also a measure of the 



total li^ht flux emitted by a luminou.- 



