324 ELEMENTS OF ELECTRICAL ENGINEERING. 



upon the ceiling of the room and the diffused light reflected 

 from the ceiling produces a beautiful soft illumination in the lower 

 portion of the room. 



Influence of absorption upon illumination. An illuminated 

 surface, such as the wall, ceiling, or floor of a room, or the sur- 

 face of an object in the room, absorbs a definite fractional part of 

 the light which falls upon it. This fraction is called the coef- 

 ficient of absorption of the illuminated surface. The lamps in a 

 room emit a given flux of light and at the instant the lamps are 

 turned on the intensity of the illumination in the room is quickly 

 increased by repeated reflections of the light from the illuminated 

 surfaces until the rate of absorption of light by the illuminated sur- 

 faces is equal to the rate of emission of light by the lamps. Given, 

 for example, two rooms, A and B, illuminated by the same number 

 of lamps. Suppose that the two rooms have the same area of 

 walls and objects to be illuminated, but suppose that the illumi- 

 nated surfaces in room A absorb an average of 40 per cent, of the 

 light which falls upon them, whereas the illuminated surfaces in 

 room B absorb an average of 80 per cent. In both rooms the 

 same amount of light is emitted by the lamps and therefore the 

 same amount of light is absorbed by the illuminated surfaces ; 

 but this absorbed light is only 40 per cent, of the mean intensity 

 of illumination in room A whereas it is 80 per cent, of the mean 

 intensity of illumination in room B ; therefore the mean intensity 

 of illumination in room A is twice as great as it is in room /?.* 



Flux of light required for effective illumination. The total flux 

 of light, in spherical-candles, required for the effective illumination 

 of a given-sized room depends upon the manner in which the 

 light is distributed, upon the composition (color) of the light, and 

 upon the mean coefficient of absorption of the illuminated surfaces 

 as pointed out above. Interior lighting is usually accomplished 

 by lamps which give a soft yellow light, the lamps are usually 



* In general the total flux of light required to produce a given mean intensity of 

 illumination in a room is proportional to ab, where a is the combined area of all the 

 surfaces to be illuminated and b is the mean coefficient of absorption of these surfaces. 



