THE PRODUCTION AND USE OF LIGHT 



TABLE IL -ACCEPTED LIGHT INTENSITIES FOR DIFFERENT SERVICE 



Auditoriums, Churches General Illumination. 1 to 3 foot-candles 



Reading Rooms General Illumination 2 to 3 foot-candles 



Residences General Illumination 1 to 2 foot-candles 



Desk Illumination 3 to 5 foot-candles 



Bookkeeping 3 to 5 foot-candles 



Drafting and Engraving 5 to 10 foot-candles 



If desk illumination is to have an intensity of 4 foot-candles 

 and the light is to be hung 4 ft. above the desk, it is necessary 

 to provide for one 64-candlepower light, or two 32-candlepower 

 lights, or four 16-candlepower lights. While reading and 

 studying, the student should not hold his book more than 2 

 or 3 ft. from a 16- or 20-candlepower lamp nor more than about 

 5 ft. from a 60- or 80-candlepower lamp, such as an ordinary 



A B 



FIG. 55. A. A bad position and poor light. There is a bright spot before 

 the eye and the light is reflected from the book to the eye. B. A good posi- 

 tion and good light. The light reflected from the ceiling is diffused light. 



Welsbach gas mantle lamp. Moreover, the book should be 

 held in such a position that the reflected light from the page 

 will not fall upon the eye. The best position for the light is 

 above and behind the reader (Fig. 55). 



76, Controlling the Distribution of Light. It is probable 

 that fully 50 per cent, of the light produced in ordinary resi- 

 dence lighting is wasted. Light is wasted unless it is used to 

 illuminate the surfaces which need to be illuminated. When 

 reading a book or paper, a person needs to have that page 



