CHAPTER XIII 

 ILLUMINATION AND PHOTOMETRY 



Light is a form of radiant energy and is probably due to vibra- 

 tions set up in the ether by luminous bodies. It has the property 

 of producing the sensation of vision on the retina of the eye and 

 so enables objects to be seen and distinguished. 



Illumination means specifically the light incident on a surface 

 or object, but in a broader sense it has come to signify that 

 branch of engineering having to do with the distribution and 

 utilization of light. The measurement of light and light dis- 

 tribution is called photometry. 



179. Candlepower. The brightness of a light source is called 

 its luminous intensity. The luminous intensity of a body is 

 measured in terms of the light intensity in a horizontal direction 

 given by a standard candle, and is called candlepower. Candle- 

 power is denoted by 7. 1 That is, if a light source, such as an 

 incandescent lamp, were replaced by 14 standard candles without 

 altering cither the total light emitted or its distribution, the in- 

 cande-cent lamp would have a luminous intensity in a horizon- 

 tal direction of 14 candlepower. 



Candles of standard dimensions, burning under standard 

 conditions, have in the past been used as standards of luminous 

 intensity. (hvint: to the difficulty of reproducing such a stan- 

 dard with a suflicieiitly high degree of precision and owing to 

 iriation of its limiinoii- intensity with atmospheric condi- 

 .. the candle has not proved an acceptable standard, 

 particularly at the present time when a hin'h decree of precision 

 in light measurement- i- Deoeesaiy, 



No perfect] 'mlanl of luminous intensity has 



as yet been devised. At pn-ent the Bureau of Standards 

 maintains incandescent lamp- which con-mute a standard of 

 luminous inten-ity at some known voltage. These lamps are 



1 Photon.. .,,|s will l.r fotiml oftrli to <luplir:itr rlrrtlir.-tl s\ ! 



ilnplr. / r:H)dl ; ,-;il unite / CUFFCnt. Plioti- 



m<l electric unite :i 



ua 



