ILLUMINATION AND PHOTOMETRY 435 



or dirt. It does not require trimming. Its maintenance is 

 high. Tungsten lamps have driven it from the lighting field. 

 iiriency is about 1.3 watts per mean hemispherical candle- 

 power. 



As a maximum, the foregoing lamps omit as light energy from 

 2 to 5 per cent, of the electrical energy supplied to them. The 

 remainder of the energy is liberated as heat. On the Other hand, 

 the firefly has a light-giving efficiency of 96 per cent. It emits 

 very little heat energy. Although great improvements have 

 been made in the field of illuminants during the past few years, 

 there is still a tremendous opportunity for further advances. 



PHOTOMETRY 



198. Photometry. Photometry is the measurement of light. 

 Light measurements are nearly all made by direct comparison. 

 The chief source of inaccuracy in making such comparisons is the 

 question of color. Unless the lamps under comparison are of 

 almost exactly the same color, only an approximate photometric 

 balance can be obtained. This balance varies with different 

 persons owing to the etTect of different colors upon the eye. 



199. The Bunsen Photometer. The Bunsen photometer is 

 the simplest type of photometric measuring device and is illus- 

 trated in Kig. 'Ml. Assume that it is denied to measure the 

 candlepower of the incandescent lamp />, using the candle as a 

 standard for comparison. The two lights are placed some 10 to 

 20 ft. apart and a moval n N is placed between them. 

 The screen S consists of a piece of paper or parchment with a 



grease Spot in the Center. The grease Spot on the screen is 

 translucent and so allows light to pass. If viewed 



de which is illuminated, the spot will appeal-darker than the 

 rest of the -creen, .mini: to the fact that light passes through the 

 tran^hn-ent L r rea-e spot more readily than through the surround- 

 ing part of til* 



On the other hand, if thi- -ame screen be viewed from the 

 non-illuminated side, the <:rea>e s| M ,t \\ill appear brighter than the 



analuoeni than the rest of 



i. Now if both >ide- of the screen receive e<|ual illumi- 

 ii, the grease spot will look the same in companion with the 



