PHOTOMETRY AND ELECTRIC LIGHTING. 3 I I 



The intensity of a horizontal beam of light from a Hefner lamp 

 is called a hefner unit or a hefner. If a lamp were to give one 

 hefner unit of light intensity in every direction, the amount of 

 light, or the so-called flux of light emitted by the lamp, would 

 be what is called one spheric al-hefner. * 



The great reliability of the Hefner lamp as compared with the 

 standard sperm candle has led to the definition of the candle in 

 terms of the hefner unit. The candle or the candle unit is a beam 

 of light of which the intensity is 1.136 hefner units and the spher- 

 ical-candle is equal to 1.136 spherical-hefners. The standard 

 sperm candle is no longer used in photometric work. 



128. Conical intensity and sectional intensity of light. The 



expression, intensity of a beam of light, which is used in the above 

 definitions of the hefner and the candle, refers to the amount of 

 light in a unit-sized cone of rays. This conical intensity, which 

 it may be called for brevity, is expressed in hefners or candles ; 

 it is independent of distance, since the light in a given cone of 

 rays remains in that cone ; and it depends only upon the area and 

 brightness of the luminous surface of the lamp. 



The intensity of a beam of light may also refer to the amount 

 of light per unit sectional area of the beam. This sectional inten- 

 sity of a beam of light, which it may be called for brevity, de- 

 creases as the square of the distance from the lamp increases, and 

 it is expressed in terms of a unit called the lux, which is the 

 sectional intensity of a horizontal beam from a Hefner lamp at a 

 distance of one meter from the lamp. The sectional intensity, /, 

 of a given beam in luxes at a distance of d meters from a lamp is 

 given by the equation : 



' 



in which h is the conical intensity of the beam in hefner units. 



*The spherical -hefner is not a kind of hefner. The word spherical is not used as 

 an adjective, but the term stands for a distinct conception ; it is one word, and it 

 should be written with a hyphen. 



