MAR. 4, 1923 CRITTENDEN: MEASUREMENT OF LIGHT 10 
followed the foot-candle and the meter candle (also called lux) which, 
as the names show, were originally defined in terms of candlepower 
and distance rather than of flux density. The unit of flux itself, the 
lumen, is in practice defined as the flux through a unit solid angle 
from a source of unit candlepower, and going farther backward on 
our scheme the unit of “‘light’’ is not even dignified by a distinctive 
name, but is called the lumen-hour. 
TABLE I.—PHOTOMETRIC QUANTITIES 
QUANTITY DERIVATION SYMBOL SPECIAL NAMES UNITS 
Light Energy x V Q Lumen-hour 
Luminous flux dQ F Lumen 
dt 
Density of flow dF E Illumination | Foot-candle 
Flux per unit area ds (on surface) | Meter-candle 
Lux 
Divergence of flow dF I Intensity Candle 
Flux per unit solid dw Candlepower 
angle (of source) 
Flux per unit solid d2F B Brightness Candle per sq. in. 
angle per unit pro-| dw dS cos @ Candle per em? 
jected area (Lambert = 1/7 candle 
Candlepower per unit per cm?.) 
projected area 
(of source) 
Note.—The nomenclature of this table departs from recognized practice in the use 
of the term light and the symbol Q for it. The practice approved by the American 
Engineering Standards Committee is set forth in Illuminating Engineering Nomencla- 
ture and Photometric Standards, a pamphlet obtainable from the Illuminating Engi- 
neering Society, 29 West 39th Street, New York City. 
Brightness is of course most simply specified in candles per square 
inch or per square centimeter; but another unit, the lambert, has 
found considerable use. The lambert is 1/7 candle per square centi- 
meter. The reason for using this apparently peculiar unit is most 
readily explained by an example. If a perfectly diffusing white wall 
is illuminated it becomes a secondary source of light, but the reflected 
light is so distributed throughout a hemisphere that the brightness 
of the wall in candles per unit area is only 1/7 times the illumination. 
The lambert is the brightness of a perfectly diffusing, completely 
reflecting surface under a unit illumination, that is one centimeter- 
candle, which means receiving one lumen per square centimeter. The 
