TABLE 299. 



PHOTOMETRIC DEFINITIONS AND UNITS. 



Luminous flux, F = radiant power according to visibility, i.e., capacity to produce sensation 

 of light. Unit, the lumen = flux emitted in a unit solid angle (steradian) by point source of one 

 candle power. 



Visibility, A\ , of radiation of wave-length X = ratio luminous flux to radiant power (energy) 

 producing it. Mean visibility, K m , over any range of X or for whole visible spectrum of any 

 source = ratio total flux (lumens) to total radiant power (erg/sec, or watts). 



Luminous intensity, 7, of (approximate) point source = solid angle density of luminous flux 

 in direction considered = dF/du or F/u if intensity is uniform. o> is the solid angle. Unit, 

 the candle. 



Illumination on surface is the flux density on the surface = dF/dS or F/S when uniform. 

 5 is the area of the surface. Units, meter-candle, foot-candle, phot, lux. 



(Lux = one lumen per m 2 ; phot => one lumen per cm 2 .) 



Brightness, b, of element of surface from a given point = dl/dS cos 0, where 6 is the angle 

 between normal to surface and line of sight. Unit, candles per cm 2 . Normal brightness, 60 

 = dl/dS = brightness in direction normal to surface. Unit, the lambert. 



Specific luminous radiation, E' = luminous flux density emitted by a surface, or the flux 

 emitted per unit of emissive area, expressed in lumens per cm 2 . For surfaces obeying Lam- 

 bert's cosine law, E' = TT& O . 



The lambert, the cgs unit of brightness, is the brightness of a perfectly diffusing surface radiat- 

 ing or reflecting one lumen per cm 2 . Equivalent to a perfectly diffusing surface with illumina- 

 tion of one phot. A perfectly diffusing surface emitting one lumen per ft 2 has a brightness of 

 1.076 millilamberts. Brightness in candles per cm 2 is reduced to lamberts by multiplying by ir. 



A uniform point source of one candle emits 4?r lumens. 



One lumen is emitted by .07958 spherical candle power. 



One lumen emitted per ft 2 = 1.076 millilamberts (perfect diffusion). 



One spherical candle power emits 12.57 lumens. 



One lux = i lumen incident per m 2 = .0001 phot = .1 milliphot. 



One phot = i lumen incident per cm 2 = 10,000 lux = 1000 milliphots. 



One milliphot = .001 phot = .929 foot-candle. 



One foot-candle = i lumen incident per ft 2 = 1.076 milliphots = 10.76 lux. 



One lambert = i lumen emitted per cm 2 of a perfectly diffusing surface. 



One millilambert = .929 lumen emitted per ft 2 (perfect diffusion). 



One lambert = .3183 candle per cm 2 = 2.054 candles per in 2 . 



One candle per cm 2 = 3.1416 lamberts. 



One candle per in 2 = .4968 lambert = 486.8 millilamberts. 



Adapted from 1916 Report of Committee on Nomenclature and Standards of Illuminating 

 Engineering Society. See Tr., Vol. u, 1916. 



SMITHSONIAN TABLES. 



