TABLE 70.— PHOTOMETRIC DEFINITIONS AND UNITS 93 



(Adapted from Reports of Committee on Nomenclature and Standards of Illuminating 

 Engineering Society, 1942.) 



Apostilb = 0.1 millilambert. 



Brightness of a luminous surface may be expressed in two ways : 



(1 ) bi =. dl/dA cos 9 where d is the angle between normal to surface and the line of 

 sight ; normal brightness when is zero. 



(2) b F = dF/dA assuming that the surface is a perfect diffuser, obeying cos law of 

 emission or reflection. Unit, the lambert. 



Candle per cm 2 = 3.1416 lamberts = 1 stilb. 



Candle per in. 2 = .4868 lambert = 486.8 millilamberts. 



Foot-candle = 1 lumen incident per ft" = 1.076 milliphots = 10.76 lux. 



Illumination on surface = E = flux density on surface = dF/dA (A is surface area) = 

 F/A when uniform. Units, meter-candle, foot-candle, phot, lux. 



Lambert, the cgs unit of brightness, is the brightness of a perfectly diffusing surface 

 radiating or reflecting one lumen per cm 2 . Equivalent to a perfectly diffusing surface with 

 illumination 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 v. 



Lambert = 1 lumen emitted per cm 2 of a perfectly diffusing surface. 



Lambert = .3183 candle per cm 2 = 2.054 candles per in 2 . 



Lumen is emitted by .07958 spherical candle. 



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



Luminous efficiency = F/* expressed in lumens/watt. 



Luminous flux = / ; or <k = rate of flow of radiation measured according to power to 

 produce visual sensation. Although strictly thus defined, for photometric purposes it may 

 be regarded as an entity, since the rate of flow for such purposes is invariable. Unit is the 

 lumen, the flux emitted in a unit solid angle (steradian) by a point source of unit candle 

 power. 



Luminous intensity of (approximate) point source = / = solid-angle (w) density of 

 luminous flux in direction considered = dF/du>, or F/w when the intensity is uniform. 

 Unit, the candle. 



Luminosity factor of radiation of wave-length X = A'x = ratio of luminous to radiant 

 flux for that X, = Fx/*x. 



Lux = 1 lumen incident per m 2 = .0001 phot = .1 milliphot. 



Mechanical equivalent of light = ratio of 4>/F for the X of max. visibility expressed in 

 (ergs/sec) /lumen or watts/lumen; it is the reciprocal of max. luminosity. See Table 58. 



Millilambert = .929 lumen per ft 2 (perfect diffusion). 



Milliphot = .001 phot = .929 foot-candle. 



Phot = 1 lumen incident per cm 2 = 10,000 lux = 1000 milliphots. 



Photon = small bundle of energy (hv), also called a quantum. 



Radiant flux = $ = rate of flow of radiation as energy, measured as ergs per second 

 or watts. 



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 

 Lambert's cosine law, E' = irb». 



Spectral luminous flux at wavelength X = (A'x)(#x). Spectral luminous curve ex- 

 presses this as a function of X and is different for various sources. 



One spherical candle emits 12.57 lumens. 



Uniform point source of one candle emits 47r lumens. 



TABLE 71.— RELATIVE MAGNITUDES OF UNITS OF BRIGHTNESS 



1 candle per ft. 2 = 3.142 foot-lamberts. 

 1 stilb = 1 candle per cm 2 

 1 apostilb = 0.1 millilambert. 



SMITHSONIAN PHYSICAL TABLES 



