Table 299. 259 



PHOTOMETRIC DEFINITIONS AND UNITS. 



Radiant flux = * = rate of flow of radiation as energy, measured as ergs per second or watts. 



Luminous flux = F or ^ = 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. 



Visibility of radiation of wave-length X = K\ = ratio of luminous to radiant flux for that 

 X, = Fa/*a. 



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. visibility. See p. 261. 



Luminosity at wave-length X = (Ka) (4>a). Spectral luminosity curve expresses this as a func- 

 tion of X and is different for various sources. 



Luminous efficiency = F/4> expressed in lumens/watt. 



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

 flux in direction considered = dF/dw, or F/w when the intensity is uniform. Unit, the candle. 



Illumination on surface = E = flux density on surface = dF/dS (S is surface area) = F/S 

 when uniform. Units, meter-candle, foot-candle, phot, lux. 



Lux = one lumen per m^; phot one lumen per cm^. 



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



''i) bi = dl/'JG. cos 6 where 6 is the angle between normal to surface and the line of sight; 



normal brightness when is zero. 

 (2) bp = dF/dS' assuming that the surface is a perfect diffuser, obeying cos. law of emission 

 or reflection. 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'^. For surfaces obeying Lambert's 

 cosine law, E' = irbg. 



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

 ing or reflecting one lumen per cm^. Equivalent to a perfectly diffusing surface with illumination 

 of one phot. A perfectly diffusing surface emitting one lumen per ft^ has a brightness of 1.076 

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



A uniform point source of one candle emits 4ir lumens. 



One lumen is emitted by .07958 spherical candle power. 



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



One spherical candle power emits 12.57 lumens. 



One lux = I lumen incident per m'' = .0001 phot = .1 milliphot. 



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



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



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



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



One millilambert = .929 lumen emitted per if^ (perfect diffusion). 



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



One candle per cm^ = 3.1416 lamberts. 



One candle per in^ = .4868 lambert = 486.8 millilamberts. 



Adapted from Reports of Committee on Nomenclature and Standards of Illuminating Engineer- 

 ing Society. 1916 to 1918. 



Smithsonian Tables. 



k 



