Tables 303-306. 261 



BRIGHTNESS OF BLACK BODY. CROVA WAVE-LENGTH. MECHANICAL EQUIVALENT 

 OF LIGHT, LUMINOUS INTENSITY AND EFFICIENCY OF BLACK BODY. 



The values of L, the lumiaous intensity, are given in light watts/steroradian/cm^ vi radiating surface 

 » (i/tt) /* °° V-<E\d\ where V\ is the visibility of radiation function. 



Mechanical equivalent. The unit of power is the watt; of lumininous flux, the lumen. The ratio of these two quan- 

 tities for light of maxunum visibility, A = 0.556 ju, is the stimulus coefficient Vm; its reciprocal is the (least) mechanical 

 equivalent of light, i.e., least since applicable to radiation of majdmum visibiUty. A better term is " luminous equiva- 

 lent of radiation 01 maximum visibility." One lumen =0.001496 watts (Hyde, Forsythe, Cady); or 1 watt of radiar 

 tion of maximum visibility (X = 0.556 «) = 668 lumen<?. 



White light has sometimes bee c defined as that emitted by a black body at 6000° K. 



The Crova wave-length for a black body is that wave-length, X, at which the luminous intensity varies by the 

 same fractional part that the total luminous intensity varies for the same change in temperature. 



TABLE 303. — Brightness, Crova Wave- 

 length of Black Body, Mechanical 

 Equivalent of Light.* 



Hyde, Forsythe, Cady, Phys. Rev. 13, p. 45, 



TABLE 304.^ — Luminous, Total Intensity and 

 Radiant Luminous Efficiency of Black Body.* 



* Coblentz, Emerson, Bui. Bureau of Standards, 14, p. 255, 

 1917. 



Note. — Minimum energy necessary to produce the sensation of light: Ives, 38 X lo"!"; Russell, 7.7 X lo"!"; 

 Reeves, 19.5 X lo"**; Buisson, 12.6 X lo'i" erg. sec. (Buisson, J. de Phys. 7, 68, 1917.) 



Color temperature (temp, black-body same color) 500 w. gas-filled lamp (22 l./w) 3o82°k; goo w. gas-filled movie 

 lamp, 22.7 1/w, 3o86°k, crater 65V. 10 amp. arc, solid carbon, 378o°k; cored carbon 342o°k. Priest, 1922. 



TABLE 305. — Color of Light Emitted by Various Sources.* 



Source. 



Sunlight 



Average clear sky 



Standard candle 



Hefner lamp 



Pentane lamp 



Tungsten glow lamp, 1 . 25 wpc 

 Carbon Jow lamp, 3 . 8 wpc ... 



Nernst glower, i . 50 wpc 



N-filled tungsten, x . 00 wpc 



Source. 



N-filled tungsten, o. 50 wpc. , . 

 N-filled tungsten, 0.3s wpc. , . 



Mercury vapor arc 



Helium tube 



Neon tube 



Crater of carbon arc, 1 . 8 amp, 

 Crater of carbon arc, 3. 2 amp, 

 Crater of carbon arc, j.o amp, 

 Acetylene flame (flat) 



Color, 



per cent 

 white. 



4S 

 53 

 70 



67 

 36 



Hue. 



584 

 584 

 490 

 598 

 60s 

 58s 

 S8S 

 583 

 S86 



* Jooes, L. A., Trans. HL £ng. Soc., VoL 9 (1914). 



Smithsonian Tabues. 



