Tables 352-354 



335 



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

 OF LIGHT. LUMINOUS INTENSITY AND EFFICIENCY OF BLACK BODY 



The values of L, the luminous intensity, are given in light watts/steroradian/cm 2 of radiating surface 

 = (i/7r) f" 33 V\E\<1\, 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 maximum visibility, X = 0.556 /a. is the stimulus coefficient V m; its reciprocal is the (least) mechanical 

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

 lent of radiation ot maximum visibility." One lumen =0.001406 watts (Hyde, Forsythe, Cady); or 1 watt of radia- 

 tion of maximum visibility (A = 0.556 U) -- 668 I'imen* 



White light has sometimes bee 1 defined us th.it 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 352. — Brightness, Crova Wave- 

 length of Black Body, Mechanical 

 Equivalent of Light * 



TABLE 353. — Luminous and Total Intensity and 

 Radiant Luminous Efficiency of Black Body * 



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



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

 1919. 1917. 



Note. — Minimum energy necessary to produce the sensation of light: Ives, 38 X io~ 10 ; Russell, 7.7 X io -10 ; 

 Reeves, 19.5 X io -10 ; Buisson, 12.6 X io~ 10 erg. sec. (Bubson, J. de Phys. 7, 63, 1917.) 



Color temperature (temp, black-bodv same color) 500 w. gas-filled lamp (22 1/w) 3o82°k; qoo w. gas-filled movie 

 lamp, 22.7 1. w, 3oS6°k, crater 65V. 10 amp. arc, solid carbon, 378o°k cored carbon 3420 ]; Priest, 1922. 



TABLE 354. — 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 ,,low lamp, 3 . 8 wpc . . . 



Nernst glower, 1 . 50 wpc 



N'-fiUed tungsten, 1 .00 wpc. . . 



Source. 



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

 N -filled tungsten, 0.35 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, 5.0 amp, 

 Acetylene flame (flat) 



Color, 

 per cent 

 white. 



Hue. 



584 

 584 

 490 

 598 

 605 

 585 

 5S5 

 583 

 5S6 



* Jones, L. A., Trans. 111. Eng. Soc, Vol. 9 (1914). 



Smithsonian Tables. 



