95 



TABLE 74.— APPARENT CANDLEPOWER OF DISK OR LINE SOURCE AT 



VARIOUS DISTANCES 



d = distance ; L = length or diameter of (disk) source. 



Candlepower, percent Candlepower, percent 



d/L 

 5 



10 



Line 



99.31 

 99.83 



Disk 



99.0 

 99.74 



d/L Line 



12 99.88 



15 99.94 



20 99.98 



Disk 



99.83 

 99.90 

 99.95 



TABLE 75.— SPECTRAL LUMINOUS INTENSITIES 



From Planck's equation and constants given in Table 53 and the relative luminosity 

 factors (Table 58) the spectral luminous intensities were calculated for a series of wave- 

 lengths ((/\ = .01/x), and for a number of temperatures and then reduced to equal total 

 luminous intensities. These relative values tor the brightness (photometric) of the black- 

 body at different temperatures hold for measurements made with a field brightness above 

 about 1 millilambert but do not hold for measurements made for low field brightness. 

 Some time ago some engineers engaged in photometry found a need for agreement for a 

 standard for low intensity. It was then decided 32 to use a source at a color temperature of 

 2360 °K. Recently 33 the International Committee on Weights and Measures adopted the 

 blackbody at the freezing point of platinum (2042° K) as the standard for low-intensity 

 brightness in photometry. 



S2 Weaver, K. S., Journ. Opt. Soc. Amer., vol. 38, p. 278. 1949; vol. 40. p. 60, 1950. 

 :,3 Terrien, Journ. Opt. Soc. Amer., vol. 39, p. 888, 1949. 

 * Platinum point. 



SMITHSONIAN PHYSICAL TABLES 



