JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. V FEBRUARY 4, 1915 No. 3 



FWY^ICS.^— Characteristic equations of tungsten filament lamps 

 and their application in heterochromatic photometry.^ G. W. 

 MiDDLEKAUFF AND J. E. Skogland. Coiiimunicated by the 

 Bureau of Standards. 



The most difficult problem in ordinary photometry is the 

 comparison of the intensity of light sources differing widely 

 in color. In such comparisons it is practically impossible, even 

 for the most experienced observers, to agree in their measure- 

 ments, this disagreement being due principally to difference in 

 color vision and to difference in judgment as to when two colors 

 viewed in the photometer are of equal intensity. Hence, especi- 

 ally, to establish standards, a large number of observations by 

 different experienced observers must be taken to average out the 

 personal errors. 



One of the most convenient methods of avoiding color dif- 

 ference in practical photometry is by the use of colored glasses 

 to bring the lamps to a color match; but by this method the 

 principal difficulties are merely shifted to the problem of cali- 

 brating the glasses, a large number of which are necessary to 

 meet the present requirements of colored light photometry. 



By the method described in this paper, color differences are 

 avoided by the use of tungsten standard lamps which are ad- 

 justed in voltage to color match the light source to be measured, 

 and the candlepower values of the standards at the voltage 



1 To be published in full as Scientific Paper No. 238 of the Bureau of Standards. 



61 



