nutting: precision colorimeter 183 



PHYSIC'S. — A new precision colorimeter. P. G. Nutting, 

 Bureau of Standards. 



It is well known that any color may be analyzed and specified 

 in either of two different ways: (1) in terms of three primary com- 

 ponents, red, green and blue (trichromatic analysis) or (2) in 

 terms of wave length of dominant hue or its complementary and 

 per cent white (monochromatic analysis). In mathematical 

 terms, the color point may be located by either trilinear or by 

 polar coordinates. Of the three elements of color, hue, tone and 

 luminosity, hue and tone are determined with a colorimeter, 

 luminosity with a photometer or a photometric part of the color- 

 imeter. 



Colors to be analyzed consist of light either emitted from some 

 source, transmitted through some selective screen or reflected 

 from some mat or semi-mat surface. Any of these colors to be 

 analyzed may be either spectral or purple according to whether or 

 not its dominant hue lies in the visible spectrum. Any analyzing 

 colorimeter must then be applicable to emitted, transmitted or 

 reflected light of either spectral or purple dominant hue. 



A trichromatic analyzer, the Ives 1 colorimeter, has been in 

 successful use for several years. A monochromatic method 

 of analysis was devised and used by Abney 2 in a laboratory 

 investigation as early as 1890. The colorimeter here described 

 is a monochromatic analyzer designed to be a practical working- 

 instrument of wide range, high precision and of the utmost sim- 

 plicity. 



The advantage of monochromatic analysis lies in the elimina- 

 tion of the arbitrary reference standards (red, green and blue), 

 readings being given directly in wave length and per cent white. 

 Both methods involve the definition and adoption of some stand- 

 ard of white. The trichromatic method may be used for either 

 spectral or purple hues indifferently, the use of the monochro- 

 matic method involves an interchange of two sources or arms in 

 passing from spectral to purple hues. Both methods give read- 

 ings varying somewhat with the observer involving in some cases, 



1 F. E. Ives: J. Franklin Inst., 421-3. 1907. 



- W. de W. Abney: Color Measurements and Mixtures, pp. 162-6. 1S91. 



