APPENDIX 



47 



narrow wavelength intervals if they are to be 

 meaningful. Two such spectral distribution 

 curves are shown in Fig. 38. They are important 

 for exhibiting the quality of the radiant energy 

 in a visual stimulus. 



Radiant Purity. Fig. 38 is also convenient 

 for defining the concept of radiant purity. Radi- 

 ant purity between the wavelengths Xi and X2 is 

 the ratio of the shaded areas marked A to the 

 entire area under the line Em- The higher the 

 ratio, the greater is the radiant purity. In 

 Fig. 38, for example, the distribution of radiant 

 flux in the upper figure is more pure than the 

 distribution in the lower figure. This is a way of 

 saying that a greater proportion of the radiant 



by the American Standards Association in 1942 

 (100). Since the latter defines the units which 

 most people are familiar with, it may be worth- 

 while to review them briefly. 



Luminous Intensity. The unit of luminous 

 intensity or candle-power is the International 

 Candle, an arbitrarj' intensity agreed upon and 

 maintained by the three national standardizing 

 laboratories of France, Great Britain and the 

 United States. This concept is unchanged in the 

 new nomenclature. 



Another unit of luminous intensity is the Hefner 

 candle, equal approximately to 0.90 of an inter- 

 national candle. Although this unit is commonly 

 encountered in visual literature, particularly 



TABLE III 



Comparable Units and Measurements in Radiometry and Photometry 

 Adapted from a Report of the Committee on Colorimetry of the Optical Society of America (18) 



flux is concentrated in a relatively narrow part 

 of the spectrum. 



Other Concepts in Radiometry . Other concepts 

 in radiometry, together with their defining 

 equations, are listed in Table III. 



Photometry 



Corresponding to each of the radiometric units 

 there is a photometric unit for measuring the 

 amount of light. The names, units, and defining 

 equations recommended by the OSA Committee 

 are shown in Table III. This table also shows the 

 parallelism between the two kinds of units. 

 Every photometric unit in this table can be 

 defined in terms of its corresponding radiometric 

 unit simply by substituting the word "luminous" 

 for "radiant." The word luminous in this context 

 means radiant energy evaluated according to the 

 sensation produced in an average eye. As has 

 already been indicated, this sytem of units and 

 nomenclature differs from the one standardized 



German literature, published before 1939, it is 

 seldom used now. 



Luminous Flux. The old unit of luminous 

 flux, the lumen, was also adopted without change 

 by the OSA Committee. A lumen is equal to the 

 luminous flux emitted in a unit solid angle by a 

 point source of one international candle. 



Illumination. Illumination (illuminance in the 

 new OSA nomenclature) is the density of the 

 luminous flux deposited on a surface. The lux, 

 or meter-candle, is a convenient unit of illumina- 

 tion. It is the illumination on a surface one 

 square meter in area when this surface is receiving 

 a uniformly distributed flux of one lumen. By 

 virtue of the definition of a lumen above, a lux is 

 also the illumination deposited on the inner 

 surface of a sphere having a radius of one meter 

 with a point source of one international candle 

 at the center. 



Several other units of illumination are used 

 in visual work and Table IV shows how they 



