Page:Letter Circular 454; hues of the spectrum colors (IA lettercircular45454nati).djvu/2

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 * DBJ:AEH IV-3
 * U.S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS. WASHINGTON
 * Letter Circular LC-454
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 * }

November 6,1953

HUES OF THE SPECTRUM COLORS

The characteristic apperance of the visible spectrum is a series of chromatic colors varying from dim red through orange, yellow, brilliant yellow-green, green, blue, to dim violet. The brightest part of the normal equal-energy spectrum under usual observing conditions is at 555 millimicrons (yellowish green), and from this point toward both longer and shorter wave lengths the brightness progressively diminishes.

If color differences due to variation in brightness are left out of account, the re still remain more than one thousand parts of the spectrum which can be distinguished by hue and saturation alone under suitable observing conditions. An appreciable part of these one thousand distinguishable steps is ascribable to saturation difference. For example, spectrum red is a much more saturated color than spectrum yellow, and there are many more steps between spectrum yellow and spectrum red than there are between spectrum yellow and a red color of the same saturation; a considerable part of the difference between spectrum red and spectrum yellow is, the refore, a difference in saturation. The part of the spectrum having the least saturated color is at 570 millimicrons (greenish yellow)

Strictly speaking, the appearance of the spectrum is var- iable depending chiefly on the following factors which will be discussed in turn:

1. Color Vision of the Observer. To the totally color blind observer, the spectrum appears as a band of achromaticlight varying from very dim at one end through a maximum brilliance near the center back to very dim at the other end; it exhibits no hue whatsoever. To the observer who has inherited partial color blindness (red-green blindness), the spectrum exhibits only two hues; it consists of a long-wave band appearing yellow and of a short-wave band appearing blue, the two bands being separated by an achromatic or neutral point.

To the normal observer, the spectrum appears as indicated above, but such observers disagree somewhat in their descriptions of the spectrum and there is reason to believe that the appearance of the spectrum varies slightly from one normal observer to another.

2. Size and Location of the Retinal Region Stimulated. Portions of the normal retina moderately distant from the center (point of most distinct vision) frequently respond as if red-green blind; the extreme periphery under ordinary conditions is totally color blind.