finding by computation whether or not any two lights form equivalent stimuli. The fundamental method of color specification based upon equivalent stimuli plus spectrophotometry will be described in detail together with other methods of obtaining the same numbers. And finally some discussion will be given on the use of the Munsell color system and color dictionaries such as the ISCC-NBS (Inter-Society Color Council-National Bureau of Standards) Method of Designating Color and a Dictionary of Color Names, National Bureau of Standards Circular 553, 1956 [79].[1]
The most widely accepted technical definition of color is that given by the Committee on Colorimetry of the Optical Society of America [15]: "Color consists of the characteristics of light other than spatial and temporal inhomogeneities; light being that aspect of radiant energy of which a human observer is aware through the visual sensations which arise from the stimulation of the retina of the eye." It will be noted that this definition relates color and light to radiant energy only in so far as the energy produces a visual effect within an observer. On this account color and light are said to be psychological entities that can be evaluated by means of psychophysical quantities, and in their evaluation it is ordinarily not necessary to pay attention to energy of wavelength shorter than 380 nm, nor longer than 770 nm because the eye is relatievly insensitive to such energy.
If an observer with normal color vision attempts to adjust one element of his visual field whose color is under his control so that it matches a neighboring element, he will ultimately discover that three independent adjustments have to be made. If he is using the red, yellow, and blue paints frequently found in primary grade schools, only by chance will he obtain a match from a mixture of two of them. Even a brown color requires blue in addition to red and yellow. Within the color gamut of the three paints, an exact match for a given paint color is easily possible, but three is the irreducible minimum. Similarly, if he is trying to color-match one spot of light by shining several spotlights of different colors onto the same neighboring spot of a screen, he finds, in general, that either three lights of fixed spectral composition are required, or, if only two lights be added together, not only the amounts of both but also the spectral composition of at least one has to be adjustable. The same rule applies to rotary mixture on a sector disk; four sectors, giving three independent adjustments, are necessary and sufficient.
As the color vision of a normal observer is at least tridimensional, it follows that there must be at least three independent excitations in the optic nerve fibers corresponding to each patch of the visual field. Theories of color vision have been, derived mostly from speculation as to the character of these excitations. It also follows that a color specification is expressible by three numbers. For normal observers three numbers are necessary; for partially color-blind observers only two numbers are necessary; and for totally color-blind observers only one is necessary.
In the examples given (paints, spotlights, sector disks), the observer by adjustment of three variables obtains a color match, that is, he has to set up a second stimulus equivalent to the first. Except by accident, however, the ternary or binary mixture does not match the unknown in spectral composition. In the usual case the mixture is equivalent to the unkonwn in color but not in spectral composition, and the unknown and the mixture are said to form a metameric pair. There are, however, degrees of difference in spectral composition. If one painted panel be matched by a mixture of red, yellow, and blue paints, the degree of metamerism is likely to be only moderate; but if the paint panel illuminated by daylight be matched by shining on a white card three spotlights each of which contains energy restricted to a narrow wavelength band (such as spectrum red, green, and blue), the degree of metamerism will ordinarily be large.
Studies of extremely metameric pairs in which mixtures of two parts of the spectrum are set up to color-match other two-part spectrum mixtures have yielded our most valuable knowledge regarding the properties of the average normal eye [1, 44, 84, 99, 158]. An outstanding fact derived from observation of such metamers is that the center of the retina (fovea centralis) has somewhat different properties from that part of the retina immediately surrounding it; that is, a color match set up for the central 2 or 3 deg of the retina becomes an easily detectable mismatch if the eye be turned so as to allow the stimuli forming the metameric pair to affect a portion of the retina, say, 6 deg from the fovea. Furthermore, if the metamers are compared in large patches so as to subtend 6 deg or more at the eye of the observer, this mismatch causes a central spot, known as the Maxwell spot [98], to appear temporarily on either field even though the field is physically uniform. After the spot has faded away, change of fixation to the other field will renew the spot. This dependence of metamerism on the portion of the retina used arises chiefly from the existence of a spot of brownish or yellowish pigment irregularly covering and interpenetrating the central 3 or 4 deg of the normal, retina; it is called the macula lutea or sometimes the yellow spot (see fig. 1). Figure 1 shows a horizontal cross section of the eye. Light enters the tear-film ff, passes through the cornea aa, the aqueous humor B, the pupil bb, the crystalline lens A, the vitreous humor C, and the macula p before reaching the retina i. The macular pigment acts as
- ↑ Figures in brackets indicate the literature references at the end of this paper.
2
