Page:Colorimetry104nime.djvu/15

Fig. 3.The (x,y) chromaticitiy diagram, showing the spectrum locus and the purple boundary.Wavelength is indicated in millimicrons. The hue names are those proposed by Kelly (77). specify the chromatic aspect of the light. The analogous ratio, $Z/(X+Y+Z)$, is also known as a chromaticity coordinate, $z$, but only two of the three coordinates, $x,y,z$, give independent information since by definition the sum of all three is unity. Table 2a gives the chromaticity coordinates, $x,y,x,$ of the spectrum colors for 2° and table 2b for 10° fields [59]. Figure 3 shows the points representing the spectrum colors in the ($x,y$)-chromaticity diagram. This diagram is also known as a Maxwell triangle because of Maxwell's first use of such a diagram [98]. Furthermore, it has aptly been called a mixture diagram because it indicates in a very simple way the chromaticity of the color resulting from the additive combination of any two lights. The point representing this chromaticity is found on the straight line connecting the points representing two lights. The primary lights are represented by points at the corners of a triangle, and every point within the triangle represents the chromaticity of a mixture of the primary lights whose proportions are indicated by the chromaticity coordinates, $x,y,z$. The spectrum colors are shown by a smooth curve known as the spectrum locus. A few points on this locus are identified by wavelength in nanometers. It will be noted from figure 3 that the spectrum locus is substantially straight from 540 nm to the long-wave extreme. This means that the standard observer would find binary mixtures of, say, 540 nm with 640 nm, closely equivalent to some intermediate portion of the spectrum. But the spectrum locus from 540 nm to the short-wave extreme is curved outward. This means that for the standard observer a binary mixture of 540 nm with, say, 440 nm would differ importantly in chromaticity from the intermediate parts of the spectrum. By drawing straight lines through any central point (such as x = y = ⅓, representing the so-called equal-energy stimulus) and extending them until they cut the spectrum locus, we may find the spectral complementaries relative to a stimulus represented by that point; that is, we may find the two parts of the spectrum that, when combined in proper proportions, will for the standard observer be equivalent to the central stimulus.

The straight line in figure 3 joining the extremes of the spectrum locus represents the chromaticities of the mixtures of the two extremes of the visible spectrum. The area bounded by the closed curve made up of the spectrum locus and this straight line is the locus of all physically realizable chromaticities. Note that the points representing the primaries of the CIE coordinate system, the apices of the triangle ($x=1,y=z=0$; $y=1,x=z=0$; $z=1,x=y=0$), all fall outside this area; that is, the primaries are imaginary. Note also that both the $X$ and $Z$ primaries fall on the line $y=0$, which is unassociated with luminosity and is known as the alychne or lightless line. The short-wave extreme of the spectrum locus comes close to this line; this means that, although it has the power to elicit in the standard observer a considerable $X$ and $Z$ response, resulting in a vivid bluish purple color, radiant flux of wavelength 380 to 420 nm is almost unassociated with luminosity. The areas in figure 3 corresponding to common color designations for lights are those proposed by Kelly [77] and will be discussed later.

2.2. Standard Sources

At the time of setting up the standard observer and coordinate system, the International Commission on Illumination [135], Commission International de I'Eclairage (CIE), recommended use of three standard sovirces for colorimetry; source A, representative of gas-filled incandescent lamps; source B, representative of noon sunlight; and source C, representative of average daylight such as that from a completely overcast sky. Source A is an incandescent lamp operated at a color temperature of 2854 °K, on the international temperature scale ($c_{2}$ = 14,380). Source B is obtained by using this same lamp in combination with a two-cell Davis-Gibson liquid filter designed to give a color temperature of about 5000 °K. Source C is obtained similarly and results in a source of correlated color temperature about 6800 °K. These sources are recommended for general use, or whenever there is no special reason for using some other source. Table 3 gives the relative spectral irradiance of Sources A, B, C, D₅₅₀₀, D₆₅₀₀, and D₇₅₀₀. Sources D₅₅₀₀, D₆₅₀₀, D₇₅₀₀ represent several phases of daylight, closely represented by the subscripted correlated color temperatures. Tables 4a and 4b give computation forms for evaluation of the colors of non-self-luminous specimens that transmit, scatter, or reflect incident light for the 2° standard ob-