But the spectrum doesnotinclude the color magenta, which is a combination of two colors on opposite ends
of the spectrum (red and blue). Magenta is an example of a class of colors calledpurples, that are formed by
combining blue/violet with red in different proportions. Purples arenotspectral colors, and do not appear in
the spectrum of white light.
There is an important distinction betweenpurpleandviolet. Purple is anon-spectral color formed by
combining blue/violet light with red light. Violet, however,isa spectral color, and appears at the short-
wavelength end of the spectrum.
56.4 The Chromaticity Diagram.
Figure 56.2 shows theCIE chromaticity diagram.^2 It is a figure upon which may be plotted every color visible
to the human eye. Its unusual shape is because of the way it is defined; see Appendix S for details.
The curved, horseshoe-shaped edge of the chromaticity diagram is where the pure spectral colors lie.
Colors along this edge are the brightest and most vivid that we see them. Moving from the edge toward
the center of the figure, the colors become more and more washed-out, finally becoming white at point
ED.0:3333;0:3333/, the equal-energy point.
The straight line from.x;y/D.0:17;0:00/to.0:73;0:26/is called theline of purples. The non-spectral
colors (magenta and other purples) lie along this line.
Whether you’re looking at Figure 56.2 on a color monitor or on paper (printed from a color printer),
you’re notreallyseeing the diagram the way it actually looks. That’s because both color monitors and color
printers are limited in the range of colors they can display. The white triangle in Fig. 56.2 shows the range
of colors visible on a typical color monitor. If you look at the figure on a color monitor, you’ll notice the
colors look relatively constant moving along a line the white triangle to the curved edge; this is because of
limitations in the color monitor.
Figure 56.3 illustrates some properties of the chromaticity diagram. Fig. 56.3(a) shows that if you connect
any two points (colors)AandBwith a straight line, then all points along the line represent colors that can be
formed by combining colorsAandBin different proportions. Another property is illustrated by Fig. 56.3(b):
choose any color point on the edge of the diagram, and draw a straight line from that point, through the center
pointE, to the edge of the opposite side of the figure. This point at the opposite edge of the figure is the
complementof the original point.
(^2) CIE is the International Commission on Illumination; its initials are an abbreviation for its French name, Commission Internationale
de l’Eclairage. ́