5. The Commission Internationale de l'Eclairage (CIE) chromaticity diagram and the
NTSC triangle.
In the development of a color matching and specification system,
extensive color matching tests on many observers were conducted using a device
known as a colorimeter. In order to establish an average which could be considered
as that of the standard observer, many persons were used, with each performing
similar tests. The results of these tests are known as tristimulas values for the
color mixture curves.
a. By the use of mathematical equations, the information contained in the color
mixture curves has been converted to a graphical representation of the color on a
three-dimensional plane and is referred to as the Maxwell Triangle. For practical
purposes the three-dimensional Maxwell triangle has been converted to a two-
dimensional drawing and designated as the CIE chromaticity diagram.
(1) Examining the diagram in Figure 1-9, you see that the horseshoe curve,
which is known as the spectrum locus, is graduated into numerals ranging from 400
in the left-hand corner to 700 at the extreme right. These figures represent the
wavelengths of the various colors in nanometers.
The blues extend from
approximately 400 to 490 nanometers, the greens from approximately 550 to 580
nanometers, and the reds (including orange) extend from 580 to 700 nanometers.
(2) Since fully saturated colors contain no white light, the spectrum colors
which lie directly on the horseshoe curve are 100-percent saturated. At point E in
Figure 1-9, only white light is present, so there is zero percent saturation. This
same principle applies when observing the vectorscope, which is a device used for
measuring relationships between the various color signals being transmitted as well
as the amount of saturation present.
(a) Various percentages of saturation fall along a straight line drawn
between any point on the spectrum locus and point E (fig 1-9). As you move towards
point E, the saturation is decreased.
(b) Conversely, as you move toward the curve, the saturation is increased.
Thus, you can see that a 100-percent saturated color is one that has 100-percent
purity or freedom from white and that a desaturated color is a color which contains
some amount of white light.
b. When primary colors were selected for color television work, it was found
that those primaries were limited by color phosphors that were available for the
picture tube. Figure 1-9 shows the actual location of the primaries (red, blue and
green) that are used in color television.
These points represent the primaries
selected by the NTSC, and define a triangle within the boundaries of the
chromaticity diagram.
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