Page 50 - Sensors and Control Systems in Manufacturing

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Cha p te r

these colors; rather, it is light that lacks wavelengths between red and
blue that reside here.
All of the colors humans can perceive fall inside the spectral locus.
It is an interesting feature of this chart that given two source colors,
all of the colors that can be made by blending those colors in different
amounts will fall on the line that connects them. An important exten-
sion of this is that the colors that can be made by blending three
sources will fall inside the triangle defined by those sources. The
vertices of the triangle are regarded as the primaries of that particular
color system. This is very useful in predicting the colors that can
be made by three different phosphors, as used in video displays.
The colors inside the triangle represent the color gamut of the display,
the colors that can be generated by the display.
The exact primary colors for a given display technology are
carefully selected to balance a set of tradeoffs between saturation,
hue, and brightness. Some very successful phosphor combinations
have been found over the years and are used in various broadcast
television and video standards. One successful set comprises the
Trinitron phosphors used in a vast number of television and com-
puter displays. It embodies a design choice where the extent of the
color gamut is diminished slightly in favor of a significantly brighter
image. Because of its ubiquity, it forms the foundation for the sRGB
color space, a standard used in PC and world-wide-web graphic
design.
The spectral locus represents the highest degree of purity possible
for a color. As one moves away from this boundary toward the inte-
rior, colors become less saturated. In the center area, the colors become
nearly neutral tints of gray. A display system, defined by a triangle of
primaries, will select a point in the center to be the white-point for the
display. It need not be the geometric center of the triangle, and for
many systems it can be quite arbitrary, since the visual system will
adapt, attempting to make an image look “natural.” The exact mecha-
nism of adaptation is complex and the subject of current color
research, but enough is known that it is now possible to translate
images from one display system to another, or to a hardcopy print,
and retain the natural appearance. The circle in the RGB space cube
representation shown in Fig. 1.7 represent the white-points for the
two different display systems charted.
A set of primaries and a white-point are enough to define a (lin-
ear) color space. These are usually called RGB color spaces because
the primaries for most useful systems are distinctly red, green, and
blue. A characteristic of these color spaces is that they can be imple-
mented using linear algebra. Converting from one space to another is
a matter of applying the correct 3 × 3 matrix operation.
There is one more characteristic of most displays that modifies
the color space and destroys its linearity: the gamma.