The Properties Of Color; The Physics Of Color; Cie Color Model - Xerox DocuColor DocuColor 4 User Manual

6.1
What we call "color" is really a perceptual ability unique to humans and a
small number of animal species. Color theory is an attempt to systematize
the properties of color perception, which by nature is relative and change-
able. A color appears different depending on the other colors around it, and
individuals vary in their abilities to perceive color.
6.1.1

The physics of color

The human eye can see electromagnetic radiation at wavelengths between
400 nanometers (purplish blue) and 700 nanometers (red). This range is called
the visible spectrum of light. We see pure spectral light as intensely satu-
rated or pure colors. Sunlight at midday, which we perceive as white or neu-
tral light, is composed of light from across the visible spectrum in more or
less equal proportions. Shining sunlight through a prism separates it into its
spectral components, resulting in the familiar rainbow of colors.
Like the sun, most light sources we encounter in our daily environment emit
a mixture of many light wavelengths, although the particular distribution of
wavelengths can vary considerably. Light from a tungsten light bulb, for
example, contains much less blue light than sunlight. Tungsten light appears
white to the human eye which, up to a point, can adjust to the different light
sources. However, color objects appear different under tungsten light than
6
they do under sunlight because of the different spectral makeup of the two
light sources.
The mixture of light wavelengths emitted by a light source is reflected selec-
tively by different objects. Different mixtures of reflected light appear as
different colors. Some of these mixtures appear as relatively saturated col-
ors, but most appear to us as grays or impure hues of a color.
6.1.2

CIE color model

In the 1930s, the Commission Internationale de l'Eclairage (CIE) defined a
standard color space, a way of defining colors in mathematical terms, to
help in the communication of color information. This color space is based on
research on the nature of color perception.
By mixing any two spectral colors in different proportions, we can create all
the colors found between them. For example, it is possible to create the same
gray by mixing blue-green and red light or by mixing yellow-green and blue-
violet light.
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The properties of color