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sharply than violet light as it passed through the prism, creating a spectrum of
colors [102].
Newton divided the spectrum into seven colors: red, orange, yellow, green,
blue, indigo, and violet (figure 3.1). These seven colors out of a belief derived
from the ancient Greek sophists, that there was a connection between the colors,
the musical notes, the known objects in the solar system, and the days of the
week [102].
A system that describes color is called a color system or color space. Each color
system has its own set of color models [29].
A color model is an abstract mathematical model describing the way colors can
be represented as tuples of numbers, typically as three or four values or color
components [29].
The three basic properties of the color radiation are: luminance, hue and
saturation. The correspondent perceptual parameters are brightness, color and
purity [29].
The luminance represents the visual perception attribute that makes an area to
reflect more or less light [36]. The humans have a non-linear perception of
brightness.
The hue represents visual perception attribute that makes an area to be similar
to one of the following colors: red, yellow, green, blue, or a combination of them
[36]. The colors found in nature are polychromes. They are a mixture of several
radiations with different wavelength. Each natural color has a dominant
wavelength that gives the visual perception of the hue. It can also include
components with wavelengths beyond visible spectrum. The white or grey light
appears when there are radiations from all wavelengths, in equal quantities.
The saturation is defined as the proportion of pure light with respect to white
light needed to produce the color. Any color can be represented by a combination
of white or grey light and a pure color, in a specific proportion. The ratio between
energy magnitude of the spectral component and total energy represent the purity,
or saturation. A pure color has 100% saturation, while the white or grey light has
the saturation 0.
The colors of the visible light spectrum
color wavelength interval frequency interval
red ~ 630–700 nm ~ 480–430 THz
orange ~ 590–630 nm ~ 510–480 THz
yellow ~ 560–590 nm ~ 540–510 THz
green ~ 490–560 nm ~ 610–540 THz
blue ~ 450–490 nm ~ 670–610 THz
violet ~ 400–450 nm ~ 750–670
Fig. 3.1. The colors of the visible light spectrum