Color is an important and often pleasant part of the visual domain; however, color is not a physical quantity but a human sensation. Color is the visual perception generated in the brain in response to the incidence of light, with a particular spectral distribution of power, on the retina. The retina is composed of photoreceptors sensitive to the visible range of the electromagnetic (EM) spectrum [21,36-38]. In general, different spectral distributions of power produce distinct responses in the photoreceptors, and therefore, different color sensations in the brain. See Table 1.1 for a representation of the EM spectrum and its parts related to various modalities of imaging, and Figure 1.1 for a display of the visible color spectrum as a part of the EM spectrum [1,39]. The diffraction of sunlight by water shows the visible color spectrum in the form of a rainbow; see Figure 1.2 for an example.
When a surface is illuminated with a source of light, it absorbs some parts of the incident energy and reflects the remaining parts. When a surface is identified with a particular color, for example, red, it means that the surface reflects light energy in the particular range of the visible spectrum associated with the sensation of red and absorbs the rest of the incident energy. Therefore, the color of an object varies with the illumination. An object that reflects a part of the light that is incident upon it may be considered a secondary source of light.
To reproduce and describe a color, a color representation model or color space is needed. Many color spaces have been proposed and designed so as to reproduce the widest possible range of colors visible to or sensed by the human visual system (HVS). The choice of a particular color space is determined by the application.