Tristimulus Theory of Color Perception

According to physiological studies, the rods and cones of the human eye decompose color into a combination of Red, Green, and Blue (in short, RGB ) and are called the primary colors. Current machine vision systems have implemented this natural tristimulus decomposition as an attempt to emulate human vision. These systems store the appropriate red, green and blue values needed to produce correspondingly matched color response at each point in an image.

A color model is simply a convenient way to represent color in numerical terms. Most color models use a three dimensional (3D) coordinate system. Each point within the system's subspace represents a unique color. The RGB color model, for example, can be visualized as a cube where Red is the X-axis, Blue is the Y-axis and Green is the Z-axis. Each one of the many million colors possible is described as a unique point within the cube.

There are many other color models in use.

Different Color Models

• RGB (Red, Green, Blue) model
• HSI model (Hue, Saturation, Intensity) and the HSV color model (Hue, Saturation, Value). These are frequently used in digital image processing.
• CMY (Cyan, Magenta, Yellow) model: standard used to describe color in the color printing industry, and
• YIQ color model (Y-axis, In-phase, Quadrature): used the television industry.

The RGB model is difficult to use directly because it requires three values of red, green, and blue to interpret temperature at a point – namely, a pixel. It is preferable to have a quantity that will have a single value corresponding to a particular temperature. The HSI model serves this purpose as discussed in the next section.