10.5 Color Representation - Color Mixing System

Light is perceived as color by the human eye, termed color stimulus, and corresponds to the visible region of electro-magnetic spectrum, with a specific spectral curve of radiance as shown in Figure 10.5.1.

However as the physical value such as a spectral curve, is not convenient for representing color in daily life, a psychological representation or sensitivity expression are more practical.

Color representation can be classified into two types; a color mixing system using a quantitative and physical approach, and a color appearance system using a qualitative approach, by color code or color sample.

The color mixing system can generate any color by mixing of the three primary colors. The RGB color system specified by CIE, uses three primary color stimuli; blue of 435.8 nm(B), green of 546.1 nm(G) and red of 700.0 nm(R) by which all spectral colors ranging from 380 nm to 780 nm, can be generated with the mixing combinations (termed a color matching function or spectral tristimulus values) as shown in Figure 10.5.2.

As a part of the three spectral stimuli and includes a negative region, a coordinate transformation is applied to generate the virtual three spectral stimuli and,and and with positive values, as shown in Figure 10.5.3. This is called the XYZ color system. The three stimuli, X, Y and Z, can be computed as follows.

where K: constant
L() : spectral irradiance of standard illumination
() : spectral reflectance of sample

Trichromatic coordinates (x,y) can be computed as follows.


The value of Y corresponds to brightness while the coordinates of (x,y) represent hue and saturation (or chrome) .

The fringe of the bell shape corresponds to the spectral color with high chrome, while the inside corresponds with a low chrome.


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