Notes

It is a color space defined on the base of cone pigments and sensitivity of cones in macaque monkeys. They represent the colors in a tridimensional space based on cones excitation. This space has three axis:

• Luminance (L + M), represents the overall brightness of a color, which is determined by the combined responses of the long-wavelength (L) and medium-wavelength (M) sensitive cones in the retina.
• Chromaticity "Red -- Green" (L - M), represents the difference in the responses of the long-wavelength (L) and medium-wavelength (M) sensitive cones, which corresponds to the red-green axis of color perception.
• Chromaticity "Blue -- Yellow" (S - (L + M)), represents the difference in the responses of the short-wavelength (S) sensitive cones and the combined responses of the long-wavelength (L) and medium-wavelength (M) sensitive cones, which corresponds to the blue-yellow axis of color perception.

Any color perceived by humans can be represented as a point in this system. In this coordinates system, azimuth and elevation are defined as follows:

• Azimuth (also called hue angle, is the angle on the horizonal plane) represents the hue of the color in the chromatic plane. It is the angle formed between the red-green axis (L - M) and the color's chromaticity vector projected onto the chromatic plane.
• Elevation (also called saturation angle, is the angle on the vertical plane) represents the saturation or chroma of the color. It is the angle formed between the luminance axis (L + M) and the color's chromaticity vector.