Color is an indispensable part of our daily lives, and our perception of color is often closely related to the properties of light and the functioning of the human visual system. In the field of color science, we use trichromatic stimulus systems to understand and describe colors.
A trichromatic stimulus system is a system used to visually match colors, based on describing the stimulus values of three primary colors: red (R), green (G), and blue (B). These three stimulus values are represented as X, Y, and Z, and are called tristimulus values.
For example, the tristimulus values of verdant green pigment are X = 22.7, Y = 39.1, and Z = 31.0. These values not only specify the characteristics of the color but also indicate the reflectance visually. For instance, the Y value equals the reflectance of the sample, which is 39.1% in this example. These values are calculated by visually comparing with a standard white surface. Under standard daylight, the tristimulus values can also be used to determine the visual perception main wavelength spectrum of a given sample, with the main wavelength of verdant green pigment being 511.9 nm.
This data can be graphically represented on a standard chromaticity diagram. The chromaticity diagram is based on the CIE standard formulated in 1931, and it is based on the x, y, and z values. Note that x + y + z = 1; therefore, if two values are known, the third value can be calculated, and the z value is typically omitted. The combination of X and Y values constitutes the chromaticity of a sample.
When their x and y coefficients are plotted on the chromaticity diagram, spectral colors from 400 nm to 700 nm follow a horseshoe-shaped curve. Non-spectral purple-red mixtures descend along a straight line connecting the 400 nm point to the 700 nm point. All visible colors fall within the resulting closed curve. Points on the circumference correspond to saturated colors; lighter, unsaturated colors appear closer to the center of the diagram. The color matching point is the central point where x = 1/3, y = 1/3, visually located at white.
Any straight line connecting points of relatively light beams includes all the colors formed by adding various amounts of two light beams. If a line passes through the color matching point, the colors represented by its endpoints need to form white when combined in appropriate amounts. Therefore, all lines passing through the color matching point terminate in saturated complementary colors on the closed curve.
The theory of trichromatic stimulus systems provides us with a framework for a deeper understanding of color perception and lays an important foundation for the development of color science.