Color basics – CIE standard chromaticity space

Almost all colors can be mixed with three primary colors in a specific proportion. None of the three monochromatic lights can be produced by mixing the other two. In 1931, CIE (International Committee for Standard Illumination) established a series of standards representing CIE standard chromaticity space, and defined CIE-RGB primary color system. It specifies that the wavelengths of the three primary colors of RGB system are 700nm, 546.1nm, 435.8nm red light (R), green light (G), and blue light (B). A group of curves in Figure 1 can be obtained through the color mixing experiment. They represent that when the spectral energy is the same in the range of 380nm-780nm, the spectral color of a certain wavelength is the same as the color mixing results of,, and. These three curves are called spectral tristimulus value curves.

CIE1931 RGB

However, this system has an obvious disadvantage: negative values will appear when calculating the tristimulus value of color, which brings inconvenience to a large number of calculations. Since any primary color system can be switched from one system to another, people can choose any primary color system they want to avoid negative values and use it conveniently. Based on this, CIE also recommends CIE-XYZ system, which uses the imaginary X, Y and Z primary colors, representing red, green and blue. Figure 2 shows the tristimulus value curves,, of the XYZ chromaticity system based on the isoenergetic spectrum of the 2 ° field of view. Therefore, it is also called the XYZ chromaticity system of 2 ° field of view. The imaginary observer with such tristimulus value curve is called CIE1931 standard chromaticity observer.

CIE1931 XYZ

Any color can be expressed as:, where X, Y and Z are the tristimulus values of the color. According to the provisions of CIE, the tristimulus value can be calculated by the following formula.

Tristimulus value formula

Where P（ λ) Is the relative spectral power distribution of the light source radiation, and is the spectral tristimulus value of CIE1931 standard colorimetric observer, that is, the curve value in Figure 2. K is the normalization coefficient. The physical meaning is to normalize the Y value of the light source to 100. The calculation formula is:

Y value of light source

After obtaining the tristimulus value of the color, the chromaticity coordinates of the color in CIE1931 chromaticity system can be calculated according to the following formula.

Chromaticity coordinate

It can be seen from the above formula that when x and y of the color are known, the z value will also be determined. Therefore, in order to express the color in two-dimensional coordinates, x and y are used to describe it. Calculate the color coordinates of each monochromatic light of 380nm-780nm visible light, draw a rectangular coordinate map with x and y as rectangularcoordinates, and connect all points to form a light. Spectral locus, called chromaticity diagram, is shown in Figure 3. Where, x=0.333, y=0.333 are isoenergetic white points.

CIE1931 chromaticity diagram

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CD-320PRO Portable Colorimeter

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