The light emitted by various sources shows various colors due to the difference in spectral power distribution. Everything in the world has different spectral reflectance (or transmittance), which is why nature appears colorful in the light of the sun. How to express the color of light sources and objects, the following two systems are introduced.
Munsell system is a color classification and calibration system developed by A.H. Munsell according to the visual characteristics of colors. It uses a spheroid model (see FIG. 7.15) to express the three basic characteristics of various surface colors: hue H, lightness V, and chromaticity C. Each part of the diorama represents a specific color and has a label.
The horizontal distance of a particular color from the central axis represents saturation (that is, the depth of the color), known as Munsell chromaticity, which indicates the degree to which colors with the same lightness value leave the neutral color. The neutral color on the central axis has a chroma of 0, and the farther away it is from the central axis, the greater the chroma value. The maximum chroma of each color is not the same, individual color chroma can reach 20.
The Angle projected horizontally from the central axis represents the hue. Figure 7.16 shows the horizontal sections of Munsell’s color diorama, with each horizontal section corresponding to a lightness value. Each central corner on the horizontal profile represents a different hue. There are 5 main colors of red (R), yellow (Y), green (G), blue (B), purple (P) and 5 intermediate colors of yellow red (YR), green yellow (GY), blue green (BG), purple blue (PB), red purple (RP). Each tone can be divided into 10 grades from 1 to 10, and the grades of each main tone and intermediate tone are set as 5.
HV/C= tonal lightness/chroma
For example, the color labeled 5Y8/12 is yellow (Y), with a lightness of 8 and a chromaticity of 12, which is a bright yellow with a high degree of saturation.
For colorless black and white series (neutral colors) denoted by N, the degree value V is indicated after N, and the color is not written after the slash line.
NV/= Neutral chromaticity value /
For example, the meaning of the label N5 is that the lightness value is 5 gray.
The color mixing experiment of CIE shows that all colors of light can be mixed by three monochromatic lights in a certain proportion. None of the three monochromatic lights can be produced by mixing the other two. These three monochromatic lights are called primary colors. In 1931 CIE stipulated that the wavelengths of the RGB system were λr =700.0nm, λg =546.1nm and λb =435.8nm for red light (R), green light (G) and blue light (B). However, when using RGB system, it is found that in some cases, some quantities appear negative values, which brings great inconvenience to the calculation. So in 1931 CIE introduced a new system, the CIE XYZ System (Figure 7.17).
The maximum range over which color changes are not perceived by the human eye is called the color breadth, or just perceptible difference. Mac Adam et al. showed that the width of color varies at different positions in the X-Y chromaticity map. The blue part has the smallest width and the green part has the largest capacity. That is to say, equal distances between different parts of the X-Y chromaticity map do not represent visually equal chromaticity differences, so it is a serious drawback when using this map to measure or represent chromaticity differences. To overcome this shortcoming, CIE established the U-V chromaticity map (see Figure 7.18) in 1960, which is a relatively ideal uniform chromaticity scale (UCS) map.
