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2025-02-11
Colorimetry is a tool used to predict whether two stimulus color samples with different spectral power distributions have the same color under specific observation conditions. Its ultimate goal is to establish a prediction model that conforms to the color vision discrimination characteristics. This article introduces the concept of colorimetry and the CIE standard colorimetry system. Interested friends can learn more!
The quantitative description of color not only involves the color vision characteristics of the observer, but is also affected by many complex factors such as lighting and physical observation conditions. Color scientists from various countries have been exploring under the coordination and guidance of the International Commission on Illumination (CIE) for a long time, and are committed to developing color physical measurement theories and technologies that are consistent with color vision characteristics to meet the needs of industrial production for quantitative and standardized color representation. In 1931, CIE recommended the CIE1931 standard colorimetry system, three standard light sources (A, B and C), and standard lighting and observation conditions, laying the foundation for modern colorimetry.
In a narrow sense, colorimetry is a tool used to predict whether two stimulus color samples with different spectral power distributions have the same color under specific observation conditions. If two stimulus color samples have the same tristimulus values, they are considered to have matching colors for a standard observer with normal color vision. In a broad sense, colorimetry also includes methods for estimating the color appearance of stimulus color samples under complex observation conditions.
The ultimate goal of colorimetry is to establish a prediction model that conforms to the color visual discrimination characteristics, including the following points:
1. Each unique color should have a unique chromaticity parameter, that is, two stimulus color samples with the same chromaticity parameter should have the same color appearance, and two stimulus color samples with different chromaticity parameters should have different color appearances.
2. Use a coordinate system with mutually perpendicular dimensions to express the corresponding chromaticity parameters, and should reflect the main visual attributes of color, such as brightness, chroma and hue.
3. The calculated color difference between two stimulus color samples should be proportional to the corresponding difference in visual perception.
The first goal has been achieved by adopting the CIE1931 standard observer color matching function. The matching or mixing of three primary colors is the physical basis of the CIE standard colorimetric system. Almost all colors can be obtained by mixing three primary colors in a certain proportion. Based on the phenomenon of additive mixing of colors, Grassmann (H. Grassmann) proposed the Grassmann color mixing law in 1854, laying the theoretical foundation for color measurement and matching. The basic content of Grassmann's color mixing law is:
1. The human eye can only distinguish three changes in color, namely brightness, saturation and hue.
2. In an additive mixture of several colors, if one of the colors changes continuously, the appearance of the mixed color will also change continuously.
3. In additive color mixing, the mixed color depends on the appearance of the colors involved in the mixing, and has nothing to do with their respective spectral composition.
4. The total brightness of a mixed color is equal to the sum of the brightness of the colors that make up the mixed color.
1.CIE1931XYZ standard colorimetric system
Since negative values appear in the CIE1931 RGB colorimetric system during calculation, CIE uses the imaginary tristimulus values X, Y, and Z to establish a new colorimetric system, namely the CIE1931 XYZ standard colorimetric system. By selecting appropriate three primary colors, the spectral tristimulus values x (x), y (y), and z (z) are not negative, and Y is proportional to the brightness value. The calculation methods of the tristimulus values X, Y, Z and their corresponding chromaticity coordinates x, y, z are shown in the following formulas:
In the formula, φ (λ) is the spectral energy distribution emitted by the light source (light source color) or reflected or transmitted by the object (object color) and entering the human eye, which is called the color stimulus function; x (λ), y (λ) and z (λ) are the spectral tristimulus values of the CIE1931 standard colorimetry system, and their data can be obtained by looking up the table; k is the corresponding normalization coefficient. It should be noted that when it is necessary to predict the tristimulus values under different lighting bodies, the color appearance model should be used for calculation.
The CIE1931 XYZ standard colorimetric system mainly reflects the role of the cone cells in the fovea of the human visual system, so it is suitable for central visual observation conditions with a 2° field of view, and can also be used for color measurement in a 1°~4° field of view, but it is no longer effective for color observation in extremely small fields of view. For color measurement of fields greater than 4°, the CIE1964 XYZ standard colorimetric system should be used.
The CIE1931 XYZ standard colorimetric system is an international standard for color measurement and characterization, which evaluates the response of the human eye based on extensive experiments. It is the basic theoretical basis for the design and manufacture of almost all color calculations and color measurement instruments.
2. CIE196 4 XYZ standard colorimetric system
Since the CIE1931 XYZ standard colorimetry system is obtained based on the color vision matching experimental data under the condition of 2° field of view, many years of application practice have shown that the CIE1931 XYZ standard colorimetry system is suitable for 1°~4° field of view. When the field of view increases to more than 4°, since the imaging area on the human eye retina exceeds the range of the fovea, the rod cells will also participate in the effect, causing a certain change in color perception. The field of view when observing objects in daily life is usually large, so it is necessary to establish a new standard colorimetry system to meet the needs of large field of view observation and measurement.
Based on two sets of experimental data obtained by Stiles and Burch and Speranskaya under 10° field of view, CIE recommended a set of CIE1964 standard chromaticity observer spectral tristimulus value curves for large field of view in 1964, namely CIE1964XYZ standard chromaticity system. The calculation equations of the tristimulus values X10, Y10, Z10 and chromaticity coordinates x10, y10, z10 are:
Where φ(λ) is the corresponding spectral energy distribution, x10(λ), y10(λ) and z10(λ) are the spectral tristimulus values of the CIE1964 standard colorimetry system, and k10 is the normalization coefficient.
