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2024-12-10
CIELab and Hunter Lab are commonly used color measurement spaces for colorimeter. Both color spaces use the three parameters L, a, and b to express color, but their meanings are different. This article analyzes the difference between CIELab and Hunter Lab.
CIELab color space
CIELab is the abbreviation of CIE1976L*a*b* uniform color space, which is obtained by nonlinear transformation of CIEXYZ color system. It uses a three-dimensional coordinate system consisting of lightness index L*, chromaticity index a* and b* to express color perception. The conversion formula is as follows:
Among them, F(X/Xn), FY/Yn), and F(Z/Zn) are piecewise functions and have the same expression:
Where X, Y, and Z are the three stimulus values of the color sample; Xn, Yn, and Zn are the three stimulus values of the CIE standard illuminant irradiating the surface of a completely diffuse reflector, representing the color of the illuminant. The purpose of introducing Xn, Yn, and Zn in the formula is to simulate the effect of color adaptation, that is, no matter what the color of the illuminant is, when it is irradiated on a completely diffuse reflector and the eyes adapt to the light source, the color sensation produced should be white, and the chromaticity index a* and b* should be 0, that is, the color of the light source itself is offset.
Schematic diagram of CIElab color space
In the figure above, it can be seen from the geometric relationship that for any color sample with a coordinate position of p, its three sensory characteristics of lightness, hue and chroma can be calculated and expressed by the following formula:
Among them, the value range of lightness L* is 0~100, 0 corresponds to black, and 100 corresponds to white. The range of hue angle h*ab is 0°~360°, with the positive a* axis as the 0° direction, and counterclockwise rotation is positive. Chroma C*ab is the distance or radius from the coordinate point of the sample on the a*-b* plane to the origin. The above formula transforms the conversion formula and the rectangular coordinate system shown in the above figure, from the rectangular coordinate system to the cylindrical coordinate system, and uses lightness L*, hue angle h*ab and chroma C*ab to represent color perception.
Hunter Lab color space
Hunter Lab color space is a three-dimensional rectangular space based on the theory of opposing colors, as shown in the figure below. Similar to the CIE L*a*b* color system, the values of the same color in these two color spaces are different. Hunter Lab is contracted in the blue part of the color space, and CIE L*a*b* is over-expanded in the yellow part of the color space; both are derived from the CIE XYZ system.
L (lightness) axis: 0 represents black and 100 represents white.
a (red - green) axis: positive values are red and negative values are green.
b (blue - yellow) axis: positive values are yellow and negative values are blue.
The larger the L value, the whiter the object is; the smaller the L value, the darker the object is. Red gradually deepens along the (+a) direction, and green gradually deepens along the (-a) direction. Yellow gradually deepens along the (+b) direction, and blue gradually deepens along the (-b) direction.
Hunter Lab chromaticity parameter technical formula is as follows:
Among them: L is brightness; a and b are chromaticity; X, Y, and Z are the three stimulus values of the object; Xn, Yn, and Zn are the three stimulus values of the completely reflecting surface; Ka and Kb are adjustment factors.
CIEL*a*b* (CIELAB) is the most complete color model commonly used to describe all colors visible to the human eye. The asterisk (*) after L, a, and b is part of the full name because they represent L*, a*, and b*. Hunter Lab generally refers to Lab, which is a three-dimensional rectangular coordinate system based on the theory of color oppositions. It does not have an asterisk, unlike L*, a*, and b*. These two color systems are both based on the theory of color oppositions and are widely used. It is still necessary to distinguish between these two color systems.
Hunter Lab and CIELAB color systems have different calculation formulas. Although they are both derived from X, Y, and Z through mathematical calculations, the values of chromaticity are different. For example, for the same tile color - yellow, the color parameter values of the Hunter Lab system are L=61.42, a=+18.11, b=+32.23, while the color parameter values of the CIELAB system are L*=67.81, a*=+19.56, b*=+58.16.
There are differences in the uniformity of the two chromaticity spaces and visual perception. For example, although the CIELAB color space is a uniform color space recommended by CIE, in fact, the CIELAB space is still uneven for the color perception of the human eye. Take two color sample points in a certain area of the space (such as the red area) and compare them with two color sample points of the same distance in another area (such as the green area). It will be found that the visual difference between the two color samples in the red area is different from the visual difference between the two color samples in the green area, that is, the color width capacity values are not equal in different color areas.
