Features of the CIE 2000 Color Difference Formula
The CIE 2000 color difference formula is not an attempt to build a color space. Instead, it defines a calculation so that the color difference calculated by color meters becomes close to the color discrimination threshold of the human eye on the solid color space of CIE Lab (L*a*b* color space). Specifically, weight is assigned to the lightness difference ΔL*, saturation difference ΔC*, and hue difference ΔH’ by using weighting coefficients SL, SC, and Sh respectively. These weighting coefficients SL, SC, and Sh include the effects of lightness L*, saturation C*, and hue angle H. Consequently, the calculation incorporates the characteristics of the color discrimination threshold of the human eye on the color space of CIE Lab (L*a*b* color system): 1) Saturation dependency, 2) Hue dependency, and 3) Lightness dependency.
With the CIE Lab (L*a*b* color space) color difference formula, the color difference evaluation ranges represented by color difference ΔE*ab and chromaticity index difference Δa*b* were a perfect circle or a square in the L*a*b* solid color space. With CIE 2000, color difference ΔE00 is represented with an ellipse having the major axis in the direction of saturation similar to the shape of the color discrimination threshold of the eye. In the region with lower saturation, the weighting coefficients SL, SC, and Sh approach 1 respectively, making the ellipse more circular.
In the region with higher saturation, the weighting coefficient SC becomes greater compared with other coefficients SL and Sh so that the ellipse becomes longer in the direction of saturation (lower sensitivity to saturation differences).
With the CIE 2000 color difference formula, the effect on the hue angle is also considered. As a result, the formula can cope with another characteristic of the color discrimination threshold of the human eye on the color space of CIE Lab (L*a*b* color space): 4) Change in the direction of the color discrimination threshold around the hue angle of 270 degrees (blue) (deviation from the direction of saturation).
