CIE76

CIE76, formally Delta E*ab, is the first widely adopted color-difference formula published by the International Commission on Illumination (CIE) in 1976. It computes the difference between two colors as the Euclidean distance between their CIELAB coordinates, effectively measuring how far apart two colors are in L*a*b* space. The standard calculation uses the same illuminant and observer for both colors, typically the CIE standard illuminant D65 and the 1931 2° (or 1964 10°) standard observer, though implementations may vary.

Given two colors with L*, a*, and b* values, the difference is defined as ΔE*ab = sqrt[(L1* − L2*)^2 + (a1* − a2*)^2 + (b1* − b2*)^2]. This single numeric value is intended to reflect perceptual similarity: smaller values indicate similar colors, larger values indicate greater differences. In practice, values around 1 are often considered barely perceptible under controlled viewing conditions, while higher values denote increasingly noticeable changes.

Limitations of CIE76 include its assumption of perceptual uniformity across the Lab space. It does not account for the nonlinearities of human color perception, leading to misestimates in certain regions of the spectrum—especially for very saturated colors, extreme lightness, or specific hues. Consequently, it can yield inconsistent judgments across colors that should be perceptually similar. As a result, many applications have shifted to perceptually uniform formulas such as CIEDE2000, or to adjustments like CMC l*a*b*, for more accurate color-difference assessments.

Despite these limitations, CIE76 remains a historical standard, a reference point in color science, and is still used for quick, rough comparisons and education, as well as in some legacy workflows.