CIELABverdier

CIELAB values, often written as L, a, and b, constitute the coordinates of the CIELAB color space. Defined by the CIE in 1976, the space is designed to be perceptually uniform and to serve as a device-independent representation of color derived from the CIEXYZ tristimulus values under a specified reference illuminant and observer. In this system, L represents lightness, ranging from 0 (black) to 100 (white). The a axis encodes red-green information, with positive values toward red and negative values toward green; the b axis encodes yellow-blue information, with positive values toward yellow and negative values toward blue. In practice, the numerical range for a and b can extend beyond the -128 to 127 interval in some measurements, and digital storage often maps these values to signed or unsigned encodings.

CIELAB values are obtained from CIEXYZ values using a defined transformation that depends on the reference white (Xn, Yn, Zn) for a given illuminant and observer. The standard equations are L = 116·f(Y/Yn) − 16, a = 500·[f(X/Xn) − f(Y/Yn)], and b = 200·[f(Y/Yn) − f(Z/Zn)], where f(t) is a piecewise function that follows a cube root for larger t and a linear segment for small t. The choice of reference white determines the exact Lab coordinates.

Uses and limitations: CIELAB values are widely used to specify colors in printing, design, textiles, and digital imaging, and to compute color differences via Delta E (e.g., ΔEab, ΔE00). While more perceptually uniform than many other spaces, Lab is not perfect; true perceptual uniformity is only approximate, and results depend on measurement geometry, illumination, and the reference white.

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