CIELAB Color Space

Overview

CIELAB (also written CIE L\a\b\) is a color space defined by the International Commission on Illumination (CIE) in 1976. It expresses color with three coordinates: L\ for perceptual lightness, and a\ and b\ for the two chromatic (color-opponent) axes. It was designed as an approximately perceptually uniform space, derived by a nonlinear transformation of CIE 1931 XYZ tristimulus values.

CIELAB serves two dominant roles in imaging and printing: as a device-independent reference for specifying color, and as the basis for color-difference (ΔE) measurement. It is standardized as ISO/CIE 11664-4.

ICC profiles, CMYK, halftoning, and the raster image processor (RIP) each have their own reference pages; this page cross-references them rather than restating them.

What it is / definition

CIELAB is a three-dimensional, opponent-color model:

• L\* — lightness, ranging from 0 (black) to 100 (a diffuse white).
• a\ — a red–green axis: +a\ toward red, −a\* toward green.
• b\ — a yellow–blue axis: +b\ toward yellow, −b\* toward blue.
• a\ = 0, b\ = 0 is the neutral (achromatic, gray) axis.

The a\ and b\ axes have no fixed numeric limits; their range depends on the color and on the reference white. Because CIELAB is computed from CIE XYZ — which is grounded in the CIE standard observer derived from color-matching experiments, not in any physical device — CIELAB colors are device-independent: the same L\a\b\* values denote the same intended color regardless of monitor, camera, or press.

A common cylindrical restatement is CIELCh (L\, C\ = chroma, h = hue angle), the same space expressed in polar form; C\* = 0 is the gray axis.

History

In 1976 the CIE recommended two coordinate systems, CIELAB and CIELUV, both nonlinear functions of X, Y, and Z. The recommendation was an attempt to unify the then-diverse practice of uniform color spaces and color-difference formulae.

The space was later published as a formal standard, jointly adopted by ISO and the CIE as Colorimetry — Part 4: CIE 1976 L\a\b\ colour space*. The 2007/2008 text (CIE S 014-4/E:2007 / ISO 11664-4:2008) was subsequently superseded by ISO/CIE 11664-4:2019, cataloged as a minor revision of the joint standard.

How it works

CIELAB is computed from CIE XYZ relative to a reference white (Xn, Yn, Zn):

• L\* = 116 · f(Y/Yn) − 16
• a\* = 500 · ( f(X/Xn) − f(Y/Yn) )
• b\* = 200 · ( f(Y/Yn) − f(Z/Zn) )

where the nonlinearity f(t) is piecewise, with δ = 6/29:

• f(t) = t^(1/3) for t > δ³
• f(t) = t / (3δ²) + 4/29 for t ≤ δ³

The cube-root branch approximates the compressive, nonlinear response of human lightness perception; the linear branch near black avoids the infinite slope of the cube root at t = 0 and is chosen to match the value and slope of the cube-root branch at the transition point.

Reference-white values commonly used (2° / CIE 1931 standard observer):

• D65: Xn = 95.0489, Yn = 100, Zn = 108.8840
• D50: Xn = 96.4212, Yn = 100, Zn = 82.5188

Because the transform depends on the chosen white, any L\a\b\* specification is only fully defined together with its reference white/illuminant and observer. Values for the 10° observer differ.

Where it sits in the color / print pipeline

CIELAB functions as an interchange and reference space rather than a rendering space. Scanners, cameras, and displays produce device RGB; presses and printers consume CMYK. CIELAB provides the vendor-neutral coordinate system in which colors are specified, measured, and compared — for example, a spot color or a print-target aim point expressed as L\a\b\* values read from a spectrophotometer.

Process-control print standards use CIELAB tolerances to judge conformance: the ISO 12647 family (process control for the production of halftone color separations, proofs, and production prints) expresses aim values and tolerances in CIELAB / ΔE terms.

Relationship to ICC profiles and CMYK

Within ICC color management, the Profile Connection Space (PCS) — the neutral hub through which device profiles connect — is defined on a D50 reference and may be encoded as either CIEXYZ or CIELAB. The ICC specifies the PCS white point as XYZ ≈ (0.9642, 1.0000, 0.8249), a D50 approximation.

CIELAB is often preferred for PCS lookup tables because its perceptual spacing makes interpolation of color LUTs more even. This is how device-dependent CMYK acquires a device-independent meaning: a CMYK output profile maps ink combinations to PCS (XYZ/LAB) values and back, so CMYK numbers gain an absolute colorimetric interpretation.

Relationship to printer technologies

CIELAB is printer-technology-neutral: it describes the intended color, not the marking method. Offset lithography, flexography, inkjet, dye-sublimation, and electrophotography (toner) all produce color through halftoning or ink/toner mixtures, but each process is characterized and controlled by measuring printed patches in CIELAB and comparing them to aim values via ΔE.

Because different processes and substrates achieve different gamuts, CIELAB is also the space in which gamut extent and gamut mismatches are commonly visualized and quantified. It supports process control — measuring printed reference charts and computing ΔE to targets — independent of the underlying marking engine.

Common problems

• Perceptual non-uniformity in practice: CIELAB is only approximately uniform. Equal Euclidean distances do not correspond to equal perceived differences everywhere, most notably in saturated and blue regions — which motivated later difference formulas (CIE94, CIEDE2000).
• Reference-white / illuminant dependence: L\a\b\* values are undefined without stating the reference white and observer; mixing D50- and D65-referenced values causes errors.
• Measurement conditions: Instrument geometry, illumination, and observer (2° vs 10°) affect measured L\a\b\*; comparisons require consistent conditions.
• Naive ΔE (CIE76): The plain Euclidean ΔE\*ab can overstate or understate perceived difference in some regions, so it can misjudge pass/fail tolerances.

Advantages

• Device independence — a single, vendor-neutral reference for specifying and communicating color.
• Perceptual orientation — lightness and opponent chroma axes align with intuitive dimensions (lighter/darker, red/green, yellow/blue) and roughly with the perceived magnitude of difference.
• Basis for color difference — enables a single-number ΔE metric for tolerancing and quality control.
• Full-gamut coverage — encompasses all colors of the standard observer, so it can reference colors outside any given device gamut.
• Standardized — defined by the CIE, published as ISO/CIE standards, and used as an ICC Profile Connection Space encoding.

Limitations

• Not truly uniform, especially in saturated and blue regions.
• Requires a specified white point and observer to be well defined.
• Not a display or rendering space — it is a reference/interchange model, not used directly to render pixels or inks.
• Simple ΔE\*ab is imperfect; accurate tolerancing generally uses CIEDE2000 or CMC.
• Does not model appearance phenomena (surround, adaptation, contrast) captured by full color-appearance models such as CIECAM.

Color difference (ΔE)

CIELAB is the coordinate basis for the standard color-difference formulas:

• CIE76 (ΔE\ab): the original 1976 Euclidean distance, ΔE\ab = √( (ΔL\)² + (Δa\)² + (Δb\*)² ). A value of roughly 2.3 is often cited as an approximate just-noticeable difference (JND), though this varies by region and viewing conditions.
• CIE94 (1994): adds parametric weighting factors (kL, kC, kH) for lightness, chroma, and hue; it is asymmetric (a quasimetric).
• CIEDE2000 (ΔE00, published 2001): adds further corrections for lightness, chroma, hue, chroma–hue interaction, and a hue-rotation term for the blue region. It is standardized as ISO/CIE 11664-6 (2014 edition; superseded by a 2022 edition), with input L\a\b\* computed per ISO/CIE 11664-4.
• CMC l:c (1984): developed by the Colour Measurement Committee of the Society of Dyers and Colourists, used in L\C\h form with weighting ratios such as 2:1 and 1:1.

Modern relevance

CIELAB remains the dominant reference space for specifying and measuring color in printing, packaging, textiles, and general color management. It underpins spot-color specification, print process control (the ISO 12647 family's aim/tolerance work), and the ICC PCS.

CIEDE2000 is the current recommended color-difference formula for most industrial tolerancing, computed on CIELAB coordinates. Newer perceptual and appearance models (for example CAM16-UCS and CIECAM-based uniform spaces) address CIELAB's uniformity shortcomings, but they build on the same CIE colorimetric foundation and have not displaced CIELAB as the everyday interchange and measurement space.

Timeline

1. 1976

   CIE recommends CIELAB and CIELUV.

2. 1984

   CMC l:c color-difference formula (Society of Dyers and Colourists).

3. 1994

   CIE94 color-difference formula (published as CIE 116-1995).

4. 2001

   CIEDE2000 color-difference formula published (CIE technical report).

5. 2007 / 2008

   CIELAB standardized as CIE S 014-4/E:2007 and ISO 11664-4:2008.

6. 2014

   CIEDE2000 standardized as ISO/CIE 11664-6:2014.

7. 2019

   ISO/CIE 11664-4:2019 supersedes the 2007/2008 text (minor revision).

8. 2022

   ISO/CIE 11664-6:2022 edition published.