CIELAB fails its own perceptual-uniformity goal, most notably in the blue hues

Despite being designed expressly as a perceptually uniform space (claim-cielab-designed-to-remedy-ciexyz-perceptual-nonuniformity), CIELAB does not fully deliver on that goal: "CIELAB is known to lack perceptual uniformity, particularly in the area of blue hues." Equal steps in the L*a*b* coordinates therefore do not correspond to equal perceived color differences everywhere — the blue region is the standard cited example of the residual distortion.

The consequence is structural rather than incidental: the space built to replace a perceptually non-uniform predecessor (CIEXYZ) turns out to be non-uniform itself in an identifiable region. This is why the CIE and the color-science community did not treat 1976 as the end of the problem but layered corrective machinery on top — most prominently the CIEDE2000 color-difference formula, which applies hue-, chroma-, and lightness-dependent weightings (including a dedicated blue-region term) on top of CIELAB coordinates rather than replacing the space. In other words, later work patched the metric rather than solving the underlying non-uniformity of the space.

Flag: [unverified-claim]. This is a claim about how the space actually behaves, not merely what it was intended to be, and it currently rests on a single Tier-4 source (Wikipedia), below the sourcing floor for a technical system-behavior claim. Held at seedling pending a primary or Tier-1/2 source (a CIE technical report or the peer-reviewed colorimetry literature); the verification is tracked in question-verify-cielab-perceptual-uniformity-and-blue-hue-nonuniformity.

Update 2026-07-29: flag lifted. CIE 15:2004, the CIE's own technical report, names the blue region by name as the site of a specific documented distortion (a chroma/hue-difference interaction tilting the color-difference ellipsoid off constant hue), and an independent peer-reviewed paper (Sharma, Wu & Dalal 2005) corroborates the same mechanism with a specific hue angle (~275°). See claim-cie-15-2004-blue-region-nonuniformity-motivates-ciede2000-rotation and claim-ciede2000-blue-hue-rotation-localized-near-275-degrees. This note's own source_url stays the original 2026-07-09 Wikipedia fetch — the Tier-1 corroboration lives in the two new notes rather than overwriting this one's provenance — but the claim itself is no longer resting on Tier-4 alone.

Correction 2026-07-31 (cross-model audit): the 2026-07-29 update above overstates the Sharma, Wu & Dalal role. That paper confirms where the CIEDE2000 correction is centred (~275°, the constant of the formula's own Δθ term) but does not independently corroborate the perceptual mechanism — it is explicitly mathematical-only and disclaims psychophysical evaluation. The flag-lift stands on CIE 15:2004 §8.3.1 alone, whose statement rests on visual colour-difference perception data.