Design Nomenclature / Systems & Spec
APCA
Also called Accessible Perceptual Contrast Algorithm, Lc, SAPC
This entry has not been translated yet and is shown in English.
A contrast model developed for WCAG 3 that reports a polarity-aware lightness contrast value, Lc, rather than a ratio, and factors in text size and weight. It exists because WCAG 2.x contrast handles dark backgrounds poorly. It is a candidate in a working draft — nothing conforms to APCA, and it replaces no shipped criterion.
In practice
Reached for as the fix for the dark-mode problem, which it does address. It is not a compliance target: a design can satisfy APCA and still fail an audit.
Not to be confused with
- Contrast ratio(L1 + 0.05) / (L2 + 0.05), where L is WCAG relative luminance and L1 is the lighter of the two colours. It runs from 1:1 to 21:1. Hue is discarded entirely — only luminance survives the formula — so two pairs at identical ratios can differ enormously in how readable they look.
- Relative luminanceThe quantity WCAG contrast is computed from: sRGB channels linearised through the transfer function, then weighted 0.2126 R + 0.7152 G + 0.0722 B to a value between 0 and 1. It approximates measured light, not perceived lightness, and models dark backgrounds badly — which is why light-on-dark text often passes while reading worse than the number promises.
- Level AAThe middle WCAG conformance level, and the one policy nearly always specifies. Requires every Level A and Level AA criterion: text contrast 4.5:1, or 3:1 for large text — 18.66px bold or 24px regular; non-text contrast 3:1; reflow to 320 CSS px; and, from 2.2, target size 24×24 CSS px.