Design Nomenclature / Systems & Spec
Relative luminance
Also called WCAG relative luminance, WCAG L, Y
The 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.
In practice
Confused with lightness and with luminance proper. It is neither — it is one formula in one standard, and WCAG 2.x still carries an sRGB threshold constant the sRGB spec later corrected.
Not to be confused with
- LuminanceRadiance weighted by the eye's photopic sensitivity and integrated over wavelength — candela per square metre. It measures light leaving a surface toward the observer, is what a display's peak-brightness spec quotes, and is linear in energy, so equal steps in it are not equal steps in appearance.
- ValueHow light or dark a color is, independent of hue and chroma — the one attribute that survives conversion to greyscale. Value does the structural work: legibility, depth, and figure-ground separation all rest on value difference far more than on hue difference, and a composition that fails in greyscale was leaning on hue to hide it.
- 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.
- APCAA 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.