Relative Luminance
A weighted measure of perceived brightness on a 0-1 scale, used in the WCAG contrast formula.
Definition
Relative luminance approximates how bright a color appears to the human eye. It weights red, green, and blue differently because our eyes are most sensitive to green light, less to red, and least to blue. Used as the input to the WCAG contrast ratio formula.
Formula
L = 0.2126 × R + 0.7152 × G + 0.0722 × B, where each channel is first linearized: v = (v_sRGB / 255 ≤ 0.03928) ? v/12.92 : ((v + 0.055)/1.055)^2.4.
Example
Pure black has L=0. Pure white has L=1. Mauve (#E0B0FF) has L≈0.55 — slightly brighter than mid-gray.