Luminance

The relative brightness of a color as perceived by the human eye, measured on a scale from 0 (black) to 1 (white). In accessibility, relative luminance is the foundation of WCAG color contrast ratio calculations, which compare the luminance of foreground text against its background. The contrast ratio formula is (L1 + 0.05) / (L2 + 0.05), where L1 is the lighter color's luminance and L2 is the darker. WCAG 2.x requires a minimum contrast ratio of 4.5:1 for normal text (Level AA) and 7:1 for enhanced contrast (Level AAA). Understanding luminance is essential for designers working with accessible color palettes, as two colors can have very different hues but similar luminance, resulting in poor contrast.