What is Luminance?
Also known as: Lightness, Value
The perceived brightness of a color, independent of hue and saturation. The L in CIE-LAB and HSL color models. Critical for accessibility and for color matching across substrates.
In detail
Luminance is how bright or dark a color reads. CIE-LAB L* axis runs 0 (pure black) to 100 (pure white). Two colors with the same hue and saturation but different luminance look like darker and lighter shades of each other. Luminance contrast between foreground and background is the primary factor in text legibility and colorblind accessibility — WCAG 2.1 mandates a 4.5:1 luminance contrast ratio for body text. In textile work, luminance is especially substrate-sensitive: a navy blue at L*=20 prints at L*=25-30 on cotton voile (lighter than designed) because the fabric reflects more light than the dye absorbs.
Example
WCAG accessibility check: pale gray text (L*=85) on white background (L*=100) has only 1.5:1 contrast — fails the 4.5:1 threshold. Same gray on dark navy (L*=15) has 11:1 contrast — passes easily. Designers reading WCAG specs work in luminance, not hex codes.