APCA contrast

APCA (Accessible Perceptual Contrast Algorithm) is UDTS's primary contrast model. It reports contrast as Lc (lightness contrast) on a 0–105+ scale and is perceptually uniform across light and dark modes — unlike WCAG 2.x's luminance-ratio model, which over-reports contrast for dark colors and under-reports for light ones.

When to use

• Picking a foreground color for a known background at a specific readability target.
• Auditing a token catalog's contrast budget.
• Generating a palette ladder where each stop targets a specific Lc band.
• Code review: flag any contrast pick that uses WCAG 2.x as the primary model instead of as a cross-check.

When NOT to use

• Free-class tokens (illustration, decorative, brand-spot) that have no pairing obligation. APCA doesn't apply.
• Single-shot demos or one-off prototypes outside a token catalog where the result won't ship.

Lc target table