Measurements & formulas

APCA

(Accessible perceptual contrast algorithm)

An algorithm in progress set to replace Web Content Accessibility Guidelines' (WCAG) relative luminance method for contrast calculation.

APCA takes into account various aspects of contrast & color perception, such as spatial frequency, simultaneous contrast, and elements of light & dark adaptation.

Contrast: 23.88

WCAG 2 equivalent: 1.81:1

How it affects UI design

APCA is not an official standard formula and should not be used in replacement of WCAG relative luminance until it has been officially released. However, there is a backwards-compatible version available if you want to use APCA while still meeting WCAG 2 criteria.

Calculated contrast for dark on light color will be different from light on dark color. Dark mode or themes should be independently evaluated for contrast and may require modification to pass compliance.

APCA formula is based on typographic shapes, which exist in a higher spatial frequency. However, there are additional criteria regarding which contrast value can be used for various combinations of font weight and size.

Sources:

  1. Andrew Somers. Myndex Color & Contrast Resources. Myndex Research. https://git.myndex.com/
  2. Andrew Somers. APCA contrast calculator. Myndex Research. https://www.myndex.com/APCA/
  3. Visual Contrast of Text Subgroup/Whitepaper. The World Wide Web Consortium (w3c). https://www.w3.org/WAI/GL/task-forces/silver/wiki/Visual_Contrast_of_Text_Subgroup/Whitepaper
  4. Visual Contrast of Text Subgroup. The World Wide Web Consortium (w3c). https://www.w3.org/WAI/GL/task-forces/silver/wiki/Visual_Contrast_of_Text_Subgroup