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🔙 APCA Readability Criterion

Tests: Bronze Simple Mode

Test Methods - Introduction



  1. Summary
  2. How It Solves User Needs
  3. Objective
  4. Detailed Description
  5. Dependencies


Visual readability of text requires good visual contrast. Visual contrast is a product of the text characteristics, such as font weight (thickness) and font size, the lightness/darkness difference of the colors used for the text and the background, and other factors.

Bronze Simple Mode is designed as a special introductory mode to using perceptually uniform contrast. It does not require the use of any mass or look up tables or matching to a reference font. Instead is designed as a simple set of threshold levels, similar to how WCAG 2 works, but using perceptually uniform math.

How It Solves User Needs

All sighted users need adequate text size and weight, coupled with ample luminance contrast (the perceived lightness/darkness difference) between background and text colors, in order to read the text easily.


This test method supports the objective:

Detailed Description

The visual contrast needed for easy reading of text is substantially higher than the contrast needed to simply "make out" words. At least ten times more contrast is needed for good readability than the bare minimum contrast needed for legibility.

Perception of contrast is affected not only by the lightness and darkness of the colors used for the text and background, but also by the weight (thickness) of the font, the font's size, and the relative spacing of lines and letters. Additionally, the perceived contrast can be different when the text is a darker color than the background, or vice versa.

A person's contrast sensitivity shifts throughout their lifetime, and visual impairments can cause a substantial reduction in contrast perception, requiring higher contrast text for the same readability. Some impairments can make it difficult to see certain hues, so it is important to ensure enough luminance contrast (lightness/darkness), and not to rely on color contrasts of hue and saturation.

Because a person's contrast perception is so variable, it is difficult for any one individual to judge what makes a good readable contrast for all. The solution is to use objective guidelines and tools to evaluate font stroke width (weight), font size, background and font colors to ensure good visual contrast and readability.


  • An APCA-W3 compliant perceptual contrast calculator
  • A defined reference font family, for comparison purposes
  • The sRGB color space, the default color space for web content