WAI/WCAG Contrast Research

"Contrast Perceptual Uniformity"

Grid Compare, November 1 2019

These are tests of the contrast measurements by the SAPC algorithm(s). These test plots are for normal polarity (dark text on a light background, aka BoW). The series on this page is a small sampling of the test plots. he contrasts shown are SAPC levels 5%, 10%, 25%, 40%, 60%, 80%, 100%.

There are three versions, the only thing different in each is the brightness of the background of the panel. The test patch colors are the same for each of the three test panel versions.

Evaluation Considerations

When viewing, try different ambient lighting conditions: outside, in the shade, indoors, in a dark room, etc. and also try different setting of your screen brightness. Notice how much it changes the contrast perception. In some situations, especially a dark room with the display set bright, you may notice some "extra" contrast in the very dark colors, this is an intentional contrast bump near black, to compensate for bright ambient environments and displays with poor black level capability.

Viewing the Chart

Each column is intended to have the same perceived contrast from top to bottom, that is best exemplified by the readability of same specific line of text on each test patch. The top line is largest and in bold, then a normal weight, and finally smaller weight and very thin fonts. This should demonstrate at which point each font size and weight become visible.

Readability vs Legibility

Important Notes for Designers

"Legibility" refers to the point where you can just barely read what the item says. But "readability" refers to the point where it is actually easy to read. Designers should be aware that the contrast needed for readability is at least twice that needed for barely legible. For instance, if you can just read "ATOM sized gap" at 25% contrast, then in practice in a design it needs to be at least at 50% contrast, with 75% recommended. If the "just barely" for "Little Moat" is 40%, then that needs to be at least at 80%, and preferably 120%.

The levels generated by the SAPC-6 algorithm are intended to make such assessments simple by being perceptually uniform to the degree that a certain contrast percentage is consistent in legibility or readability regardless of if the colors are bright or dark. An additional advantage is that to get from "barely legible" to easily readable, simply triple the legible contrast percentage for that particular font size, weight, and design. (triple for best readability, double for basic readability for non-content items such as copyright notices).