Perception Research

We are currently researching perceptual contrast for assessing legibility and accessibility on graphically rich web pages and computer displays.

Supervising Color Science Researcher: Andrew Somers

There is a live issue and discussion at GitHub (#695 relating to the WCAG), and also on ResearchGate. Below are links to some of our live experimental pages, evaluating the issue(s) in detail:

Color Vision Deficiency Simulator (CVD Sim)

NEW: CVD Simulator. We developed this new web app to simulate how someone with Color Vision Deficiency** sees the world. We accurately simulate Protanopia, Deuteranopia, and Tritanopia. We also present an experimental model of Blue Cone Monochromacity. www.myndex.com/CVD/

**Color Vision Deficiency is sometimes referred to as "color blind," though that is not accurate except in some very rare cases.

Please be advised: The experiment pages listed below are using fonts sized in REM and are thus NOT responsive. They are intended for desktop/laptop use, or use in our test app.

Live Contrast Experiments 2019

   • Initial Page describing the problem with the current standard. Discussion of the genesis of this project, the discovery of the issues with the WCAG contrast standard math. NOTE: This page contains some of the initial reactions, but the preliminary conclusions are superseded by more recent experimental research.
   • Some initial equation evaluations and considerations. Early experiments solving the perceptual uniformity problem. (link is off to avoid confusion but available on request).
   • Experiments CEX and CEXI (link is off to avoid confusion but available on request) — A layout of the basic experiments but using the current WCAG math for a standard of comparison .


   • Experiment CE09 — Comparing several different contrast assessment equations.
   • Experiment CE10 — SAPC3 Evaluation (Dark on Light). Sample DIVs with sRGB values that are approximately perceptually uniform with in each target range.
   • Experiment CE11 — SAPC3 Evaluation (Light on Dark). A straight inversion of the values used in CE11, see that when inverted the assessment does not indicate perceptual uniformity from bright to dark. See experiment CE12.
   • Experiment CE12 — SAPC3 Evaluation (Light on Dark v02) Perceptual uniformity Experiment. The sRGB values were adjusted for uniformity. Useful to note how polarity inversion affects perceptual contrast in terms of uniformity.
   • Experiment CE14 — SAPC5 Evaluation (Dark on Light) Further perceptual uniformity Experiment. CE14 and CE15 are examine a new formula/algorithm for a perceptually uniform contrast assessment. OF NOTE: This equation is showing overall promise in real-world ambient light, especially on mobile devices in daylight. (Please note this page is NOT yet responsive, and thus has usability issues on mobile devices).
   • Experiment CE15 — SAPC5 (INV) Evaluation (Light on Dark) Further perceptual uniformity Experiment. See CE14, this is the light text/dark BG version.
   • Experiment CE14size NEW! How Stimulus Size affects perceptual uniformity Experiment, also contains parts of CE14 for reference. The examples in this page demonstrate that stimulus size is a CRITICAL FACTOR in perceptual contrast. (Please note this page is NOT yet responsive, and thus has usability issues on mobile devices).
   • Experiments CE16 thru 19 — In development, based on findings of CE14 & CE15 and related research.

Brief Commentary on Contrast

and the WCAG Accessibility Standards.

Interestingly, the W3C/WCAG math is similar to a basic Weber contrast in terms of a perception curve when in a totally DARK environment (such a bedroom at night). Of course the described problems are present in typical and high ambient lighting.

One of the curiosities with the WCAG math is the addition of a 0.05 "flare" component to both sides of the simple contrast equation: (L1 + 0.05) / (L2 + 0.05) This appears to do little except distort contrast by marginally pushing ratios slightly toward 1:1 (if you eliminate the L luminance terms, you get 0.05/0.05 = 1:1).

It is not a particularly useful way to boost contrast or limit low contrast. While it does help to slightly enhance contrast in dark color pairs, dark color pairs is where the overall WCAG still fails most, especially in high ambient lighting conditions. Indeed this math generates some contrast figures that are ultimately illegible in real world conditions. (Paradoxically, the math also rejects some color pairs that seem perfectly legible).

Hwang-Peli's Modified Weber adds the flare component to only one side, the denominator, as:
   (Llight - Ldark) / (Llight + 0.05) 
In our experiments in real world conditions (including mobile devices) we've found that an offset of 0.1 is more useful than 0.05 :
   (Llight - Ldark) / (Llight + 0.05) 
I discuss this modified Weber in this post in the GitHub discussion. This method provides a superior and more robust contrast assessment than the current WCAG methods.

Advances in Perception: Our ongoing research is working with a new set of equations & algorithms that are based on human visual perception. Beyond providing a reliable and robust assessment of contrast for accessibility guidelines for graphically rich web content, we anticipate further development of the concepts will lead to better environmental responsiveness for computer display devices in general.


We are presenting a new set of articles and help files for web design, web app development, and user interface design (GUI).

Click here for our new Hue Contrast Color Chart. Use this chart as you would a color wheel — only this chart presents the 12 colors in a view matrix so you can easily judge relative color contrast. Also, this chart is adjusted for perceptual uniformity, so the colors are more like pigments in the real world instead of harsh "computer" colors.

It is important to note however, that this chart presents colors with a similar luminance — remember that luminance contrast is more important that color contrast for information. When selecting colors here, it is important to alter the luminance of at least one of them to provide adequate luminance contrast.