WebAIM - Web Accessibility In Mind

E-mail List Archives

Re: How were the WCAG contrast ratios determined

for

From: Jared Smith
Date: Feb 11, 2016 8:10PM

Sean Curtis wrote:
> Does anyone have any historical information about how the ratios for A, AA,
> and AAA were determined? Were they based on the contrast of screens, or on
> how light and vision works?

The formulas were derived from the sRGB color space proposal document
[1] and IEC/4WD 61966-2-1 (couldn't find a copy of this). These were
both authored in 1996.

Think for a moment about the screen you were using in 1996. There's no
doubt that modern screens are brighter, have higher contrast, better
color resolution, and MUCH higher pixel resolution. All of these
things can decrease the impact on readability of lower luminosity
contrast as derived from RGB color values.

We regularly conduct an informal poll as part of our trainings by
showing various colors on screen and ask participants to guess which
pass and fail WCAG. Almost universally folks choose several colors
that fail as being the ones that pass.

To help get some measurable data on the actual user impacts (with a
focus on users with low vision and color blindness) of color
combinations (and font sizes, faces, etc.), WebAIM will be conducting
some research on this topic in the near future to see, in part, how
well the WCAG contrast formula thresholds relate to actual
accessibility on modern devices.

Jared

[1] https://www.w3.org/Graphics/Color/sRGB.html