WebAIM - Web Accessibility In Mind

E-mail List Archives

Re: How were the WCAG contrast ratios determined

for

From: Tim Harshbarger
Date: Feb 12, 2016 7:02AM

The "Understanding" page for success criteria 1.4.3 (Contrast [Minimum]) provides rationale for the numbers:
http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html


I just reread the entry. To me, it seems like the ratios would stay the same despite changes in luminance of monitors. What might change is how the ratio is calculated.

-----Original Message-----
From: WebAIM-Forum [mailto: <EMAIL REMOVED> ] On Behalf Of Sean Curtis
Sent: Thursday, February 11, 2016 9:18 PM
To: WebAIM Discussion List
Subject: Re: [WebAIM] How were the WCAG contrast ratios determined

Thanks Jared,

The main thing that interests me is whether or not the higher contrast of
modern displays (LCD vs the old CRT displays prominent in 1996) would
affect the ratios if the formulas were derived today.


On Fri, Feb 12, 2016 at 2:10 PM, Jared Smith < <EMAIL REMOVED> > wrote:

> 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
> > > > >