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Thread: How were the WCAG contrast ratios determined

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Number of posts in this thread: 7 (In chronological order)

From: Sean Curtis
Date: Thu, Feb 11 2016 7:30PM
Subject: How were the WCAG contrast ratios determined
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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? I couldn't find any information about how these
were initially determined. Can anyone fill me in?

Cheers,

Sean

From: Maxability Accessibility for all
Date: Thu, Feb 11 2016 7:38PM
Subject: Re: How were the WCAG contrast ratios determined
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Hello Sean,

The below link is what I am refering for years when someone ask me the same
question.
http://juicystudio.com/article/luminositycontrastratioalgorithm.php
It may help you.

Thanks & Regards
Rakesh

On Fri, Feb 12, 2016 at 8:00 AM, Sean Curtis < = EMAIL ADDRESS REMOVED = > 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? I couldn't find any information about how these
> were initially determined. Can anyone fill me in?
>
> Cheers,
>
> Sean
> > > > >

From: Jared Smith
Date: Thu, Feb 11 2016 8:10PM
Subject: Re: How were the WCAG contrast ratios determined
← Previous message | Next message →

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

From: Sean Curtis
Date: Thu, Feb 11 2016 8:17PM
Subject: Re: How were the WCAG contrast ratios determined
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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 ADDRESS 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
> > > > >

From: Jonathan Avila
Date: Thu, Feb 11 2016 8:21PM
Subject: Re: How were the WCAG contrast ratios determined
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> 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.

Keep in mind there are new confounding factors such as using your mobile in daylight that also come into play. Not everyone has a new LCD monitor and my current LCD monitor isn't really any brighter than my CRT was. In my experience the Mac screens such as on the MacBook AIR have great contrast but they also introduce other issues such as glare because of their glossiness.

Jonathan

--
Jonathan Avila
Chief Accessibility Officer
SSB BART Group
= EMAIL ADDRESS REMOVED =

703-637-8957 (o)
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-----Original Message-----
From: WebAIM-Forum [mailto: = EMAIL ADDRESS REMOVED = ] On Behalf Of Sean Curtis
Sent: Thursday, February 11, 2016 10:18 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 ADDRESS 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
> > > archives at http://webaim.org/discussion/archives
> >

From: Tim Harshbarger
Date: Fri, Feb 12 2016 7:02AM
Subject: Re: How were the WCAG contrast ratios determined
← Previous message | Next message →

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 ADDRESS 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 ADDRESS 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
> > > > >

From: Patrick H. Lauke
Date: Fri, Feb 12 2016 7:14AM
Subject: Re: How were the WCAG contrast ratios determined
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On 12/02/2016 14:02, Tim Harshbarger wrote:
> 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.

From my reading, no particular display technology was actually
considered for the ratios, but rather on some form of idealised model of
color rendition.

Though I don't think anybody's suggestion this, I'd be careful if this
line of reasoning moves towards the idea that these ratios should be
measured "on the screen", so to speak. There are many factors - type of
display, color correction profiles, user-set contrast/brightness,
ambient light condition, etc - which are outside of the control of
developers, are extremely variable across even the same device
type/model, and cannot (reliably, or at all) be programmatically
detected. Even just for auditing/testing, this would very quickly result
in the same color values returning different ratios (and potentially
differences between a pass and a fail) simply based on the auditor's
current screen/setup.

As such (as with the note on large text, which has exactly the same
issues of variability in the real world) the only reliable, measurable
and stable reference to calculate ratios has to be the values "obtained
from the user agent" - and then assume the device/display render those
values in whatever they deem is the most faithful/appropriate way (and
knowing that users may still change this themselves anyway).

P
--
Patrick H. Lauke

www.splintered.co.uk | https://github.com/patrickhlauke
http://flickr.com/photos/redux/ | http://redux.deviantart.com
twitter: @patrick_h_lauke | skype: patrick_h_lauke