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RE: avoid "color contrast"

From: Andrew Kirkpatrick <akirkpat@adobe.com>
Date: Mon, 17 Mar 2014 15:31:06 +0000
To: CAE-Vanderhe <gregg@raisingthefloor.org>, Shawn Henry <shawn@w3.org>, "Eric Eggert" <ee@w3.org>, "EO 'w3c-wai-eo@w3.org'" <w3c-wai-eo@w3.org>, "Joshue O Connor" <joshue.oconnor@cfit.ie>
Message-ID: <b22f7712560b4fdd9c3c9d0c3a757d16@BY2PR02MB171.namprd02.prod.outlook.com>

   There are a number of reasons for not using the term Color Contrast.   Some of them are

  1.  For WCAG - the success criteria do not use "color contrast" but rather use "contrast"  or "contrast ratio" - and the definitions make it clear this is "luminosity contrast".   Changing the requirement to any other type of contrast in the WCAG support documents, is changing the SC - which can not (should not ) be done.
The definitions do make it clear that this is "luminance contrast".
I do disagree with your point that changing any of the words in the supporting documents changes the SC.
So, I wrote to Aries Arditi about this topic (he is a vision scientist formerly from Lighthouse) to ask him his thoughts.  His answer convinces me that this area is rather difficult to deal with and is not so simple.
My objection to "luminance contrast" is that this phrase does not appear in WCAG.  I completely agree that we are determining the contrast level between two colors based on luminance, but I don't think that using a term that isn't defined explicitly and isn't part of the regular language of our readers is helpful.  Of course, we don't define or use "color contrast" either, but this at least is familiar to people and every time I've ever used the phrase "color contrast" they indicate that they understand what the idea is and then ask "how do we measure that?".  I'm ok sticking to the "contrast ratio" language that we do have in WCAG when we talk about this, but I do have the concern about saying that "color contrast is incorrect", as based on Dr. Arditi's comments it isn't correct, it just isn't specific enough.
I suggest a note like this for your http://www.w3.org/WAI/eval/preliminary#contrast document:
(This accessibility requirement is sometimes called "color contrast" but this is a general term - to be more specific what is measured is ratio of the luminance of the two colors. On this page we use "contrast ratio" because that is the phrase used in the WCAG 2.0 document.) There is much more to know about contrast; we've just introduced the basics here.

  1.  Luminosity Contrast is not the same as color contrast.
  2.  Color Contrast is poorly defined - and there are several different definitions of what it is.   And none of them are what is specified in WCAG.
  3.  Even the definition of color is obscure.   The same color (hue) with different darkness/lightness is considered the same color by some and different colors by others.       And   For those who say they are the same color (hue)   Red and Pink would have not color contrast but would have luminosity contrast and would be pass WCAG just fine.       So do  light gray and dark gray.      And contrasting colors may be the same for people with some types of color perception differences.   (again there is argument between people with different definitions of color  and of color contrast).
  4.  WCAG used luminosity because it works for color deficiencies that cause different colors to become equal.  Color difference or contrast differs among users.     Even with WCAG luminosity - the value can be different visually than for people with full color vision . But the VALUES chosen in WCAG were chosen so that those with color perception differences would still have the same contrast or better than is recommended for people with no color deficiency.

Most of the discussion above is why WCAG chose luminosity contrast ratio rather than another measure for contrast.   However once WCAG did choose this-  we should not be talking about other types of contrast in our documents - or using 'color contrast' as a synonym for luminosity contrast - and confusing people I would not think.


On Mar 13, 2014, at 3:12 AM, Joshue O Connor <joshue.oconnor@cfit.ie<mailto:joshue.oconnor@cfit.ie>> wrote:

Hi Shawn and all,

Actually, *the wording used throughout those WCAG pages* is "contrast
ratio" and "luminance contrast". "Color contrast" is only used once in
the main text, and twice in the future techniques -- which EOWG assumed
was just an oversight.

In conclusion, EOWG's comment to WCAG WG was to correct the couple of
places where "color contrast" seemed to be a mistake. :)

Ok, good stuff. Thanks for putting the issue on our radar and I look forward to working it out.



On 3/12/2014 3:55 PM, Joshue O Connor wrote:

Hi Shawn,

Please avoid the phrase "color contrast".

<chair hat off>
I'm happy to discuss this but it could be a big ask - essentially you
are technically right but there is a common vernacular amongst
developers and the term 'colour contrast' is well established as 'it'
- largely based on the language traditionally used by WCAG it's worth

Also it's worth noting that even if light intensity is measured in
lumens, and variations within RGB values represent (on screen) what we
know as colour - as a 'cowpath' the term 'colour contrast' already has
a lot of traction and common understanding so to switch now - or
attempt to switch could create more dissonance than it is worth.

My 2 cents


Some places we've found it:


Additionally, several pages include "color contrast" in the Resources
listings. We wonder if it would be worthwhile to add a note there, e.g.,
something along the lines of what we have at
<http://www.w3.org/WAI/eval/preliminary#contrast>: This accessibility
requirement is sometimes called sufficient "color contrast"; however,
that is incorrect - technically it's "luminance contrast"...

Shawn for EOWG

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