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Luminance / Luma confusion

From: Tobias Bengfort <tobias.bengfort@posteo.de>
Date: Sun, 6 Mar 2016 10:35:16 +0100
To: w3c-wai-ig@w3.org
Message-ID: <56DBF9D4.5070807@posteo.de>

I recently filed a bug report with Less[1] because I thought they may
have confused the terms "luminance" and "luma". They use luma to
calculate the color contrast although the spec clearly states that you
have to use "relative luminance"[2].

We came to the conclusion that Less is actually right and the spec may
be wrong. From an article by Charles Poynton[3] I understand that the
term "luminance" has been used ambiguously in the past and the term
"luma" has been specifically invented to allow a clear distinction. The
wrong wording in the spec may therefore be a relic of that ambiguous past.

So am I correct in thinking that the spec actually refers to luma? If
that is the case, how can it be fixed?

I assume that the definition of color contrast was taken from another
specification, so it may not necessarily be a good idea to change the
wording there. In that case, there should at least be a note about the
luminance/luma ambiguity in the "Understanding WCAG 2.0" document[4].


[1]: https://github.com/less/less.js/issues/2829
[2]: https://www.w3.org/TR/WCAG20/#contrast-ratiodef
[3]: http://www.poynton.com/PDFs/YUV_and_luminance_harmful.pdf

Received on Sunday, 6 March 2016 09:36:00 UTC

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