Edit comment LC-580 for Accessibility Guidelines Working Group

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Comment LC-580: Comment Shortname:; Commenter: Chris Lilley <chris@w3.org> on behalf of W3C; Message-id:; Section of the document concerned:

or; Refinement of the section concerned:; Status:
assigned to; Type:; Resolution status:
Response drafted Resolution implemented Reply sent to commenter; Response status:
No response from Commenter yet Commenter approved disposition Commenter objected to disposition
Commenter's response (URI):; Comment:
Part of Item:
Comment Type: TE
Comment (including rationale for proposed change):

The term "luminosity" is incorrect here (it applies only to certain Broadcast video signals). Relative luninance is the correct term. When used as a ratio, the difference between absolute and relative Luminance can be dropped, but the term luminance rather than luminosity should be used.

http://www.w3.org/TR/2006/WD-WCAG20-20060427/appendixA.html#luminosity-contrastdef
defines the coefficients for calculating luminance, and its good to see that the Rec.709 chromaticities are used (rather than, for example, the NTSC ones which do not apply to modern computer monitors at all). (the coeeficients are correct, see the "luminanceToAlpha" section of http://www.w3.org/TR/SVG11/filters.html#feColorMatrix but the specification does not explain why this is so.

However the text as it stands implies that the formula given is universally applicable. it is not. This is particularly importnt when printing Web materials.

Proposed Change:

change "luminosity" to luminance throughout.

change "The luminosity of a color is defined as" to "the luminance of an sRGB color is defined as".

change "blue RGB values" to "blue sRGB values" (the equation does not apply to other color spaces).

Remove the exponentiation operator and the 2.2 gamma approximation. Instead, use the correct sRGB transfer curve.

Reference the sRGB specification. See the SVG 1.2Tiny specification for an example of how to reference it.

You may want to note that the equation given is only correct for typicalcolor vision (the ISO standard observer). For atypical color vision, often incorrectly termed color blindness, different equations apply depending on the type of atypical color vision and the degree of severity.
For more information, please see
http://www.w3.org/Graphics/atypical-color-response; Related issues: (space separated ids); WG Notes: [TEAMA] Discussed on the 22 June 2006 teleconference Resolution: Accept issue 580 as proposed but put on hold for overall luminosity review. http://www.w3.org/WAI/GL/2006/06/22-wai-wcag-minutes.html Discussed in the 08 March 2007 telecon: resolution: accept 580 as amended http://www.w3.org/WAI/GL/2007/03/08-wai-wcag-minutes.html DONE refer to http://www.w3.org/WAI/GL/2007/03/contrast-proposals.htm for updated SC and definitions and links to related changes to Understanding docs. Internal WD updated 23 March 2007. Discussed in the 10 May 2007 telecon: Accepted by Unanimous Consent http://www.w3.org/WAI/GL/2007/05/10-wai-wcag-minutes.html Response from commenter: http://lists.w3.org/Archives/Public/public-comments-wcag20/2007May/0126.html; Resolution: Thanks for the comments and suggestions. To take them each in turn: CL: change "luminosity" to luminance throughout. Since Web content doesn't provide any light output (HTML doesn't give off photons) we can't use the word "luminance" (which means light output). However 'relative luminance' is used in the literature for the concept we are describing and we are now using this. CL: change "The luminosity of a color is defined as" to "the luminance of an sRGB color is defined as". See above regarding luminance. And, we have switched to specifying that we are talking about sRGB in our equations. CL: change "blue RGB values" to "blue sRGB values" (the equation does not apply to other color spaces). Correct and we have done so. CL: Remove the exponentiation operator and the 2.2 gamma approximation. Instead, use the correct sRGB transfer curve. Done. Now uses the equations from the W3C document on sRGB. CL: Reference the sRGB specification. See the SVG 1.2Tiny specification for an example of how to reference it. Done CL: You may want to note that the equation given is only correct for typical color vision (the ISO standard observer). For atypical color vision, often incorrectly termed color blindness, different equations apply depending on the type of atypical color vision and the degree of severity. For more information, please see http://www.w3.org/Graphics/atypical-color-response. We have explained this briefly in our "Understanding" document. A longer exposition of this will be released in a paper (since it is too complicated to put into How to Meet SC 1.4.3 itself. The contrast ratios were set higher than normal and in a way to account for low vision and atypical color vision. The paper "Atypical colour response" has also been added as a resource. You can find the updated SC and definitions at http://www.w3.org/TR/2007/WD-WCAG20-20070517/#visual-audio-contrast-contrast and http://www.w3.org/TR/2007/WD-WCAG20-20070517/#visual-audio-contrast7 . The Understanding SC 1.4.3 and 1.4.5 documents can be found at http://www.w3.org/TR/2007/WD-UNDERSTANDING-WCAG20-20070517/#visual-audio-contrast-contrast and http://www.w3.org/TR/2007/WD-UNDERSTANDING-WCAG20-20070517/#visual-audio-contrast7 .; (Please make sure the resolution is adapted for public consumption)