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A. Arditi, G.C. Vanderheiden; Proposal for an Accessible Color Contrast Standard for Text on the World Wide Web . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2331.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: People with low vision constitute a large portion of the disabled community who must be afforded reasonable access to the web, under legislation including the Americans with Disabilities Act, and Section 508 of the Rehabilitation Act. The World Wide Web Consortium (W3C) and other groups publish recommendations for improving web accessible, but, due to variability in color display hardware, viewing conditions, and users’ color perception, are at a loss when it comes to providing a simple, economically feasible and sensible recommendation that can help web authors select accessible colors. Methods: Consistent with a substantial literature, we began with the simplifying assumption that luminance contrast alone determines accessibility of color contrasts. While based on this assumption, only black on white or vice versa result in maximum accessibility, designers need more flexibility so they can use color for semantic and aesthetic reasons. We then surveyed and evaluated all of the popular algorithms for translating display colors to gray values, and the contrast literature to determine a reasonable contrast value above which accessibility could reasonably be assured. Finally, since contrast is a ratio, we devised a means by which high contrasts with very low lightnesses (e.g. RGB(0.01,0,0) vs. RGB(0,0,0)) could be eliminated. Results: We selected the International Telecommunications Union recommendation BT.709 for high–definition television, which contains an algorithm that yields reasonable luminance values for a range of display monitor technologies and very good international consensus for RGB to grayscale conversion. It specifies lightness values as 0.222*R + 0.707*G + 0.071*B. Log letter acuity declines with log contrast with a slope of roughly –0.5, letter acuity will fall by 2 lines (or .2 log unit) on an acuity chart when contrast is reduced (from its maximum of ∼ 1) by .4 log unit. Assuming this acuity reduction is a tolerable cost to pay for the benefit of designers’ flexibility, this suggests a Weber contrast of 40% as an appropriate recommendation. To prohibit high contrasts with low lightnesses, which can be very difficult to discriminate, we suggest requiring gray values to differ by at least half their range, i.e. 128 for an 8 bit range. Conclusions: We propose as an accessibility recommendation to web authors that gray values, as computed above, of foreground and background colors of text differ by at least half their range, and have a Weber contrast of at least 40%.
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