Thread Subject: Rationale for the contrast numbers.
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From: Gregg Vanderheiden
Date: Tue, Sep 04 2007 1:20 PM
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Here is the text from the Understanding WCAG 2.0 that deal with WCAG
provision 1.4.3 which deals with contrast.
io-contrast-contrast.html#textdef> and images of text have a contrast
io-contrast-contrast.html#contrast-ratiodef> ratio of at least 5:1, except
if the text is pure
io-contrast-contrast.html#puredecdef> decoration. Larger-scale
io-contrast-contrast.html#larger-scaledef> text or images of text have a
contrast ratio of at least 3:1.
Intent of this Success Criterion
The intent of this success criterion is to provide enough contrast between
text and its background so that it can be read by people with moderately low
vision (who do not use contrast-enhancing assistive technology). For people
without color deficiencies, hue and saturation have minimal or no effect on
legibility as assessed by reading performance (Knoblauch et al., 1991).
Color deficiencies can affect luminance contrast somewhat. However, in the
recommendation, the contrast is calculated in such a way that color is not a
key factor so that people who have a color vision deficit will also have
adequate contrast between the text and the background. Text that is
decorative and conveys no information is excluded. For example, if random
words are used to create a background and the words could be rearranged or
substituted without changing meaning, then it would be decorative and would
not need to meet this criterion.
Text that is larger and has wider character strokes is easier to read at
lower contrast. The contrast requirements for larger text is therefore
lower. This allows authors to use a wider range of color choices for large
text, which is helpful for design of pages, particularly titles. 18 point
text or 14 point bold text is judged to be large enough to require a lower
contrast ratio. "18 point" and "bold" can both have different meanings in
different fonts but, except for very thin or unusual fonts, they should be
sufficient. Since there are so many different fonts, the general measures
are used and a note regarding fancy or thin fonts is included.
The previously-mentioned contrast requirements for text also apply to images
of text (text that has been rendered into pixels and then stored in an image
format) as stated in success criterion 1.4.3.
This requirement applies to situations in which images of text were intended
to be understood as text. Incidental text, such as in photographs that
happen to include a street sign, are not included. Stylized text, such as in
corporate logos, should be treated in terms of its function on the page,
which may or may not warrant including the content in the text alternative.
Rationale for the Ratios Chosen
A contrast ratio of 3:1 is the minimum level recommended by [ISO-9241-3]
html#ISO-9241-3> and [ANSI-HFES-100-1988]
html#ANSI-HFES-100-1988> for standard text and vision. The 5:1 ratio is
used in this provision to account for the loss in contrast that results from
moderately low visual acuity, congenital or acquired color deficiencies, or
the loss of contrast sensitivity that typically accompanies aging.
The rationale is that loss of logarithm of visual acuity is generally
linearly related to loss of logarithm of contrast sensitivity, in people
with low vision such that the user with 20/40 visual acuity would need
roughly 4.5:1 contrast to have the equivalent of the 3:1 minimum contrast
standard for normal vision [ARDITI-FAYE]
html#ARDITI-FAYE> . The user with 20/47 visual acuity would require contrast
of about 5:1, and the user with 20/80 visual acuity would require contrast
of about 7:1.
Hues are perceived differently by users with color vision deficiencies (both
congenital and acquired) resulting in different colors and relative
luminance contrasts than for normally sighted users. Because of this,
effective contrast and readability are different for this population.
However, color deficiencies are so diverse that prescribing effective
general use color pairs (for contrast) based on quantitative data is not
feasible. Requiring good luminance contrast accommodates this by requiring
contrast that is independent of color perception. Fortunately, most of the
luminance contribution is from the mid and long wave receptors which largely
overlap in their spectral responses. The result is that effective luminance
contrast can generally be computed without regard to specific color
deficiency, except for the use of predominantly long wavelength colors
against darker colors (generally appearing black) for those who have
protanopia. (We provide an advisory technique on avoiding red on black for
that reason). For more information see [ARDITI-KNOBLAUCH]
The contrast ratio of 5:1 was chosen for level AA because it compensated for
the loss in contrast sensitivity usually experienced by users with vision
loss equivalent to approximately 20/40 vision. (20/40 calculates to
approximately 4.5:1 which is rounded up to 5 providing a slight additional
increase in contrast.) 20/40 is commonly reported as typical visual acuity
of elders at roughly age 80. [GITTINGS-FOZARD]
The contrast ratio of 7:1 was chosen for level AAA because it compensated
for the loss in contrast sensitivity usually experienced by users with
vision loss equivalent to approximately 20/80 vision. People with more than
this degree of vision loss usually use assistive technologies to access
their content (and the assistive technologies usually have contrast
enhancing, as well as magnification capability built into them). The 7:1
level therefore generally provides compensation for the loss in contrast
sensitivity experienced by users with low vision who do not use assistive
technology and provides contrast enhancement for color deficiency as well.
Note: Calculations in [ISO-9241-3]
html#ISO-9241-3> and [ANSI-HFES-100-1988]
html#ANSI-HFES-100-1988> are for body text. A relaxed contrast ratio is
provided for text that is much larger.
Notes on formula
Conversion from nonlinear to linear RGB values is based on IEC/4WD 61966-2-1
html#IEC-4WD> and on "A Standard Default Color Space for the Internet -
The formula (L1/L2) for contrast is based on [ISO-9241-3]
html#ISO-9241-3> and [ANSI-HFES-100-1988]
The ANSI/HFS 100-1988 standard calls for the contribution from ambient light
to be included in the calculation of L1 and L2. The .05 value used is based
on Typical Viewing Flare from [IEC-4WD]
html#IEC-4WD> and the [sRGB]
html#sRGB> paper by M. Stokes et al.
This success criterion and its definitions use the terms "contrast ratio"
and "relative luminance" rather than "luminance" to reflect the fact that
Web content does not emit light itself. The contrast ratio gives a measure
of the relative luminance that would result when displayed. (Because it is a
ratio, it is dimensionless.)
Note: Refer to related
dio-contrast-contrast-resources-head> resources for a list of tools that
utilize the contrast ratio to analyze the contrast of Web content.
Specific Benefits of Success Criterion 1.4.3:
* People with low vision often have difficulty reading text that
does not contrast with its background. This can be exacerbated if the person
has a color vision deficiency that lowers the contrast even further.
Providing a minimum luminance contrast ratio between the text and its
background can make the text more readable even if the person does not see
the full range of colors. It also works for the rare individuals who see no
Examples of Success Criterion 1.4.3
Resources are for information purposes only, no endorsement implied.
* Contrast Analyser
<http://www.wat-c.org/tools/CCA/LCRA/index.html> - Application
Analyser - online service
hp> Contrast Analyser - Firefox Extension
* Color Contrast Samples
* Atypical colour
* Colors On the Web Color
<http://www.colorsontheweb.com/colorcontrast.asp> Contrast Analyzer
REFERENCES USED ABOVE
ANSI/HFS 100-1988, American National Standard for Human Factors Engineering
of Visual Display Terminal Workstations, Section 6, pp. 17-20.
Arditi, A. (2002). Effective color contrast: designing for people with
partial sight and color deficiencies. New York, Arlene R. Gordon Research
Institute, Lighthouse International. Also available at
Arditi, A. and Faye, E. (2004). Monocular and binocular letter contrast
sensitivity and letter acuity in a diverse ophthalmologic practice.
Supplement to Optometry and Vision Science, 81 (12S), 287.
Arditi, A. and Knoblauch, K. (1994). Choosing effective display colors for
the partially-sighted. Society for Information Display International
Symposium Digest of Technical Papers, 25, 32-35.
Arditi, A. and Knoblauch, K. (1996). Effective color contrast and low
vision. In B. Rosenthal and R. Cole (Eds.) Functional Assessment of Low
Vision. St. Louis, Mosby, 129-135.
Experts Issue Recommendations to Protect Public from Seizures Induced by TV
/ Videogames. A copy of the standard is available at
Gittings, NS and Fozard, JL (1986). Age related changes in visual acuity.
Experimental Gerontology, 21(4-5), 423-433.
IEC/4WD 61966-2-1: Colour Measurement and Management in Multimedia Systems
and Equipment - Part 2.1: Default Colour Space - sRGB. May 5, 1998.
ISO 9241-3, Ergonomic requirements for office work with visual display
terminals (VDTs) - Part 3: Visual display requirements. Amendment 1.
Knoblauch, K., Arditi, A. and Szlyk, J. (1991). Effects of chromatic and
luminance contrast on reading. Journal of the Optical Society of America A,
"A Standard Default Color Space for the Internet - sRGB," M. Stokes, M.
Anderson, S. Chandrasekar, R. Motta, eds., Version 1.10, November 5, 1996. A
copy of this paper is available at
Gregg C Vanderheiden Ph.D.
Professor - Depts of Ind. Engr. & BioMed Engr.
Director - Trace R & D Center
University of Wisconsin-Madison
< <http://trace.wisc.edu/> http://trace.wisc.edu/> FAX 608/262-8848
DSS Player at http://tinyurl.com/dho6b
If Attachement is a mail.dat try <http://www.kopf.com.br/winmail/>
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