Contrast (Minimum)
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Understanding SC 1.4.3
1.4.3 Contrast (Minimum): Text and images of text have a contrast ratio of at least 5:1, except for the following: (Level AA)
Large Print: Large scale text and large-scale images of text have a contrast ratio of at least 3:1;
Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are incidental text in an image, or that are not visible to anyone, have no minimum contrast requirement.
[begin delete][end delete]Note: Success Criteria 1.4.3 and 1.4.6 can be met via a contrast control available on or from the page. [2434]
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. Therefore, 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. Nor is text that for some reason is designed to be invisible to all viewers. 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.
Note 1: Some people with cognitive disabilities require color combinations or hues that have low contrast, and therefore we allow and encourage authors to provide mechanisms to adjust the foreground and background colors of the content. Some of the combinations that could be chosen may have contrast levels that will be lower than those found in the Success Criteria. This is not a violation of this Success Criteria provided there is a mechanism that will return to the default values set out in the Success Criteria.
Note 2: Images of text do not scale as well as text because they tend to pixelate. It is also harder to change foreground and background contrast and color combinations for images of text, which is necessary for some users. Therefore, we suggest using text wherever possible, and when not, consider supplying an image of higher resolution.
Although this Success Criterion only applies to text, similar issues occur for data presented in charts or graphs. Good color contrast should also be provided for data presented in these forms.
Rationale for the Ratios Chosen
A contrast ratio of 3:1 is the minimum level recommended by [ISO-9241-3] and [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]. 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] [ARDITI-KNOBLAUCH-1996] [ARDITI].
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] and [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 [IEC-4WD] and on "A Standard Default Color Space for the Internet - sRGB" [sRGB].
The formula (L1/L2) for contrast is based on [ISO-9241-3] and [ANSI-HFES-100-1988] standards.
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] and the [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 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:
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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 color.
Examples of Success Criterion 1.4.3
Related Resources
Resources are for information purposes only, no endorsement implied.
Techniques and Failures for Success Criterion 1.4.3 - Contrast (Minimum)
Each numbered item in this section represents a technique or combination of techniques that the WCAG Working Group deems sufficient for meeting this Success Criterion. The techniques listed only satisfy the Success Criterion if all of the WCAG 2.0 conformance requirements have been met.
Sufficient Techniques
Instructions: Select the situation below that matches your content. Each situation includes techniques or combinations of techniques that are known and documented to be sufficient for that situation.
Additional Techniques (Advisory) for 1.4.3
Although not required for conformance, the following additional techniques should be considered in order to make content more accessible. Not all techniques can be used or would be effective in all situations.
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Using a higher contrast value for text that is over a patterned background (future link)
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Using a light pastel background rather than a white background behind black text (future link)
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Using Unicode text and style sheets instead of images of text (future link)
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Using a higher contrast values for lines in diagrams (future link)
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Using greater contrast level for red-black text/background combinations
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Using colors that are composed predominantly of mid spectral components for the light and spectral extremes (blue and red wavelengths) for the dark
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Using a light pastel background rather than a white background behind black text to create sufficient but not extreme contrast (future link)
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Making icons using simple line drawings that meet the contrast provisions for text (future link)
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Providing sufficient color contrast in graphs and charts (future link)
Common Failures for SC 1.4.3
The following are common mistakes that are considered failures of Success Criterion 1.4.3 by the WCAG Working Group.
Key Terms
- contrast ratio
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(L1 + 0.05) / (L2 + 0.05), where
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L1 is the relative luminance of the lighter of the [begin delete] foreground or background [end delete] colors, and
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L2 is the relative luminanceof the darker of the [begin delete] foreground or background [end delete] colors. [2548]
Note 1: Contrast ratios can range from 1 to 21 (commonly written 1:1 to 21:1).
[begin delete]Note 2: For colors that are dithered at the source, use the average values of the colors that are dithered (average R, average G, and average B). If dithered at destination, then the source value is used.
[end delete][begin change]Note 3: Because authors do not have control over user settings as to how text is rendered (for example font smoothing or anti-aliasing), the contrast ratio for text can be evaluated with anti-aliasing turned off.
[end change][begin change]Note 4: For the purpose of Success Criteria 1.4.3 and 1.4.6, contrast is measured with respect to the specified background over which the text is rendered in normal usage. If no background color is specified, then white is assumed. [2548]
[end change][begin change]Note 5: Background color is the specified color of content over which the text is to be rendered in normal usage. It is a failure if no background color is specified when the text color is specified, because the user's default background color is unknown and cannot be evaluated for sufficient contrast. For the same reason, it is a failure if no text color is specified when a background color is specified. [2548]
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- incidental text in an image
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text that is inconsequential to the meaning of the image
Example: In a photo of two men talking on a street corner, there is a sign on a store in the background.
- large scale (text)
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with at least 18 point or 14 point bold or font size that would yield equivalent stroke width for Chinese, Japanese and Korean (CJK) fonts
Note 1: Fonts with extraordinarily thin strokes or unusual features and characteristics that reduce the familiarity of their letter forms are harder to read, especially at lower contrast levels.
Note 2: Font size is the size when the content is delivered. It does not include resizing that may be done by a user.
Note 3: The actual size of the character that a user sees in dependent both on the author-defined size and the users display or user-agent settings. This Success Criterion is based on common pixel sizes available today. Users who have low vision would be responsible for choosing appropriate settings.
[begin add]Note 4: When using text without specifying the font size, the font size used on major browsers for unspecified text should be used. If a level 1 heading is rendered in 14pt bold or higher on major browsers, then it would be large text. Relative scaling can be calculated from the default sizes in a similar fashion. [2430]
[end add] - pure decoration
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serving only an aesthetic purpose, providing no information, and having no functionality
Note: Text is only purely decorative if the words can be rearranged or substituted without changing their purpose.
Example: The cover page of a dictionary has random words in very light text in the background.
- text
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sequence of characters that can be programmatically determined, where the sequence is expressing something in human language
- user interface component
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a part of the content that is perceived by users as a single control for a distinct function
Note 1: Multiple user interface components may be implemented as a single programmatic element. Components here is not tied to programming techniques, but rather to what the user perceives as separate controls.
[begin add]Note 2: User interface components include form elements and links as well as components generated by scripts. [2405]
[end add]Example: An applet has a "control" that can be used to move through content by line or page or random access. Since each of these would need to have a name and be setable independently, they would each be a "user interface component."