Accessibility for Visually Impaired

Color Blindness

Color blindness is a lack of sensitivity to certain colors. Common forms of color blindness include difficulty distinguishing between red and green, or between yellow and blue. Sometimes color blindness results in the inability to perceive any color.

To use the Web, some people with color blindness use their own style sheets to override the font and background color choices of the author.

Barriers that people with color blindness may encounter on the Web can include:

• color that is used as a unique marker to emphasize text on a Web site
• text that inadequately contrasts with background color or patterns
• browsers that do not support user override of authors' style sheets

Blindness

Blindness involves a substantial, uncorrectable loss of vision in both eyes.

To access the Web, many individuals who are blind rely on screen readers -- software that reads text on the screen (monitor) and outputs this information to a speech synthesizer and/or refreshable braille display. Some people who are blind use text-based browsers such as Lynx, or voice browsers, instead of a graphical user interface browser plus screen reader. They may use rapid navigation strategies such as tabbing through the headings or links on Web pages rather than reading every word on the page in sequence.

Examples of barriers that people with blindness may encounter on the Web can include:

• images that do not have alternative text
• complex images (e.g., graphs or charts) that are not adequately described
• video that is not described in text or audio
• tables that do not make sense when read serially (in a cell-by-cell or "linearized" mode)
• frames that do not have "NOFRAME" alternatives, or that do not have meaningful names
• forms that cannot be tabbed through in a logical sequence or that are poorly labelled
• browsers and authoring tools that lack keyboard support for all commands
• browsers and authoring tools that do not use standard applications programmer interfaces for the operating system they are based in
• non-standard document formats that may be difficult for their screen reader to interpret

Low Vision

There are many types of low vision (also known as "partially sighted" in parts of Europe), for instance poor acuity (vision that is not sharp), tunnel vision (seeing only the middle of the visual field), central field loss (seeing only the edges of the visual field), and clouded vision.

To use the Web, some people with low vision use extra-large monitors, and increase the size of system fonts and images. Others use screen magnifiers or screen enhancement software. Some individuals use specific combinations of text and background colors, such as a 24-point bright yellow font on a black background, or choose certain typefaces that are especially legible for their particular vision requirements.

Barriers that people with low vision may encounter on the Web can include:

• Web pages with absolute font sizes that do not change (enlarge or reduce) easily
• Web pages that, because of inconsistent layout, are difficult to navigate when enlarged, due to loss of surrounding context
• Web pages, or images on Web pages, that have poor contrast, and whose contrast cannot be easily changed through user override of author style sheets
• text presented as images, which prevents wrapping to the next line when enlarged
• also many of the barriers listed for blindness, above, depending on the type and extent of visual limitation