By Sarah Horton and David Sloan — Some people have disabilities that affect the visual channel and are not fully correctable through glasses or contact lenses. The impact may range from loss of functional vision to reduced visual acuity, field of vision, or color perception.
Low vision and sight loss can be caused by many different conditions, including eye disease such as macular degeneration, retinitis pigmentosa, and cataracts, and conditions such as diabetes, albinism, brain injury, and cancer. People with a specific condition may experience different combinations and severity of vision loss. Some people with low vision may experience a gradual loss of sight that may lead to blindness; others may have a specific, stable condition that does not change over time.
People who are blind have little to no functional vision. The primary user accessibility need for people who are blind is information that is available through the audio channel. Some people who are blind use braille or other tactile representations of content, so an additional user accessibility need is the ability to perceive content in tactile form in addition to or as an alternative to auditory output. Many blind people do not read braille, although finding reliable data on the percentage of people who are braille literate is challenging.
Accessibility needs of people who are blind center around the ability to receive all content in non‑visual format, most importantly through audio and tactile channels. This includes the ability to access:
- Equivalent alternatives to information provided in images.
- Equivalent alternatives to information provided through visual characteristics such as color, shape, and size of text and objects.
- Information about the structure of page content and relationships between pieces of content, necessary to support understanding the content.
- Information about each interactive user interface control, including its name, type of control, and current state or value.
- Notifications indicating changes to content or presence of errors.
People with low vision have functional vision that is impaired in some way. In some regions, such as the UK, this group may be referred to as partially sighted. Types of vision impairment include:
- Reduced visual acuity, where the ability to distinguish detail is significantly reduced. This may affect near (close) or far (long‑distance) vision, and in more significant cases may make it difficult to distinguish text or graphics.
- Reduced field of vision, where someone may have difficulty or be unable to see specific areas of the field of vision. For example, people with the condition often referred to as tunnel vision may only be able to see the center of the field of vision.
- Reduced sensitivity to light, especially bright light.
- Reduced sensitivity to contrast, affecting the ability to distinguish similar hues.
Reduced color vision or color deficit refers to a person’s difficulty or inability to distinguish specific pairs of colors. This condition is often also referred to, less accurately, as color blindness. Common instances include:
- Protanomaly and protanopia, respectively, the difficulty and the inability to perceive red light.
- Deuteranomaly and deuteranopia, respectively, the difficulty and the inability to perceive green light.
- Tritanomaly and tritanopia, respectively, the difficulty and the inability to perceive blue light.
Protanomaly, protanopia, deuteranomaly, and deuteranopia can make it difficult or impossible for someone to distinguish red and green hues; red and green colors may appear to have identical hues, such as dark green or brown. Tritanomaly and tritanopia can mean that blues and yellows appear similar or identical. The condition of monochromacy is the inability to perceive any color at all, and compared to conditions that affect the perception of specific colors, it is extremely uncommon.
Accessibility needs of people with low vision and color deficits vary depending on the nature of the impairment. Many accessibility needs of people with low vision and color deficits focus on the ability to change the appearance of an interface, including the ability to perform one or more of the following:
- Magnify or zoom in on all content, including text and images.
- Increase the text size.
- Customize the display of indications of input location, such as keyboard focus, pointers, and cursors.
- Reflow content into a single column in order to make it easier to scan.
- Change the text and background color schemes.
- Change the typeface.
- Change the text and line spacing.
- Reduce the brightness of a screen.
- Change the contrast of hues—for some people, to increase contrast, and for others, to reduce contrast.
Another low vision accessibility and color deficit user need is the ability to perceive information without needing to distinguish hue, brightness, and other visual characteristics of content.
From Horton, S., & Sloan, D. (2024). What Every Engineer Should Know About Digital Accessibility. CRC Press, Boca Raton, Florida.
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