Visual impairment represents a continuum, from people with very poor vision, to people who can see light but no shapes, to people who have no perception of light at all. However, for general discussion it is useful to think of this population as representing two broad groups: those with low vision and those who are legally blind. There are an estimated 8.6 million people with visual impairments (3.4% of the U.S. population). In the elderly population the percentage of persons with visual impairments is very high.
A person is termed legally blind when their visual acuity (sharpness of vision) is 20/200 or worse after correction, or when their field of vision is less than 20 degrees in the best eye after correction. There are approximately 580,000 people in the US who are legally blind.Low vision includes problems (after correction) such as dimness of vision, haziness, film over the eye, foggy vision, extreme near-or farsightedness, distortion of vision, spots before the eyes, color distortions, visual field defects, tunnel vision, no peripheral vision, abnormal sensitivity to light or glare, and night blindness. There are approximately 1.8 million people in the US with severe visual impairments who are not legally blind.
Many diseases causing severe visual impairments are common in those who are aging (glaucoma, cataracts, macular degeneration, and diabetic retinopathy). With current demographic trends toward a larger proportion of elderly, the incidence of visual impairments will certainly increase.
Those who are legally blind may still retain some perception of shape and contrast or of light vs. dark (the ability to locate a light source), or they may be totally blind (having no awareness of environmental light).
Those with visual impairments have the most difficulty with visual displays and other visual output (e.g., hazard warnings). In addition, there are problems in utilizing controls where labeling or actual operation is dependent on vision (e.g., where eye-hand coordination is required, as with a computer "mouse"). Written operating instructions and other documentation may be unusable, and there can be difficulties in manipulation (e.g., insertion/placement, assembly).
Because many people with visual impairments still have some visual capability, many of them can read with the assistance of magnifiers, bright lighting and glare reducers. Many such people with low vision are helped immensely by use of larger lettering, sans-serif typefaces, and high contrast coloring.
Those with color blindness may have difficulty differentiating between certain color pairs. This generally doesn't pose much of a problem except in those instances when information is color coded or where color pairs are chosen which result in poor figure ground contrast.
Key coping strategies for people with more severe visual impairments include the use of braille and large raised lettering. Note, however, that Braille is preferred by only 10% of blind people (normally those blind from early in life). Raised lettering must be large and is therefore better for indicating simple labels than for extensive text.