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Definitions Visual impairment: a functional limitation in seeing, including both those with:
Legal blindness refers to central visual acuity of 20/200 or less in the better eye with the best possible correction, as measured on a Snellen vision chart, or a visual field of 20 degrees or less. Vision correctable to 20/20 with at least 180-degree field is considered 'normal vision'. A person who is legally blind sees at approximately 20 feet what a person with 20/20 vision sees at 200 feet, or is able to see no more than a 20-degree field without scanning. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Types of vision loss General types of vision loss: Reduced acuity can refer to a large range of functional vision from vision tested as 20/20 to totally blind. Lighting and contrast affect functional vision and are not reflected in the clinical measurements. The general category of restricted fields can be further divided into central field loss and peripheral field loss. Reduced acuity The picture below represents a street crossing as it might be seen by a person with general reduced visual acuity. An overall loss of acuity, sensitivity to glare, and loss of contrast sensitivity is common in the elderly population.
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Central field loss Individuals with a central field loss usually will have difficulty seeing pedestrian signals, some signs, and details directly in front of them. Central field loss is typical of macular degeneration, the leading cause of blindness in those over 60.
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Peripheral field loss Individuals with peripheral field loss, sometimes referred to as tunnel vision, may see details directly in front of them clearly, but have difficulty with objects and signs off the side. In addition, depth perception is often impaired. Glaucoma and retinitis pigmentosa are the main causes of peripheral field loss.
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Decrease in attentional field Research by Brabyn, Haegerstr�m-Portnoy, Schneck, and Lott (2000) demonstrated that over age 60-65 the prevalence of problems detecting objects in the peripheral visual field increases dramatically. This is known as a decrease in attentional field, and it may be present with or without other types of visual impairment. By age 90, 40% of people have an attentional field of less than 10 degrees left and right. Thus, if they are looking at a pedhead, they are unlikely to be visually aware of vehicles that may be disobeying the signal, or turning across their path of travel, until it is too late to take appropriate action. Total blindness or light perception Individuals who are considered totally blind usually cannot see any difference in light and dark. Individuals who have light perception may be able to tell if it is dark or light and the direction of a bright light source, but do not have vision that is useable for discerning objects or the travel path. Prevalence of blindness Some degree of vision impairment affects 8.3 million (3.1%) Americans of all ages (Adams, Hendershot, & Marano, 1999). Approximately 3% of individuals age 6 and older, representing 7.9 million people, have difficulty seeing words and letters in ordinary newspaper print even when wearing glasses or contact lenses. This number increases to 12% among persons age 65 and older (3.9 million) (McNeil, 2001). Approximately 1.3 million Americans are legally blind. By 2010, projections are that there will be 20 million visually impaired persons over 45. Area of residence Most persons who have a vision impairment live in metropolitan areas (70%), but they are less likely to live in metropolitan areas than are persons without visual impairments (78%) (Schmeidler & Halfmann, 1998b; based on 1994 NHIS-D). 33% live in cities, 37% live in suburbs, 28% live in non-metropolitan areas (e.g., small towns) and 1% live in farm areas (Schmeidler & Halfmann, 1998b). In comparison to the general population, persons who are visually impaired are over-represented in cities and non-metropolitan areas and somewhat under-represented in the suburbs (i.e., 48% of general population live in suburbs) (Schmeidler & Halfmann, 1998b). This information is included here to clarify that people who are visually impaired or blind do not cluster in the same area of town or same type of area. top |
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