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The Definition of Legal Blindness
Legal blindness is defined by two types of visual impairment – reduced visual acuity and visual field impairment. The presence of either or both of these impairments is sufficient to consider an individual legally blind.
Reduced Visual Acuity
Visual acuity is the measurement of clarity by which one sees, whereby reduced visual acuity means reduced clarity. Visual acuity is what we refer to when we say that 20/20 is “perfect” vision.
In the United States, legal blindness can be defined as a visual acuity of 20/200 or less in the better eye with the best correction. In other words, what a person with perfect vision sees from a distance of 200 feet, a person who is legally blind sees from a distance of 20 feet or less.
Common Functional Implications:
- Less than average visual clarity and detail, especially with increased distances
- Difficulty reading standard print without magnification
- Difficulty seeing/reading/identifying signs, posters, faces, videos, overheads, etc. from “normal” distances without telescopic devices
- Possible difficulty with tasks requiring color discrimination
- High contrast may be helpful
Specific Eye Conditions:
- Most common are retinal, corneal, lens and optic nerve conditions
(including: Cataracts, Diabetic Retinopathy, Glaucoma, Retinitis Pigmentosa, Optic Atrophy, Macular Degeneration, Stargardt’s Disease, Corneal Dystrophies, Progressive Myopia, Optic Neuritis, etc.)
Visual Field Impairment
Impairment of the visual field, the area viewed by one or both eyes at a time, is associated with any one of, or a combination of, the following symptoms:
Peripheral field loss: Less than 170-180 degrees of sight from side to side or top to bottom
Central field loss: Unable to see what is “straight ahead”
Scotomas: “Holes” or “blind spots” scattered throughout the visual field
Field cuts: Unable to see pieces or an entire half of the visual field in
each eye (nasal, temporal, superior, inferior, left, right)“Islands of vision”: Visual field reduced to only small areas in one or both eyes
Legal blindness can be defined by a visual field restriction, specifically, a visual field of 20 degrees or less.
Common Functional Implications:
- Less than average field of vision
- Unable to see straight ahead
- Difficult to locate best visual fields
- Limited as to strength/power of magnification usable
- May need to organize work space based on task requirements and visual field restrictions
Specific Eye Conditions:
- Peripheral field loss: Glaucoma, Retinitis Pigmentosa
- Central field loss: Macular Degeneration, Stargardt’s disease,
- Diabetic Retinopathy
- Scattered scotomas: Diabetic Retinopathy, Optic Neuritis
- Field Cuts: Stroke (Heminopsia), Optic Neuritis, Cortical Blindness, Aneurism
- “Islands of vision”: Glaucoma, Diabetic Retinopathy, Retinit is Pigmentosa
Photophobia and Glare Sensitivity
Although not a criterion of legal blindness, photophobia (light sensitivity) and glare sensitivity are common visual impairments. For people who live with the impairments, visual exposure to specific amounts or types of light (e.g. moderate to bright, natural or unnatural, “blue light”/reflective, etc.) can be, to varying degrees, uncomfortable or painful.
Functional Implications:
- Reduced visual function/ability
- Organize work environment so light comes from behind rather than from the front
- Reduce/eliminate glare by use of lamp shades, filters, window coverings, matte paper/work surfaces rather than shiny, and reducing brightness on computer monitors
- Wear light sunfilters/glasses in appropriate colors to enhance image contrast while reducing glare
- Use visors to reduce/eliminate overhead glare
- Use task lighting
Specific Eye Conditions:
- All corneal and lens conditions, including Cataracts and Corneal Dystrophies
- Most retinal conditions, including Retinitis Pigmentosa, Diabetic Retinopathy, Macular Degeneration, Macular Dystrophy, Rod Cone Dystrophy and Glaucoma
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