Legal blindness is a specific designation created by law, not an absolute medical diagnosis of total darkness. It serves as a regulatory standard used primarily by the Social Security Administration (SSA) to determine eligibility for disability benefits, vocational training, and other assistance programs in the United States. This definition is based on measurable metrics of visual function, which are distinct from the functional experience of daily vision. The criteria establish a threshold of visual impairment that qualifies an individual for legal support, even if they retain some usable sight.
Defining Visual Acuity and Visual Field
The definition of legal blindness hinges on two measurable components: visual acuity and visual field. You are considered legally blind if your better eye meets either standard, even with the best possible correction from glasses or contact lenses. The first standard is a reduced central visual acuity of 20/200 or less. Visual acuity measures the clarity of central vision, typically tested using the Snellen eye chart. A 20/200 measurement means that the smallest letters you can clearly see from 20 feet away are letters a person with normal vision could see clearly from 200 feet away.
The second criterion involves a significant limitation in peripheral vision, known as the visual field. The visual field is the entire area you can see without moving your eyes. If the widest diameter of your visual field in your better eye is restricted to an angle no greater than 20 degrees, you are considered legally blind, regardless of your central visual acuity. This severe restriction is often described as “tunnel vision” because it limits sight to a very small central area.
The Diagnostic Steps
Determining if a person meets the legal criteria requires specialized examinations performed by eye care professionals, such as ophthalmologists or optometrists. The process begins with a comprehensive eye exam to determine the best corrected vision. This involves a refraction test to find the strongest lens prescription that yields the clearest sight, ensuring the final measurement is taken with the “best conventional correction.”
Visual acuity is measured using a standardized chart, such as the Snellen chart or the Early Treatment Diabetic Retinopathy Study (ETDRS) chart. The professional observes the smallest line of letters the patient can read from a set distance, often 20 feet.
The extent of the visual field is measured through a procedure called perimetry testing. During this test, the patient looks into a bowl-shaped instrument, called a perimeter, and focuses on a central target. Small, dim lights appear in various locations, and the patient indicates when they see them. The results create a map that shows the total area of sight, allowing the professional to confirm if the widest angle is 20 degrees or less.
Medical Conditions That Can Lead to Legal Blindness
A variety of underlying eye diseases can cause permanent, severe vision loss that meets the legal definition. Age-related macular degeneration (AMD) is a common cause, as it damages the macula, the small area of the retina responsible for sharp, central vision. This damage often results in significant loss of visual acuity, making it difficult to read or recognize faces. Diabetic retinopathy, a complication of diabetes, occurs when high blood sugar damages the blood vessels in the retina, leading to blurred vision, blind spots, and severe vision loss if left untreated.
Glaucoma is a group of conditions that damage the optic nerve, often due to increased pressure within the eye. This damage causes a gradual loss of peripheral vision, making it a common cause of legal blindness due to a restricted visual field of 20 degrees or less. Retinitis pigmentosa is another condition that causes progressive retinal damage and often results in severe “tunnel vision.” Cataracts can also lead to legal blindness if they become extremely severe and are left uncorrected, causing the lens to become so cloudy that vision is profoundly blurred.