Legal blindness represents a specific classification of vision impairment, distinct from a complete absence of sight. This classification serves various purposes, including legal definitions, medical diagnoses, and statistical tracking of visual disability. It identifies a significant level of vision loss that can impact daily activities and often qualifies individuals for support services.
Understanding the Core Criteria
In the United States, legal blindness is formally defined by the Social Security Administration (SSA) based on two primary criteria. A person is considered legally blind if their central visual acuity is 20/200 or less in their better eye, even with the best possible corrective lenses. This means what a person with normal vision can see clearly at 200 feet, a legally blind individual can only see clearly when they are 20 feet away.
Alternatively, legal blindness can be determined by a significant restriction in the field of vision. This second criterion applies if an individual has a visual field defect where the peripheral field has narrowed to an angular distance no greater than 20 degrees in their better eye. A normal visual field typically spans around 160-180 degrees horizontally, so a 20-degree field is often described as “tunnel vision,” indicating a severe limitation of side vision. An individual meets the definition of legal blindness if they satisfy either the visual acuity or visual field criterion in their better eye.
How Visual Acuity and Field of Vision are Assessed
Eye care professionals utilize specific tests to assess visual acuity and the field of vision. Visual acuity is commonly measured using a Snellen eye chart, which features rows of letters that progressively decrease in size. During this test, the patient typically stands or sits 20 feet from the chart and attempts to read the smallest line of letters they can discern. The results are then expressed as a fraction, where the top number represents the testing distance and the bottom number indicates the distance at which a person with normal vision could read the same line.
Measuring the visual field involves perimetry tests, which map out the entire area of vision, encompassing both central and peripheral sight. These tests identify any blind spots or areas where vision sensitivity is reduced. Common types of perimetry include automated static perimetry, such as the Humphrey Field Analyzer, and kinetic perimetry, like the Goldmann perimeter. These assessments provide a detailed understanding of an individual’s remaining visual span.
Distinguishing Legal Blindness from Total Blindness
A common misconception is that legal blindness implies a complete lack of sight, but this is generally not the case. Most individuals classified as legally blind retain some degree of vision. Their remaining vision can vary significantly, ranging from the ability to perceive light and darkness to distinguishing shapes, colors, or large objects. This residual vision allows many legally blind people to navigate their environment, read with assistive devices, or perceive motion.
Total blindness, on the other hand, is a distinct and much rarer condition characterized by the complete absence of light perception. The definition of legal blindness was established to identify a level of vision impairment that significantly impacts daily life and requires adaptive strategies, rather than to denote absolute sightlessness.