The correct and official designation is “legally blind.” This is a specific medical and legal definition established by the United States government, primarily for determining eligibility for disability benefits and assistance programs. The classification is based on standardized measurements of a person’s remaining sight, even after their vision has been corrected with the best possible lenses. Legal blindness is determined by one of two specific criteria related to central vision clarity or peripheral sight.
Defining Legal Blindness by Visual Acuity
The most recognized criterion for legal blindness relates to central visual acuity, which measures how clearly a person can see fine details straight ahead. This measurement is taken using the familiar Snellen eye chart during an examination. An individual is considered legally blind if their best-corrected vision is 20/200 or worse in their better-seeing eye.
The fraction 20/200 means the person must stand at 20 feet to clearly see an object that a person with normal vision can discern from 200 feet away. This measurement is only valid when the individual is wearing their optimal prescription. If corrective glasses or contact lenses restore vision to better than 20/200, the person is not classified as legally blind based on acuity.
Defining Legal Blindness by Visual Field
The alternative criterion for legal blindness involves the visual field, which is the total area a person can see without moving their eyes. Even if central vision is better than 20/200, a person may qualify as legally blind if their peripheral vision is severely restricted. This restriction is defined as having a visual field that subtends an angle of 20 degrees or less in the better eye.
A normal human visual field spans about 160 to 170 degrees horizontally. A restriction to 20 degrees or less is often described as “tunnel vision,” meaning the individual retains a small, clear central view but is blind to everything on the sides. This extreme narrowing presents profound challenges for mobility and spatial awareness, making it an independent qualification for legal status.
Living with Legal Blindness
A widespread misconception is that being legally blind means seeing nothing at all, but this is rarely the case. Total blindness is medically defined as the complete absence of light perception, and only a small percentage of those who are legally blind fall into this category. The vast majority of people who meet the legal criteria retain some level of usable sight, often referred to as low vision.
The remaining vision can be highly varied, sometimes involving the ability to perceive light, colors, or large shapes. For example, a person may have 20/200 central vision but a full visual field, allowing easier navigation than someone with severe tunnel vision. The diagnosis of legal blindness is a threshold for administrative purposes, acknowledging that the vision loss is severe enough to significantly impact daily life. Individuals with this classification utilize various tools, such as magnifiers and screen-reading software, to perform tasks impossible with unassisted sight.