Legally blind: What is the definition?

“Legally blind” means you’re essentially sightless in the eyes of the law if eyeglasses or contact lenses cannot correct your vision beyond specific standards.

How do we define who’s legally blind?

In the U.S., the standard definition of legal blindness is based on central visual acuity (what’s in front of you) and field of vision (what’s above, below and to the sides).

By this definition, you’re legally blind if your better eye — when using a corrective lens — has a central visual acuity of 20/200 or lower, or field of vision of no more than 20 degrees.

It’s good to know the definition of legal blindness because it can affect your ability to get a driver’s license or receive government disability benefits.

Measuring your visual acuity

When your eye doctor asks you to read the smallest row of letters from across the room, you’re looking at the Snellen chart, the U.S. standard for measuring clarity of eyesight.

The eye chart assumes a base viewing distance of 20 feet and compares your vision to the historical norm for most humans.

If you have 20/20 vision, then the smallest letters you can read from 20 feet away match the normal 20-foot distance. By contrast, 20/200 vision means the letters you can read from 20 feet can be read from 200 feet by people with normal vision. Thus, you see only one-tenth of the norm.

Measuring your visual field

Some people can see small letters on an eye chart but can't see the person standing next to them. That’s because they have legal blindness due to poor peripheral vision. A wide visual field is crucial for activities like driving a car or crossing a busy street.

Visual field tests determine if you have a normal field of view that has no blind spots or unusual narrowing of your peripheral vision.

Peripheral vision has two parameters: lateral (side to side) and vertical (up and down). A maximum lateral field of view is nearly 180 degrees; distant objects remain visible from the right or left of the observer.

The normal vertical field is smaller at about 135 degrees.

You are considered legally blind if testing finds your peripheral vision is 20 degrees or less (a condition often called tunnel vision).

How many people are legally blind?

About 1 million Americans were legally blind in 2015, according to the most recent estimate from the recent estimate from the National Eye Institute.

The same report noted that 3.2 million Americans had low vision (20/40 or worse with best correction), while 8.2 million had uncorrected vision problems.

As the nation’s population ages, the number of legally blind Americans is expected to double by the year 2050. 

Causes of legal blindness

Though some people become blinded in accidents, a host of conditions can cause a visual disability or legal blindness.

Four leading causes of legal blindness in the United States are age-related macular degeneration (AMD or ARMD), cataracts, diabetic retinopathy and glaucoma.

Optic neuritis and neuropathy also can cause legal blindness, as can a number of congenital conditions such as congenital cataracts, infantile glaucoma and retinopathy of prematurity.

Keratoconus, a gradual thinning of the cornea, also can cause severe vision loss to the point of legal blindness.

Resources for those who are legally blind

Legal blindness can make people eligible for special services and assistance.

The Social Security Administration provides benefits to the legally blind. States and federal taxing authorities also allow tax deductions. Multiple non-governmental resources help make it easier to lead a normal life even with severe vision impairment.

If you are legally blind or you are a guardian or care provider for someone who is, seek the services of an eye doctor who specializes in low vision.

Low-vision specialists typically are familiar with the latest vision aids such as magnifiers, telescopes and digital devices that can help you make the best use of any vision you still have.

These devices often help a person with legal blindness live more independently and enjoy activities that everyone else takes for granted.

Professionally trained guide dogs also are a great help for many people who are legally blind.

Page published on Tuesday, 28 July, 2020