What Is the Meaning of Legally Blind?

"Legally blind" is the definition of blindness used by the U.S. Social Security Administration (SSA) to determine if someone is eligible to receive disability benefits, tax exemptions, and low vision training.

The terms also may be used by health insurers to determine benefits and as part of vision screening tests required by state departments of motor vehicles (DMVs) in determining driver's license eligibility. For safely reasons, people with legal blindness or low vision typically are not eligible for a driver's license.

Meaning

To be considered legally blind, you would have to meet one of two criteria for visual acuity (sharpness of vision) and visual field (the entire scope of what you can see without moving your eyes).

Visual Acuity

Visual acuity refers to how close a person needs to be to an object that's 20 feet away in order to see it in detail. Normal vision is measured as 20/20. If you had visual acuity of 20/80, it would mean that you would be able to see details from 20 feet away the same as a person with 20/20, or normal, vision could see from 80 feet away.

A legally blind person with 20/200 vision (with the best corrective lenses) would need to be 20 feet from an object in order to see it as well as someone with 20/20 vision could see it from 200 feet away.

Another way to look at it: If someone with 20/20 vision was standing next to a person who's legally blind, in order for the legally blind person to see an object that's 200 feet away as well as the person with the normal vision they would have to get as close as 20 feet to it.

Low vision is a visual acuity of 20/40 or worse while wearing corrective lenses.

Visual Field

If a person has a visual field of only 20 degrees, they can see things that are right in front of them without moving their eyes from side to side but they can't see anything on either side (peripheral vision). A visual field of 180 degrees is considered normal. A severely limited visual field sometimes is called tunnel vision. It makes it nearly impossible to drive safely.

Eligibility

An eye doctor will measure visual acuity and visual field to determine if a person is legally blind.

A common test for visual acuity is the Snellen eye chart. Someone who is legally blind would be able to read only the top line of the chart, a capital E while wearing corrective lenses. The line below the big E is the line for 20/100. There are also tests that can measure in between 20/200 and 20/100. Someone who cannot view the line for 20/100 but sees somewhere between 20/100 and 20/200 would still meet the government's standard of legal blindness, which is why it is listed as "20/200 or less."

Visual field testing often begins with a confrontational visual field test in which you an eye doctor has you cover one eye at a time and then holds up one or more fingers in different quadrants of the visual field to see if you can see them while keeping your eyes focused on a central point in front of you. There are also more comprehensive computerized tests that use flashing, flickering, or moving lights or images to measure your visual field. It involves pressing a button when you see the light or images.

Causes

There are many conditions that can cause legal blindness, but the most common ones are age-related eye disease. Age-related eye diseases that are the leading causes of low vision and blindness are:

• Age-related macular degeneration
• Cataracts (cloudiness of the clear lens of the eye)
• Diabetic retinopathy
• Glaucoma

Eye trauma or injuries and genetic conditions, such as Usher syndrome, can also lead to legal blindness.

Treatments

Treatments for legal blindness vary depending on the cause and the stage of the disease. For age-related eye diseases, it typically involves prescription medications or eye procedures to try to delay or keep the vision from worsening.

For example, the goal of treatment for glaucoma is to reduce eye pressure. This can be achieved with prescription eye drops or oral medications, laser procedures, and, in severe cases, surgeries to try to prevent further damage. Careful monitoring of glaucoma and other age-related eye diseases is important for determining if treatment is working or needs to be adjusted.

Cataracts are the exception in that vision can be restored with surgery to remove the clouded lens and, in most cases, replace it with an implant.

A Word From Verywell

Low vision or legal blindness can be limiting to be sure, but there are many resources and assistive devices to help you live your life with the utmost independence. Depending on the cause of your vision loss, you may be able to benefit from eye exercises and strategies for participating in everyday activities. You also may find using a cane, talking calculator, special computer software, and other products designed to support people who are legally blind to be helpful.