Blindness is a lack of vision or a loss of vision that can’t be corrected. The term partial blindness indicates that you have very limited vision, while the term complete blindness indicates that you can’t see anything, including light.

The United States has had a legal definition of blindness since the mid-1930s. Beyond a total inability to see, this definition helps establish a level of vision loss that might result in the inability to perform certain everyday tasks without assistance.

According to the National Eye Institute, in 2015 about 1 million Americans were legally blind and approximately 3.2 million Americans had visual impairment. They estimate that the number of people in the U.S. with visual impairment or blindness is expected to double by 2050 to more than 8 million.

The formal description of legal blindness is: “Central visual acuity 20/200 or less in the better eye with best correction, or widest diameter of visual field subtending an angle of no greater than 20 degrees.”

This basically means that even with corrective lenses or surgery, to clearly see an object that a person with normal vision could see from 200 feet away, you would have to be 20 feet or closer.

A person is considered to have low vision or visual impairment if their best corrected vision is 20/40 or worse. This person isn’t considered legally blind, but they can have difficulty in certain visual environments.

A person is functionally blind when they have to use alternative techniques to perform tasks, such as reading using braille.

Central vision loss

Most of your fine detailed vision is in the center of your visual field. This includes reading. Eye conditions that can interfere with your central vision include:

Tunnel vision

Tunnel vision allows you to see in the central part of your vision, but not in the periphery. Tunnel vision interferes with travel vision more than it does with reading vision. Eye conditions that can cause tunnel vision include:

Blind spots

Some people have vision with blind spots. These can be caused by traces of blood in the vitreous fluid in the eye that block certain areas. The amount and location of vision impairment can change daily. Conditions that can cause blind spots include:

Partial vision

Also known as low vision, partial vision varies from person to person, depending on factors such as:

  • color
  • glare
  • movement
  • fatigue
  • lighting
  • size

Some people with partial vision could be considered legally blind. Conditions that can cause partial vision include:

  • cataracts
  • retinitis pigmentosa
  • retinopathy of prematurity

Total blindness

While some people have absolutely no vision, some people who are considered totally blind may have light perception or the ability to see bright colors or some movement. Conditions that can cause total blindness include:

Although we tend to think of blindness as total blindness, there are legal definitions in the United States of varying degrees of blindness.

These definitions, including being legally blind, detail a level of vision loss that, even with correction, requires assistance in performing certain everyday tasks.