A Functional Definition of Low Vision
Not all eye care professionals agree with an exclusively numerical (or visual acuity) description of low vision. Here’s another — more functional — definition of low vision:
- Low vision is uncorrectable vision loss that interferes with daily activities. It is better defined in terms of function, rather than [numerical] test results. (Massof and Lidoff)
- In other words, low vision is “not enough vision to do whatever it is you need to do,” which can vary from person to person.
- Most eye care professionals prefer to use the term “low vision” to describe permanently reduced vision that cannot be corrected with regular glasses, contact lenses, medicine, or surgery.
- If you have low vision, it is necessary to have a different kind of eye examination that uses different and more detailed tests to determine what you can and cannot see. You can learn more about these specialized eye charts and testing procedures at What is a Low Vision Examination?
Low Vision vs. Legal Blindness
“Legal blindness” is a definition used by the United States government to determine eligibility for vocational training, rehabilitation, schooling, disability benefits, low vision devices, and tax exemption programs. It’s not a functional low vision definition and doesn’t tell us very much at all about what a person can and cannot see.
Part 1 of the U.S. definition of legal blindness states this about visual acuity:
- A visual acuity of 20/200 or less in the better-seeing eye with best conventional correction (meaning with regular glasses or contact lenses).
This is a 20/200 visual acuity measurement, correlated with the Snellen Eye Chart (pictured above):
- If you can only read line 1 (the big “E”) from 20 feet away while wearing your regular glasses or contact lenses, the doctor records your vision (or visual acuity) as 20/200 with best correction.
- Update: In 2007, the Social Security Administration updated the criteria for measuring legal blindness when using newer low vision test charts with lines that can measure visual acuity between 20/100 and 20/200. Under the new criteria, if a person’s visual acuity is measured with one of the newer charts, and they cannot read any of the letters on the 20/100 line, they will qualify as legally blind, based on a visual acuity of 20/200 or less.
Part 2 of the U.S. definition of legal blindness states this about visual field:
- OR a visual field (the total area an individual can see without moving the eyes from side to side) of 20 degrees or less (also called tunnel vision) in the better-seeing eye.
For more information on the definitions of legal blindness, you can read Disability Evaluation Under Social Security, a publication from the Social Security Administration.
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