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What is a Vision Impairment?
Vision impairment means that a person's eyesight cannot be corrected to a "normal" level. Vision impairment may be caused by a loss of visual acuity, where the eye does not see objects as clearly as usual. It may also be caused by a loss of visual field, or a changing or degenerative eye condition.
There are different ways of describing the severity of a person's vision loss. The World Health Organization defines "low vision" as visual acuity between 20/70 and 20/400, with the best possible correction, or a visual field of 20 degrees or less. "Blindness" is defined as a visual acuity worse than 20/400, with the best possible correction, or a visual field of 10 degrees or less. Someone with a visual acuity of 20/70 can see at 20 feet what someone with normal sight can see at 70 feet. Someone with a visual acuity of 20/400 can see at 20 feet what someone with normal sight can see at 400 feet. A normal visual field is about 160-170 degrees horizontally.
Vision impairment severity may be categorized differently for certain purposes. In the United States, for example, we use the term "legal blindness" to indicate that a person is eligible for certain education or federal programs. Legal blindness is defined as a visual acuity of 20/200 or worse, with the best possible correction, or a visual field of 20 degrees or less.
Visual acuity alone cannot tell you how much a person's life will be affected by their vision loss. It is important to also assess how well a person uses the vision they have. Two people may have the same visual acuity, but one may be able to use his or her vision better to do everyday tasks. Most people who are "blind" have at least some usable vision that can help them move around in their environment and do things in their daily lives.
Adapted from the CDC. Click here for the source.
Click the following links for Vision Simulations and a glossary of Vision Conditions.
Free Ophthalmology Online Textbook - Has easily understood explanations and video lectures of eye anatomy and conditions.