skip navigation

S M L Text size
Home » Resources » Statistics and Data » Definitions


Low Vision

Vision impairment is defined as the best-corrected visual acuity less than 6/12 (<20/40) in the better-seeing eye (excluding those who were categorized as being blind by the U.S. definition.)


Blindness as defined by the U.S. definition is the best-corrected visual acuity of 6/60 or worse (=20/200) in the better-seeing eye. Blindness is defined by U.S. definition 20/200 best-corrected visual acuity in the better-seeing eye and the World Health Organization standard of < 20/400.

Open-angle Glaucoma

In open-angle glaucoma, the fluid passes too slowly through a spongy meshwork. Since the fluid builds up, the pressure inside the eye rises to a level that may damage the optic nerve. When the optic nerve is damaged from increased pressure, open-angle glaucoma-and vision loss may result.

Diabetic Retinopathy

Diabetic retinopathy is the most common diabetic eye disease and a leading cause of blindness in American adults. It is caused by changes in the blood vessels of the retina. In some people with diabetic retinopathy, blood vessels may swell and leak fluid. In other people, abnormal new blood vessels grow on the surface of the retina. The retina is the light-sensitive tissue at the back of the eye. A healthy retina is necessary for good vision.


A cataract is a clouding of the lens in the eye that affects vision. Most cataracts are related to aging. Cataracts are very common in older people.

Refractive Error

Myopia, also known as nearsightedness, is a common type of refractive error where close objects appear clearly, but distant objects appear blurry. Cases of refractive error include myopia of 1.0 diopters or more.

Hyperopia, also known as farsightedness, is a common type of refractive error where distant objects may be seen more clearly than objects that are near. The threshold for clinically significant hyperopia is somewhat higher at 3.0 diopters or more.

Age-related macular degeneration (AMD)

AMD is a leading cause of vision loss in the U.S. It destroys the macula, the part of the eye that provides sharp, central vision needed for seeing objects clearly. Prevalence estimates are on late AMD, involving neovascular AMD and geographic atrophy.

Department of Health and Human Services NIH, the National Institutes of Health