9th May 2013 .
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Various scales have been developed to describe the extent of vision loss and define blindness. Total blindness is the complete lack of form and visual light perception and is clinically recorded as NLP, an abbreviation for "no light perception." Blindness is frequently used to describe severe visual impairment with residual vision. Those described as having only light perception have no more sight than the ability to tell light from dark and the general direction of a light source.
By the 10th Revision of the WHO International Statistical Classification of Diseases, Injuries and Causes of Death, low vision is defined as visual acuity of less than 20/60 (6/18), but equal to or better than 20/200 (6/60), or corresponding visual field loss to less than 20 degrees, in the better eye with best possible correction. Blindness is defined as visual acuity of less than 20/400 (6/120), or corresponding visual field loss to less than 10 degrees, in the better eye with best possible correction.
The WHO estimates that in 2002 there were 161 million visually impaired people in the world (about 2.6% of the total population).
Of this number 124 million (about 2%) had low vision and 37 million (about 0.6%) were blind. In order of frequency the leading causes were cataract, uncorrected refractive errors (near sighted, far sighted, or an astigmatism), glaucoma, and age-related macular degeneration.
In 1987, it was estimated that 598,000 people in the United States met the legal definition of blindness. Of this number, 58% were over the age of 65. In 1994-1995, 1.3 million Americans reported legal blindness.