Total blindness is a term used to describe those who have complete lack of light perception, documented as no light perception (NLP). Only about 15% of people with eye disorders have total blindness, the majority of those with visual impairment have some level of vision.
Low vision is a term used to describe those whose vision cannot be fully corrected by conventional method such as glasses, contact lenses, medicine, surgery, magnification aids or assistive technology.
Visual impairment is a functional term to describe those whose decreased visual function interferes with the ability for one to perform their activities of daily living, such as reading, driving, and watching TV. Visual impairment is defined based on function, instead of using visual acuity or visual field cutoff values. Visual impairment includes those who have low vision or who are blind.
The types of visual symptoms you may experience include:
Blurred vision
Hazy/cloudiness
Wavy or spots in central vision
Restricted peripheral vision
Poor night vision
Difficulty seeing colors
Legal blindness is a term defined by the United States Social Security Administration (SSA) to determine those who are eligible to receive disability benefits, tax exemption programs, and rehabilitation training. SSA uses visual acuity or visual field results to determine this eligibility. A person is considered legally blind if he/she has central visual acuity of 20/200 or worse in the better-seeing eye with best correction (using glasses or contact lenses) at a distance, or if he/she has visual field restriction where the widest diameter is 20 degrees or less in the better-seeing eye. The tests used to measure visual acuity or visual fields have been outlined as follows:
Visual acuity testing for distance is carried out using Snellen visual acuity chart or another test that is comparable to the Snellen methodology. In 2007, SSA updated the criteria for measuring the visual acuity by allowing newer low vision test charts to be used instead of Snellen acuity charts alone. Under this update, if a person cannot read at least one letter on the 20/100 line, he or she will be classified as legally blind.
Acceptable tests for visual field testing include automated static perimetry such as Humphrey Field Analyzer (HFA) 30-2, HFA 24-2, and Octopus 32, kinetic perimetry such as Goldmann perimetry or HFA "SSA Test Kinetic." Screening tests including confrontation tests, tangent screen tests, and static screening tests are not accepted forms of testing to determine legal blindness.
World Health Organization classifies visual impairment into the following categories based on visual acuity or visual field of the better seeing eye.
Normal: 20/10-20/25
Near Normal visual impairment: 20/30-20/60
Moderate visual impairment: 20/70-20/160
Severe visual impairment: 20/200-20/400, or 11-20 degrees on visual field
Profound visual impairment: 20/500-20/1000 visual acuity, or 6- 10 degrees on visual field
Near total visual impairment: Counting fingers, Hand motion, Light perception, or 5 degrees or less on visual field
Total visual impairment: No light perception
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