"Blindness" is defined as visual acuity worse than 3/60 meters (10/200 or 20/400 in US terms), or a visual field of less than 10 degrees in the better eye with best possible correction. (Please Note: Since most parts of the world use the metric system, the World Health Organization uses meters in its definitions.)

Low vision ( previously referred to as 'partial sight'), is a significant reduction of visual acuity that cannot be fully corrected by ordinary eyeglasses, contact lenses, medical treatment and/or surgery.

A person with low vision will have some sight but it will be reduced and will usually interfere with everyday activities, such as cooking, reading and writing. If the visual field is reduced, the person may have difficulty seeing well enough to walk alone or drive a car. The World Health Organization defines low vision as visual acuity or worse than 6/18 (US 20/60), but better than 3/60 (10/200 in US terms), or a visual field of less than 20 degrees in the better eye with best correction.

Best possible correction. The best visual acuity achieved with spectacles, contact lenses, and/or surgery. However, when the best possible correction cannot achieve 20/20 or 6/6, the person is said to have "low vision". Low vision causes a significant reduction in visual function that affects daily activities. These people may need additional "low vision aids" such as a special magnifier or scope.

What do these visual acuity numbers mean?

Since Europe and most parts of the world use the metric system, the World Health Organization uses meters in its definitions. In the United States, visual acuity is measured at a distance of 20 feet. In Europe and the rest of the world visual acuity is measured at a distance of 6 meters.

Normal vision is therefore defined as a distance visual acuity of 6/6 (or 20/20 in US terms) with a full visual field of 180 degrees. Distance visual acuity is the most central part of vision that is used to see objects at a distance. "6/6 or 20/20" means they can see a specific line of letters when they are standing or seated 20 feet or 6 meters away from an eye chart.

The loss of visual acuity causes blurred vision and you cannot see objects at a distance clearly.