Definition of Blindness
Definition of Blindness
According to the World Health Organization International Classification of Diseases there are four levels of visual function:
1. normal vision
2. moderate visual impairment
3. severe visual impairment
4. blindness.
Moderate visual impairment and severe visual impairment together comprise “low vision.” Low vision together with severe visual impairment and blindness represents all visual impairment.
Most people believe that blindness is the total inability to see anything. The fact is most people considered blind have some vision. The National Federation of the Blind consider a person blind if their sight is bad enough, with corrective lenses, that they must use alternative methods to engage in any activity that persons with normal vision would do using their eyes.
The United States Bureau of the Census say that “Significant vision loss” encompasses both total or near-total blindness and “trouble seeing, even when wearing glasses or contact lenses.”
The statutory definition of “legally blind” is that central visual acuity must be 20/200 or less in the better eye with the best possible correction or that the visual field must be 20 degrees or less.
20/200 means that a person who is legally blind must stand 20 feet (6 meters) away from an object to see what can be seen by a person with normal vision standing 200 feet away.
In the standard charts used to test reading vision, called ETDRS Chart R, the top line represents 20/400, the fourth line 20/200, what is considered legally blind. The second line from the bottom is 20/20 and the bottom line is 20/15, above average vision. Near vision charts such as the one below can obtained from Lighthouse International. ETDRS eye charts have become the standardized format for testing visual acuity. ETDRS acuity tests are now used in numerous clinical studies worldwide. The entire scope of vision is 180 degrees; an arc formed by making a half circle from one ear to the other is 180 degrees. Looking through a ring created by closing your index finger and thumb will give you 20 to 30 degrees of vision.
