It is estimated that out of the total world population, approximately 9 million are blind. Out of this India’s share is estimated at 4.5 million, i.e. 50% of the population. But except for their challenge of vision, a large number of the blind are able bodied men and women who, given the right opportunity, can make significant contributions to the society. But that would require sensitivity and understanding of facts on the part of the sighted and the larger society. So here are some quick facts.
In normal practice, a person who has normal vision is referred to as having central visual acuity of 6/6. By central visual acuity is meant the ability of the eye to perceive and discriminate the shape or form of object in direct line of vision. 6/6 implies that the person is able to perceive clearly at 6 meters what an average normal vision person perceive clearly at the same distance, i.e. 6 meters. If we say a person has central visual acuity of 6/18, it means that what object a person with normal vision perceives clearly at 18 meters, can be perceived clearly by this person (having visual acuity of 6/18) only by going nearer up to 6 meters. The testing of central visual acuity of a person is normally carried out by using suitable eye charts.
So we can now understand part of the formal definition of legal blindness as defined by the World Council for the Welfare of the Blind which is: ‘Central visual acuity of 6/60 or less in the better eye after correction.’ This means that what object a person with normal vision is able to perceive clearly at 60 meters, the blind or visually impaired can do so by getting as close as 6 metres to that object.
There is yet another state of blindness. This is related to Field Of Vision or Visual Field. A person may have good central visual acuity, i.e., more than 6/60 or even 6/6, but may suffer from narrow vision. That is, a normal person sighted person looking to his front, can perceive objects over an arc of 170 degrees from left to right. For a person to be declared legally blind on account of visual field, the widest diameter of the visual field must subtend an angle no greater than 20 degree at the eye. Another name for this defect is tunnel vision.
The principal causes of preventable visual impairment vary across regions. In developing countries like India, visual impairment associated with infections, nutritional deficiencies and trauma is much more common than it is in developed countries. Some of the eye conditions that contribute to visual impairment include:
Milestones in Mobility Training for the Visually Impaired