Total blindness is a simple inability to see light or form. Legal blindness is defined differently in different countries. In most of Europe and in North America, it is defined as having less than 6/60 (20/200) vision in the better eye despite correction with lenses. 6/60 means that the person concerned would have to be 6 feet away from something an ordinary person could see from 60 feet. In the UK, a very restricted field of vision (20 per cent rather than 180 per cent) would represent legal blindness even if acuity is better than 6/60.

Partial sight has no statutory definition, but there are official guidelines for registering a person as partially sighted: generally these are a corrected visual acuity of 3/60 or less in the better eye with some contraction of the peripheral field, or better with gross field defects. In the UK nearly 150,000 people are registered as legally blind and a similar number partially sighted. The World Health Organisation has estimated that there are over 40 million binocularly blind people in the world. The causes of blindness vary with age and degree of development of the country. In Western society the commonest causes are glaucoma, diabetic retinopathy, other retinal diseases and senile cataract. (See also VISION.)