Vision experts assess an individual's sight using two measurements: visual acuity and visual field. Visual acuity is the ability to see details, such as symbols or letters of specific sizes. Normal vision is described as 20/20. A person with any degree of sight loss has a visual acuity with a higher second number, such as 20/200. Visual field refers to the space around the center of vision—the peripheral area. A normal visual field is said to be 180 degrees in diameter, or half a circle.
In the United States, legal blindness is defined as a person with a visual acuity of 20/200 or less in the better eye with the best optical correction, such as eyeglasses or contact lenses, or a visual field whose widest diameter is no greater than 20 degrees. An individual with a visual acuity of 20/200 must stand at 6 m (20 ft) to see objects that a person with normal sight can see at 60 m (200 ft). An individual with a visual field of 20 degrees or less has a limited visual range sometimes referred to as tunnel vision that is likened to viewing the world through a toilet paper roll. A person declared legally blind in the United States is eligible for government benefits.
PREVALENCE AND INCIDENCE
About 750,000 Americans are considered legally blind and about 50,000 new cases of blindness occur each year. Approximately 8.9 million people have low vision. Of these, about 1.5 million cannot read ordinary newspaper type even with the aid of eyeglasses or other optical aids that provide significant magnification . Although it is very difficult to determine the global prevalence of blindness, the World Health Organization (WHO) estimates that 40 to 45 million people are blind worldwide and an additional 160 million individuals suffer from low vision.