The term "blindness" also applies to partial visual impairment: In North America and most of Europe, legal blindness is defined as vision of 20/200 (6/60) or less in the better eye with correction. People with normal acuity who nonetheless have a visual field of less than 20 degrees - the norm being 180 degrees - are also classified as being legally blind.
The World Health Organization (WHO) defines legal blindness as vision of 20/400 (3/60) or less in the better eye with correction. They also accept people with a visual field of less than 10 degrees under that heading.
Approximately ten percent of those classified as being legally blind are actually sightless. The rest have some vision, from light perception alone to relatively good acuity.
Many countries also have a legal provision for people who, though not legally blind, have vision poor enough to affect their performance of daily tasks using traditional methods.
Causes of blindness
Diseases of the eye
People in developing countries are significantly more likely to experience visual impairment as a consequence of treatable or preventable conditions, than are their counterparts in the developed world. Moreover, while vision impairment is most common in people over age 60 across all regions, children in poorer communities are more likely to be affected by a blinding condition than are their more affluent peers.
Conditions and disorders
Another kind of visual impairment, cortical blindness, is the result of brain injury affecting a vital area of the visual system called the occipital lobe. People with cortical blindness can, despite having perfectly normal eyes and optic nerves, still be legally or totally blind.
Other conditions, such as Optic Nerve Hypoplasia and Optic Nerve Atrophy, cause visual impairment by affecting those nerve bundles that send visual signals from the eyes to the brain for interpretation.
Alternative techniques and tools
Many blind and visually impaired people use a wide range of alternative techniques and specialized tools to accomplish tasks traditionally done using sight.
Tools used to help the blind include:
A number of visually impaired people travel independently using this mobility device, typically colored white with a red tip. This tool is used to extend the user's range of touch sensation, often being 'swept' back and forth across the intended path of travel to detect obstacles. The color of the cane is intended to promote visibility of the device to sighted people, alerting them to the presence of a blind or impaired user. In many countries, it is not permitted for sighted persons to use white canes. Additionally, some countries mandate that the right-of-way be given to users of white canes or guide dogs, and to blind persons in general.
Magnifying glasses, non-prescription "reading glasses", full page fresnel lenses, and 'large text' format books and magazines can make reading more readily-available to those with lessened visual acuity.
Modern web browsers provide the ability to increase the size of text on web pages, through browser controls, or through user-controlled style sheets, providing the same effect as large-print books, or magnification devices.
Clever adaptation of existing technologies as well as new technologies can aid the blind or visually impaired:
Visually impaired people can 'read' written material using a system of embossed print.
Used by visually impaired people, along with enlarged or marked oven dials, when they are cooking. Some blind people also use talking watches, talking clocks, talking scales, talking calculators, talking compasses and other talking equipment.
Electronic devices that enlarge and contrast many textual items
Web Accessibility Techniques
Persons with visual impairment may use text-based or aural browsers, such as Jaws and Window-Eyes to read content on the internet and worldwide web, as well as other computer programs, particularly word processing programs such as Microsoft Word. Henter-Joyce, the original makers of Jaws, and the makers of many other technology products for blind people have consolidated themselves into one company, Freedom Scientific. Window-Eyes is made by the GWMicro corporation. Legislation has been proposed to make public web space accessible in the same way that public buildings are required, in some countries, to be accessible to wheelchairs, walkers, etc. see w3.org 's Web Accessibility Initiative (WAI) page (http://www.w3.org/WAI/)
To make web pages more accessible to the visually impaired, the Worldwide Web Consortium recommends the following techniques:
A very small number of visually impaired people employ the services of guide dogs, sometimes improperly called, "seeing-eye dogs". These companions are especially trained to lead blind individuals around obstacles on the ground and overhead. Though highly intelligent, guide dogs neither interpret streets signs nor determine when the team ought to cross a street. Visually impaired people who employ these animals must, then, already be competent travelers.
Social attitudes towards blindness
Historically, blind and visually impaired people have either been treated as if their lack of sight were an outward manifestation of some internal lack of reason, or as if they possessed extra-sensory abilities. Stories such as The Cricket on the Hearth (Dickens) provided yet another view of blindness, wherein those affected by it were ignorant of their surroundings and easily deceived.
The authors of modern educational materials (see: blindness and education for further reading on that subject), as well as those treating blindness in literature, have worked to paint a truer picture of blind people as three-dimensional individuals with a range of abilities, talents, and even character flaws. Certain individuals are gifted, and others licentious, but nothing definitive can be said of the blind as a class but that they cannot see well.
cy:Dallineb de:Blindheit es:Ceguera nl:Blindheid ja:失明 pt:Cegueira simple:Blindness