Blindness can be defined physiologically as the condition of lacking visual perception. The definition as it applies to people thus legally classified is, however, more complex.
"Blindness" also applies to partial visual impairment: In North America and most of Europe, legal blindness is defined as visual acuity (vision) of 20/200 (6/60) or less in the better eye with best correction possible. This means that a legally blind individual would have to stand 20 feet from an object to see it with the same degree of clarity as a normally sighted person could from 200 feet. In many areas, people with average acuity who nonetheless have a visual field of less than 20 degrees (the norm being 180 degrees) are also classified as being legally blind.
Diseases
Most visual impairment is caused by disease and malnutrition. According to WHO estimates in 2002, the most common causes of blindness around the world are:
- cataracts (47.8%),
- glaucoma (12.3%),
- age-related macular degeneration (AMD) (8.7%),
- trachoma (3.6%),
- corneal opacity (5.1%), and
- diabetic retinopathy (4.8%), among other causes.
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. 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 blinding diseases than are their more affluent peers.
The link between poverty and treatable visual impairment is most obvious when conducting regional comparisons of cause. Most adult visual impairment in North America and Western Europe is related to age-related macular degeneration and diabetic retinopathy. While both of these conditions are subject to treatment, neither can be cured.
Abnormalities and Injuries
Eye injuries, most often occurring in people under 30, are the leading cause of monocular blindness (vision loss in one eye) throughout the United States. Both of these conditions, injuries and cataracts, affect the eye itself. Abnormalities such as optic nerve hypoplasia affect the nerve bundle that sends signals from the eye to the back of the brain, which can lead to decreased visual acuity.
People with injuries to the occipital lobe of the brain can, despite having perfectly normal eyes and optic nerves, still be legally or totally blind.
Genetic Defects
People with albinism often suffer from visually impairment to the extent that many are legally blind, though few of them are actually sightless.
Visually impaired and blind people have devised a number of techniques that allow them to complete daily activities using their remaining senses. These might include the following:
- Adaptations of banknotes so that the value can be determined by touch. For example:
- In some currencies, such as the euro and pound sterling, the physical size of a note increases with value.
- Canadian banknotes have a tactile feature to indicate denomination in the upper right corner. This tactile feature is a series of raised dots, but it is not standard Braille.
- It is also possible to fold notes in different ways to assist recognition.
- Labeling and tagging clothing and other personal items
- Placing different types of food at different positions on a dinner plate
- Marking oven, dishwasher, and dryer dials for ease of use
Most people, once they have been visually impaired for long enough, devise their own adaptive strategies in all areas of personal and professional management.
Designers, both visually impaired and sighted, have developed a number of tools for use by blind people.
Mobility
People with serious visual impairments can travel independently using a white cane, the international symbol of blindness.
A long cane is used to extend the user's range of touch sensation, swung in a low sweeping motion across the intended path of travel to detect obstacles. However, some visually impaired persons do not carry these kinds of canes, opting instead for the shorter, lighter identification (ID) cane. Still others require a support cane.
Each of these is painted white for maximum visibility, and to denote visual impairment on the part of the user. In addition to making rules about who can and cannot use a cane, some governments mandate the right-of-way be given to users of white canes or guide dogs.
Reading and Magnification
Most blind and visually impaired people read print, either of a regular size or enlarged through the use of magnification devices. A variety of magnifying glasses, some of which are handheld while others rest on desktops, can make reading easier for those with decreased visual acuity.
The rest read Braille and Moon type or rely on talking books and readers. They use computers with special hardware such as scanners and refreshable Braille displays as well as software written specifically for the blind, like optical character recognition applications and screen reading software.
Some people access these materials through agencies for the blind, such as the National Library Service for the Blind and Physically Handicapped in the United States, the National Library for the Blind or the RNIB in the United Kingdom.
Closed-Circuit Televisions, equipment that enlarge and contrast textual items, are a more high-tech alternative to traditional magnification devices. So too are modern web browsers, which can increase the size of text on some web pages through browser controls or through user-controlled style sheets.
Computers
Access technology such as Freedom Scientific's JAWS for Windows screen reading software enable the blind to use mainstream computer applications. Most legally blind people (70% of them across all ages, according to the Lighthouse for the Blind) do not use computers. Only a small fraction of this population, when compared to the sighted community, have internet access. This bleak outlook is changing, however, as availability of assistive technology increases, accompanied by concerted efforts to insure the accessibility of information technology to all potential users, including the blind.
The movement towards greater web accessibility is opening a far wider number of websites to adaptive technology, making the web a more inviting place for visually impaired surfers.
Experimental approaches such as the seeing with sound project are beginning to provide access to arbitrary live views from a camera.
Other Aids
People may use talking thermometers, enlarged or marked oven dials, talking watches, talking clocks, talking scales, talking calculators, talking compasses and other talking equipment.
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 extrasensory abilities. Stories such as The Cricket on the Hearth by Charles 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.
