Blindness can be defined physiologically as the condition of lacking sight. The definition as it applies to people thus legally classified is, however, more complex.
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. 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.
The World Health Organization (WHO) defines legal blindness as vision of 20/400 (3/60) or less in the better eye with correction, or a visual field of less than 10 degrees.
Approximately ten percent of those deemed legally blind, by any measure, are actually sightless. The rest have some vision, from light perception alone to relatively good acuity. Those who are not legally blind, but nonetheless have serious visual impairments, possess Low vision.
Causes of blindness
Serious visual impairment has a variety of causes:
Most visual impairment is caused by disease and malnutrition: The most common causes of blindness around the world are cataracts (43% in 1997, according to WHO), Glaucoma (15%), Trachoma (11%), and Vitamin A deficiency found in children under 5 (2%) and Coloboma (3%).
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.
In developing countries, wherein people have shorter life expectancies, cataracts and water-borne parasites - both of which can be treated effectively - are most often the culprits. Of the estimated 40 million blind people located around the world, 70-80% can have some or all of their sight restored through treatment.
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, and 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.
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.
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, being 'swept' back and forth across the intended path of travel to detect obstacles. Some visually impaired people do not carry these kinds of canes, however; 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.
A small number of people, about one percent, employ guide dogs. These companions are trained to lead blind individuals around obstacles on the ground and overhead. Though highly intelligent, guide dogs neither interpret street signs nor determine when the team ought to cross a street. Visually impaired people who employ these animals must already be competent travelers.
Reading and magnification
Most blind and visually impaired people read print, either of a regular size or enlarged through the use of magnification devices. The rest either read Braille and Moon type, use specially adapted computers with scanners, optical character recognition applications, refreshable Braille displays, screen reading software or rely on talking books and readers.
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.
A variety of magnifying glasses, some of which are hand-held while others rest on desktops, can make reading easier for those with decreased visual acuity.
Closed-Circuit Televisions - pieces of 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.
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 thanks to the availability of assistive technology 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.
People may use Talking Thermometers , enlarged or marked oven dials, talking watches, talking clocks, talking scales, talking calculators, talking compasses and other talking equipment.
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.