Blindness is an important health care issue that exacts both economic and social costs. It is easy to understand that blindness is a vision problem of lacking visual perception due to physiological or neurological factors. Blindness is a kind of vision loss. The most common eye disease causes of blindness including: cataracts, glaucoma and age-related macular degeneration. Besides, abnormalities, injuries, genetic defects and certain chemicals may also lead to blindness.
In order to determine which people may need special assistance because of their visual disabilities, various governmental jurisdictions have formulated complex definitions for legal blindness. In 1934, the American Medical Association (AMA) adopted the following definition of legal blindness: central visual acuity of 20/200 or less in the better eye with corrective glasses or the widest diameter of the visual field subtends an angular distance no greater than 20 degrees in the better eye. And now, blindness is defined according to the World Health Organization (WHO) and North American criteria for the better-seeing eye. The WHO divide vision problems into five levels: moderate low vision, severe low vision, profound vision loss (central visual acuity from 20/500 to 20/1000 or visual field < 10 degrees), near total vision loss, and total vision loss in which clinically recorded is “no light perception”(NLP). The last three levels are defined as blindness. Note that low vision does not meet criteria for a definition of blindness. While North American and most of Europe still uses the previous definition by AMA since 1934.
Blind people with undamaged eyes may still use light in order to keep the 24-hour light and dark cycle.