From the physiological point of view blindness can be defined as the state of sight deficiency. However, the definition, as it is applied to people who are legally classified as blind, is more complex. Total blindness is a permanent and complete loss of sight when the visually handicapped person cannot perceive the light. The significant decrease of vision functions is labeled as practical blindness.
The seriousness of sight handicap is defined by the vision sharpness expressed by Snellen ratio. For example strong shortsightedness is defined as the quality of vision expressed in Snellen ratio 1/10 or less. This means that practically blind individual has to stand 6 meters from the object to see it as clearly as the normally sighted person would see it from the distance of 60 meters.
The World Health Organization (WHO) defines legal blindness as the vision 20/400 (3/60) or less in stronger eye with correction or visual field smaller than 10 degrees.
Approximately 10 percent of people considered to be legally blind by any measurements are actually without the sight. Others have at least some degree of vision, from the perception of light to relatively good sharpness. Those, who are not legally blind but in spite of this they have a serious visual disruption, are considered to be short-sighted.
A serious eyesight disorder - blindness has many causes:
Sight disorder is usually caused by diseases and malnutrition. Majority of general causes of blindness in the world are:
People in developing countries suffer from eyesight disorder more often due to the conditions which could be prevented. Whereas eyesight disorder is everywhere more common in people older than 60 years, the children from poor regions will suffer a lot more from eye diseases than the children living in developed countries.
The connection between poverty and treatable eyesight disorder is expressed finely when we compare the regions. The most of eyesight disorders in adults from North America and Western Europe arise in connection with senile macular degeneration and diabetic retinopathy. Even though both of these diseases can be treated with medicine, at the moment none of them is curable.
Eye injuries, which are the most common in people under 30 years of age, are the main cause of monocular blindness (the loss of sight in one eye). Injuries and cataracts affect the eye itself. Abnormalities (e.g. optic nerve hypoplasia) affect the nerve bundle, which transmits signals from the eye to the posterior part of the brain, and can lead to the decrease of vision sharpness. Injuries of the occipital lobe can cause blindness even when the eyes or optic nerves are not damaged.
People with albinism often suffer from the eye disorder in such extent that many of them are practically blind even though only couple of them is completely without the sight.