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Visual or vision impairment (VI or VIP) is the partial or total inability of visual perception.In the absence of treatment such as corrective eyewear, assistive devices, and medical treatment, visual impairment may cause the individual difficulties with normal daily tasks, including reading and walking. [6]
In the study of visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. [1] The term comes from the Greek skotos, meaning 'darkness', and -opia, meaning 'a condition of sight'. [2] In the human eye, cone cells are nonfunctional in low visible light.
Low myopia usually describes myopia between −0.50 and −3.00 diopters. [65] Moderate myopia usually describes myopia between −3.00 and −6.00 diopters. [65] Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome or pigmentary glaucoma. [84] High myopia usually describes myopia of −6.00 or more.
Low vision is both a subspeciality and a condition. Optometrists, Opticians and Ophthalmologists after their training may undergo further training in Low vision assessment and management. There are various classifications for low vision, this varies from country to country and even from state to state.
Degrees of vision loss vary dramatically, although the ICD-9 released in 1979 categorized them into three tiers: normal vision, low vision, and blindness. Two significant causes of vision loss due to sensory failures include media opacity and optic nerve diseases, although hypoxia and retinal disease can also lead to blindness. Most causes of ...
Peripheral vision is also relatively good at detecting motion (a feature of Magno cells). Central vision is relatively weak in the dark (scotopic vision) since cone cells lack sensitivity at low light levels. Rod cells, which are concentrated further away from the fovea, operate better than cone cells in low light.
At the low end of the range is the absolute threshold of vision for a steady light across a wide field of view, about 10 −6 cd/m 2 (0.000001 candela per square meter). [22] [23] The upper end of the range is given in terms of normal visual performance as 10 8 cd/m 2 (100,000,000 or one hundred million candelas per square meter). [24]
Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process input from one eye and over time favors the other eye. [1] It results in decreased vision in an eye that typically appears normal in other aspects. [1]