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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]
No light perception is considered total visual impairment, or total blindness; see Visual impairment#Classification: NPC: Near point of convergence or no previous correction NRC Normal retinal correspondence NV Near vision NWT Normal wearing time o symptoms Zero symptoms ϕ Horizontal orthophoria θ Vertical orthophoria ⊕
In order to see a clear image, the eye must focus rays of light on to the light-sensing part of the eye – the retina, which is located in the back of the eye.This focusing – called refraction – is performed mainly by the cornea and the lens, which are located at the front of the eye, the anterior segment.
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
In the visual system, images captured by the eye are translated into electric signals that are transmitted to the brain where they are interpreted. As such, in order to overcome presbyopia, two main components of the visual system can be addressed: image capturing by the optical system of the eye and image processing in the brain.
The visual acuity is assigned in the form of a fraction. Visual acuity is recorded as "20/20" (or another fraction like 20/40) when all optotypes (letters or symbols) on a specific line of the eye chart are correctly identified. When an individual correctly identifies additional 2 letters in the next 20/30 lines, then they will be assigned 20/40+2.
Glasses are the simplest and safest, although contact lenses can provide a wider field of vision. Refractive surgery can eliminate the need to wear corrective lenses altogether by permanently changing the shape of the eye but, like all elective surgery, comes with both greater risk and expense than the non-invasive options.
This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment. [72] Pseudomyopia is the blurring of distance vision brought about by spasm of the accommodation system. [73]