How to Gauge Visual Impairment


Before we can explore various causes of blindness and how to cope with the loss of eyesight, it is necessary first to define exactly what is meant by the medical designation ‘blind’. There are essentially three levels of vision commonly accepted in Western medicine: Normal Vision, Low Vision, and Blindness. Each has its own guidelines and parameters based on the vision’s acuity and field of vision.

Acuity

Acuity is the amount of detail a person can see from a given distance. The Snellen Vision Chart, created by Herman Snellen in 1862, is commonly seen in any optometrist’s office and measures the acuity of the person’s eyesight. The Snellen chart is made up of differently-sized letters beginning with largest on the top line representing a distance of 70 feet (21 metres) and ending on the tenth line with very small letters representing 4 feet (1.2 metres). To test your vision’s acuity, stand 20 feet away from the chart and read as accurately as possible the smallest line you can see. Average visual acuity is 20/20 (6/6); this means that from 20 feet away you can accurately read something from a further 20 feet’s distance represented by the sixth line down on the chart.  20/20 is only the average acuity, however. If the vision is 20/30, for example, then the person sees something at 20 feet what a person with average normal vision could see from 30 feet away, thus making the acuity slightly worse. In contrast, if the vision is 20/15 then sight is better than average as that person is able to see from 20 feet away what a person with normal vision would have to be closer at 15 feet to see.

Field of Vision

Vision is not only gauged be acuity, however, as the person’s visual field must also be taken into account. The visual field is the scope of a person’s eyesight, or peripheral vision, and it is measured by various different perimetry tests. An optometrist can do a basic confrontational test with a finger or pencil coming in from the sides of the patient’s eyes while the patient tells the doctor when the object can be seen. Lights are often employed for similar purposes with machines like the Humphrey Field Analyzer or a Goldmann field test which utilise points of light at various places on the visual field to measure the range of the person’s peripheral vision.
Both acuity and field of vision must be taken into account when defining vision as Normal, Low, or Blind.

Normal to Near-Normal Vision:

Although it is generally common knowledge that normal vision is 20/20 feet (6/6 metres), there is actually a range of ‘normal’ eyesight which varies from 20/10 to 20/25. Near-normal vision ranges from 20/25 to 20/60. All of these measurements are based off of the Snellen Vision Chart and only represent the acuity of the eye. A normal person’s vision field is generally around 170 degrees. Vision is considered normal or near-normal if it does not need correction or can be corrected through glasses, lenses, or Lasik surgery, none of which can reverse low vision or blindness. 

Low Vision (Moderate Visual Impairment to Severe Visual Impairment):

The range for moderately impaired vision goes from 20/70 to 20/200. Severely impaired vision, also referred to in the United States as ‘legal blindness,’ is designated by both the American Optometric Association and the World Health Organisation as being beyond 20/200 (3/60) on the Snellen chart or with a restricted visual field of 20 degrees or less. Although being severely visually impaired is a form of legal blindness, its range ends at 20/400 and is not yet considered complete blindness as some light is still received and processed in the eye.

Blindness: (Profound Visual Impairment, Near-total Visual Impairment, to Total Visual Impairment)

Profound visual impairment, or moderate blindness, is ranged from 20/500 to 20/1000 with 10 degrees or less of visual field. According to the American Optometric Association’s guidelines, there are other qualifications for people with blindness than acuity and degree of visual field. These are: being able to count fingers from a distance or the ability to recognise hand motion from a certain distance. For moderately blind people, their ability to count fingers should be from 10 feet (3 metres) or less.  Severe blindness or near-total visual impairment is around 20/1000 and the visual field is only 5 degrees or less. They can count fingers from 3 feet (1 metre) or less and can recognise hand motion from 15 feet (5 metres) or less. The final category is total blindness in which no light is processed or perceived at all.

Causes of Blindness>>>

<<<Blindness & Low Vision Guide