info@shroffeye.orgBlindness means the inability to see, but there are various degrees of blindness. It does not necessarily mean that one cannot see anything at all!
Also, ‘blindness’ causes can be reversible and irreversible; the commonest reversible cause being Cataract (over 60%) and about 20% of cases even spectacle refractive errors come in this terminology but can be corrected with prescription glasses..
Someone who is legally blind has a corrected vision of 20/200 or 6/60 in their best seeing eye, after correction with glasses, which simply means that despite best efforts with prescription glasses, vision does not improve beyond 20/200 or 6/60.
The standard definitions from the World Health Organization (WHO) are listed in the table below. This definition is internationally accepted and incorporated into the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10).
WORLD HEALTH ORGANIZATION (WHO) DEFINITION
| Category | Worse than: | Equal to or better than: |
| Visual impairment – Mild or none (Score: 0) |
6/18 20/60 |
|
| Visual impairment – Moderate (Score: 1) |
6/18 20/60 |
6/60 20/200 |
| Visual impairment – Severe (Score: 2) |
6/60 20/200 |
3/60 20/400 |
| Blindness (Score: 3) | 3/60 20/400 |
1/60 20/1200 Counting fingers at 1 meter |
| Blindness (Score: 4) | 1/60 20/1200 Counting fingers at 1 meter |
Light perception |
| Blindness (Score: 5) | No light perception | |
These criteria are based upon testing of the vision with the Snellen chart.
With the standard Snellen chart, vision is tested at a distance of 6 meters or 20 feet. The ‘normal’ visual acuity is 6/6 or 20/20.
Terms to understand:
6/6: seeing at 6 meters what the average person sees at 6 meters
20/20: seeing at 20 feet what the average person sees at 20 feet
6/18: seeing at 6 meters what the average person sees at 18 meters
20/60: seeing at 20 feet what the average person sees at 60 feet
6/60: seeing at 6 meters what the average person sees at 60 meters
20/200: seeing at 20 feet what the average person sees at 200 feet
3/60: seeing at 3 meters what the average person sees at 60 meters
20/400: seeing at 20 feet what the average person sees at 400 feet
1/60: seeing at 1 meters what the average person sees at 60 meters
20/1200: seeing at 20 feet what the average person sees at 1200 feet
When the vision very poor that one cannot read the Vision Chart at all, then the estimation of vision is done with counting fingers at 1 meter. One or two fingers are placed 1 meter in front of the patient, and the patient is asked to count the number of fingers that are shown.
Light perception is used when vision is worse than counting fingers. A bright light with a torch is shone on to the eye and the patient is asked if any light is noticed. The eye is considered to be completely not functional if it cannot perceive any light that is directly shone at it.
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