Poor vision, often referred to clinically as visual impairment or low vision, is defined by specific criteria related to how clearly a person can see and their range of peripheral vision, even after using corrective lenses or undergoing treatment.
Understanding Visual Impairment Categories
While "poor vision" is a general term, standardized definitions help classify the severity of visual impairment. The World Health Organization (WHO) provides widely accepted criteria that distinguish between "low vision" and "blindness."
World Health Organization (WHO) Definitions
According to the reference provided, the World Health Organization defines categories of visual impairment based on visual acuity and visual field, evaluated with the best possible correction.
Here's a breakdown based on the WHO criteria:
| Category | Visual Acuity (with best correction) | Visual Field (with best correction) |
|---|---|---|
| Low Vision | Between 20/70 and 20/400 | 20 degrees or less |
| Blindness | Worse than 20/400 | 10 degrees or less |
Note: These definitions apply after the person has used their best possible correction, such as glasses, contact lenses, or had surgery.
Breaking Down the Numbers
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Visual Acuity (e.g., 20/70, 20/400): This common measurement indicates how clearly you see from a distance. The first number (20) represents the distance you are standing from the eye chart (20 feet). The second number indicates the distance at which a person with normal vision could read the same line. For example:
- 20/70: Means you must be 20 feet away from something to see it as clearly as a person with normal vision could see it from 70 feet away. This falls into the low vision range.
- 20/400: Means you must be 20 feet away to see something as clearly as a person with normal vision could see it from 400 feet away. Visual acuity between 20/70 and 20/400 is considered low vision. Visual acuity worse than 20/400 is classified as blindness.
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Visual Field: This refers to the entire area a person can see without moving their head or eyes. A limited visual field can make it difficult to navigate or detect things in peripheral vision, even if central vision (acuity) is relatively good.
- A visual field of 20 degrees or less is classified as low vision.
- A visual field of 10 degrees or less is classified as blindness.
In summary, when discussing "poor vision," it typically refers to having visual acuity between 20/70 and 20/400, with the best possible correction, or a visual field of 20 degrees or less, as defined by the WHO for "low vision." More severe impairment, with acuity worse than 20/400 or a field of 10 degrees or less, is defined as "blindness."
