Table 1.
WHO definitions for vision impairment
| Distance visual acuity worse than | Distance visual acuity equal to or better than | |
|---|---|---|
| 0 - no vision impairment | ||
| Snellen, metres | NA | 6/12 |
| Snellen, feet | NA | 20/40 |
| LogMAR | NA | 0·30 |
| Decimal | NA | 0·5 |
| 1 - mild vision impairment | ||
| Snellen, metres | 6/12 | 6/18 |
| Snellen, feet | 20/40 | 20/60 |
| LogMAR | 0·30 | 0·50 |
| Decimal | 5/10 (0·5) | 0·3 |
| 2 - moderate vision impairment | ||
| Snellen, metres | 6/18 | 6/60 |
| Snellen, feet | 20/60 | 20/200 |
| LogMAR | 0·50 | 1·00 |
| Decimal | 3/10 (0·3) | 0·1 |
| 3 - severe vision impairment | ||
| Snellen, metres | 6/60 | 3/60 |
| Snellen, feet | 20/200 | 20/400 |
| LogMAR | 1·00 | 1·30 |
| Decimal | 1/10 (0·1) | 0·05 |
| 4 - blindness | ||
| Snellen, metres | 3/60 | 1/60* |
| Snellen, feet | 20/400 | 5/300 |
| LogMAR | 1·30 | 1·80 |
| Decimal | 1/20 (0·05) | 0·02 |
| 5 - blindness | ||
| Snellen, metres | 1/60* | Light perception |
| Snellen, feet | 5/300 | Light perception |
| LogMAR | 1·80 | Light perception |
| Decimal | 1/50 (0·02) | Light perception |
| 6 - blindness | ||
| Test for light perception | Light perception | No light perception |
Sourced from WHO, 2019.5 There are several visual acuity chart types, which differ in the number and type of characters (optotypes), spacing, and reporting formats. The participant is asked to read down the chart, which has multiple rows with progressively smaller characters, at a specific distance. The visual acuity is the line with the smallest characters correctly identified. Snellen is expressed as a fraction: the numerator is the test distance and the denominator is the smallest line size correctly read. For each vision impairment category, the equivalent visual acuity thresholds are presented. NA=not applicable. LogMAR=Logarithm of the Minimum Angle of Resolution.
*
Or counting fingers at 1 metre.