|Purpose||Snellen chart is used to estimate visual acuity|
A Snellen chart is a standard method of measuring visual acuity. A chart is rated for the distance it should be viewed at, and the lowest line that can be read has a visual acuity result number next to it. The standard distance for Snellen charts is 20 feet or 6 meters. Smaller versions are available for use in smaller indoor spaces. Generally, most myopes can be corrected to the 20/20 line unless they have visual acuity problems other than myopia. There is no direct correlation between visual acuity and myopia. The Snellen chart is the most commonly used way of testing if someone has sufficient corrected visual acuity to drive legally. Your optometrist will use the Snellen chart as a reference, to see what refraction will allow you to read the lowest on the chart.
Definition of the numbers
The acuity is expressed as the ratio of the distance (usually 20ft or 6m but can be less) to the letter size, expressed as the distance at which the letter subtends an angle of 5 arc-minutes. (Or equivalently, as multiples of the height at which the letter subtends 5 arc-minutes at a distance of 1 unit of distance). 5 arc-minutes just means 5/60 of a degree. So
- 20/20 means 5 arc-minutes at 20 feet
- 20/16 means 4 arc-minutes at 20 feet
- 6/12 means 10 arc-minutes at 6m
The critical gaps that need to be resolved are 1 arc-minute, so that's really what defines the required acuity.
How to use a Snellen Chart to measure visual acuity
- Read the documentation that came with your chart to determine what distance it is designed to be viewed at. This is usually 10 feet, 20 feet, 3 m or 6m. It may say right on the chart, on the back, or in the description of the product where you got it.
- Post the chart at eye level
- Make a mark on the floor at the distance the chart is rated for.
- Stand so that your face is above that line.
- Read the lowest line you can see clearly.
- Attempt to read the next line down.
- Have an assistant tell you if you got the letters right or move forward to check. If you got more than half of the letters right on that line, then the ratio marked for that line is your visual acuity.