Difference between revisions of Snellen chart
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→Definition of the numbers: add that the details require 1 arc-minute resolution.
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(→Definition of the numbers: add that the details require 1 arc-minute resolution.) |
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==Definition of the numbers== | ==Definition of the numbers== | ||
The | 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 | * 20/20 means 5 arc-minutes at 20 feet | ||
* 20/16 means 4 arc-minutes at 20 feet | * 20/16 means 4 arc-minutes at 20 feet | ||
* 6/12 means 10 arc-minutes at 6m | * 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== | ==How to use a Snellen Chart to measure visual acuity== | ||