Snellen Eye Chart

What is Snellen Eye Chart?

The Snellen Eye Chart is a tool used by eye care professionals to measure visual acuity. It is named after Dutch ophthalmologist Hermann Snellen, who developed the chart in 1862. The chart consists of multiple lines of letters or symbols, with each line decreasing in size. By reading the letters or identifying the symbols on the chart from a specific distance, an eye doctor can determine the sharpness of a person's vision.

What are the types of Snellen Eye Chart?

There are several types of Snellen Eye Charts available, each designed for specific purposes. The most common type is the standard Snellen Eye Chart, which is used to assess visual acuity at a distance of 20 feet. However, there are also charts available for testing near vision and for use with non-English-speaking individuals. Additionally, modern technology has allowed for the development of digital versions of the Snellen Eye Chart, which can be displayed on screens or printed out.

Standard Snellen Eye Chart
Near vision Snellen Eye Chart
Non-English Snellen Eye Chart
Digital Snellen Eye Chart

How to complete Snellen Eye Chart

Completing a Snellen Eye Chart requires the following steps:

01
Stand or sit the recommended distance away from the chart (usually 20 feet for the standard chart).
02
Cover one eye with a suitable occluder (e.g., palm of hand, eye patch).
03
Read or identify the letters or symbols on each line of the chart from top to bottom.
04
Note the line at which the individual can read or identify at least 50% of the letters or symbols correctly.
05
Repeat the process with the other eye.
06
Record the results for each eye separately, noting the distance at which the individual achieved 50% accuracy.

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Video Tutorial How to Fill Out Snellen Eye Chart

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Questions & answers

The visual acuity is then recorded as Test distance (m)/Letter size read (M units). For example, if the patient could read the 32 line at 1 meter, their acuity would be recorded as 1/32. 1/32 is equal to a Snellen acuity of 20/640. You can use Snellen equivalent numbers with a LogMAR chart.
Recording Snellen Results Top number equates to the distance (in metres) at which the test chart was presented (usually 6m), Bottom number identifies the position on the chart of the smallest line read by the 'patient'. Eg. 6/60 means the subject can only see the top letter when viewed at 6m.
If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can only see what a normal human can see when standing 40 feet from the chart.
With 20/20 vision, the top number represents the distance away from the object (eye chart) – 20 feet – and the bottom number represents the distance at which a person with average vision or can read the same line.
Eye caregivers use numbers and letters on a chart at a given distance to measure visual acuity, typically the Snellen Eye chart. If you can read the second line on the chart from 20 feet away, you have a 20/100 visual acuity.
20/20 vision refers to “normal” vision, not “perfect” vision. Visual acuity is a term that means clarity or sharpness of vision and that the objects you see are crisply outlined and not blurry. Calling normal vision “20/20 vision” is true for eye care professionals in the U.S., but not everywhere in the world.