Usually, when you visit an ophthalmologist for an eye test, he shows you a chart of alphabets and asks you to read them. These are called Snellen eye charts and their use in determining your eye sight is of a great deal.
Snellen Charts were first developed by the Dutch ophthalmologist Hermann Snellen in 1862.
Snellen Chart: Most of you might be knowing what a Snellen chart is. But still, you have a picture of it on the right of your screen.
A traditional Snellen chart consists of eleven lines of alphabets in varying sizes. The top most line consists of only one large alphabet and can be any alphabet out of the 26 available, like E in the figure you see. As you scroll down the chart, you can see that the number of alphabets per line increases and so does the size of the alphabets.
While testing your eyes, you are usually asked to read the letters aloud with one of your eyes closed and based on your readings, your visual acuity is determined.
Snellen Eye Charts and their Use: Snellen when designing his chart set a few parameters for deciding a person’s visual acuity. According to him, any person can recognise the alphabet on his chart, only when the chart is at an angle of 5 minutes from the person’s eye and there exists an arc of 1 minute. These parameters allow the person viewing to discriminate the spatial pattern between the lines and alphabets.
In the U.S, the Snellen charts are usually placed at a distance of 20 feet from the person’s eye. The line which says 20/20(the most visible bottom line in the chart) is the smallest line which a person can read from a distance of twenty feet. Any person who does this is supposed to be having a 20/20 visual acuity.
Snellen eye charts and their use in determining a person’s visual acuity play a very important role in many military recruitment programs. Most popularly, they help an optician prescribe the right glasses to the individual.
