The Snellen Eye Chart used by eye care professionals to determine how well a patient can see was invented in 1862 by a Dutch Ophthalmologist named Herman Snellen.
Visual acuity is typically measured by eye care professionals using a standard Snellen eye chart. The chart displays letters of progressively smaller size, with "normal" vision being measured as 20/20.
The first line of the chart consists of one very large letter which may be one of several letters, though typically it is the "big E". After the first large letter, the chart displays letters of progressively smaller size, with "normal" vision being measured as 20/20.
The letters used on an acuity chart are block letters with a particular height, width and thickness. The thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C". The height and the width of the letter is five times the thickness of the line.
Only the ten Sloan letters C, D, E, F, L, N, O, P, T, Z are used in the traditional Snellen chart.
"Normal" vision is generally indicated by 20/20. But, what does that mean? Basically, it means that the patient can see the 20/20 line of letters from a distance of 20 feet. Similarly, a patient who has 20/40 vision means that the patient sees the 20/40 line at 20 feet away while the person with the "normal" vision of 20/20 can see that same line from 40 feet away.
If you'd like to play around with making your own personalized eye chart with a personalized message, we found this fun tool online. Try it out and have some fun!
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