Most of us have had to undergo an eye exam. We are told to cover one eye and repeat which letters we see in front of us. While we've mindlessly read the alphabet soup in front of us, we usually don't think about the meaning or history of those letters.
What exactly is the Chart?
The chart was developed by a Dutch ophthalmologist named Herman Snellen in 1862 to measure the decrease of vision in diabetes patients. Now it is used to test what is called visual acuity, or the accuracy of vision.
What do the Letters Mean?
The letters used on a traditional Snellen chart are C, D, E, F, L, O, P, T and Z. These were picked because they were easiest to distinguish and patients often find them easier to tell apart. The top letter, usually an E, stands at 88mm with each letter gradually decreasing in size and increasing in number underneath them.
How its Used
In the United States, a patient stands 20 feet away from the chart and usually covers one eye. If you wear eyeglasses, you will probably be asked to read the Snellen chart with and without your corrective lenses. The top line, roughly represents vision at 20/200, meaning that most people with 20/20 vision can see the top letter (typically an E) at 200 ft. The vision field halves and it goes down initially, and then begins to decrease by increments of 30, then 10. The chart usually has one letter at the 20/200, two at 20/100, three at 20/70 and so on.
Better than 20/20?
"Perfect 20/20 vision" is, in fact, not perfect. 20/20 vision was an idea of perfection established by Dr. Snellen. The chart has two lines below 20/20, the 20/15 and 20/10. Someone under the age of 60 years with perfect vision should be able to read those lines without much difficulty. It is only as people age that the perfect target vision becomes 20/20. Although most humans can only see about 20/10, some animals (such as hawks) can see at a 20/2 level.
With technology, we are aware now that the Snellen chart is not the last word in measuring people's eyes. It has come under attack several times as many people have come to memorize the chart if they go to the eye doctor often. In addition, it has been labeled confusing for children and older individuals who get mixed up as the letters decrease in size but increase in number. Now, we have several ways not only to measure eye function, but to randomize the Snellen chart if necessary. A new logMAR chart is being produced so that the letters can be shown at varying distances. However, the Snellen chart is still a favorite and standard among many doctors and professionals today.