If you’ve ever been to the eye doctor for an eye and vision exam, you’ve likely been asked to stand 20 feet away from a funny-looking chart and read off the letters until you can’t anymore. This procedure has been in practice since the middle of the 19th century to test visual acuity, and the chart was so expertly designed that it has not changed much since. Keep reading to learn more about the eye chart most commonly used by ophthalmologists.
The History of the Snellen Chart
The most popular eye chart was conceptualized by Dutch ophthalmologist Herman Snellen in 1862. According to the Academy of Ophthalmology, Dr. Snellen developed the design of the chart after watching how his colleague Dr. Franciscus Donders used a chart in his office to help diagnose patients’ vision and determine the right lens prescription for them. Dr. Donders asked Dr. Snellen to make the chart, which resulted in the most popular poster ever created: the Snellen chart.
In the late 1800s, travel had become more accessible to the middle class and industrialization workers were finding they needed vision adjustment to perform handwork and machine jobs. This standardized chart was revolutionary for patients to have their visual acuity tested by one eyecare provider or eyeglass maker and receive consistent results anywhere they traveled. While Snellen chose to use Roman letters in the chart, it has since been translated into dozens of languages so the letters or characters can match a patient’s native language.
How the Snellen Chart Works
The Snellen chart uses a geometric scale to measure the sharpness of vision and our ability to discern the shapes and details of things we see. The chart uses 11 lines of block letters descending in size. The first line is one large letter, either E, H, or N, and the next rows have an increasing number of letters that become smaller as you work your way down. There are actually only nine letters used on the chart, known as optotypes: C, D, E, F, L, O, P, T, and Z. Our eyes are great at shape recognition and can pick up on the outline of certain letters, even if the image is blurred. This is why you will never find a Q, X, or Y on any eye chart!
Patients stand 20 feet away from the chart, cover one eye, and read the letters out loud from the top row to the bottom. The smallest row you can correctly read is how the eye doctor determines your vision accuracy. The “20/20” ratio term originates from the Snellen chart and means that one can clearly see the letters standing 20 feet away. The sizing of the letters on the chart is geometrically consistent, therefore the letters representing 20/40 are twice the size of the 20/20 letters. The Snellen chart can help your eye doctor determine any vision problems you may have and find the proper vision correction for you.
Other Charts Used by Eye Doctors
While the Snellen eye chart is used for school age and above, there’s typically a picture chart on the back to test visual acuity in preliterate children or illiterate patients. The Tumbling E chart (also developed by Dr. Snellen) and the Landolt C chart also serve a similar purpose. These charts display just one letter, either E or C, in different orientations and patients can point or move the direction of their head to demonstrate the direction they see.
Developed in 1982, the ETDRS chart is similar to the Snellen chart but is better suited for smaller rooms because patients only need to stand at a distance of four meters. The chart employs a logMAR design, with the rows of Sloan letters on the chart varying in size in a logarithmic progression. The ETDRS chart is accepted by the National Eye Institution and the Food and Drug Administration as the mandated standard for clinic eye test trials. During your next eye exam, you can ask your eye doctor about the tests they conduct for you to gain a deeper insight into the process.