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Imagine stepping into the optometrist clinic for an eye exam, sitting yourself down in front of a chart with a series of letters. The unique arrangement of these letters is known as the Snellen chart, a tool used by optometrists worldwide in evaluating a patient’s vision and eye health. The deep history of the Snellen Chart begs the question of how it originated: how did the identification of letters revolutionize the optometry community? Where did the expression “20/20 vision” come from? Why does it work?
The Snellen Chart was developed by Hermen Snellen in 1862 at the height of the ocular industry as well as the Industrial Revolution, a period in which having good eyesight was crucial in order to operate machinery (in fact, this was about the same time when factory-made eyeglasses were mass-produced). Prior to its development, optometrists had their own version for patients, some using letters, simple words, or pictures of identifiable objects like flowers, wagons, or houses. Other variations played around with the orientation of certain letters, which was often used for illiterate children.
However, what makes the construction of the Chart so special is how meticulous its geometric elements are. A standard Snellen Chart has eleven rows, with each row’s letters getting progressively smaller in size. Note that not every letter is featured on it, and this is due to how the human brain can recognize the shape of some letters, like Q, where its tail creates a shadow, even if it is blurry. As a result, an accurate eye chart features the letters C, D, E, L, O, P, T, and Z. Each row is categorized with a fraction, where the numerator represents the number of feet in which the patient is standing from the Chart, and the denominator represents the number of feet a person with perfect eyesight can stand and still read the line the patient is identifying. The fractions increase numerically since the letters get smaller per row, suggesting that a person with 20/20 vision is able to read up to the eighth row. However, the remaining three rows are reserved for those with vision better than 20/20, and their vision is categorized as 20/15 or 20/10 because they are able to read beyond the eighth row!
Using the Snellen Chart is very simple: a patient is required to sit at a 20-feet distance from the chart. In smaller clinic spaces, a mirror with the Chart projected on it may also be used. Lighting is also key, and it is required for the room as well as the chart to be well-lit. Testing one eye at a time, the patient has to read the letters from top to bottom, using their most recent corrective lenses or a pinhole. As the patient continues to read, the smallest line that they can read represents their visual acuity.
Today, the Snellen Chart is a key tool in the optometrist’s office, and is often used with other tests to assist in diagnosing eye disorders. By creating the Snellen Chart, optometrists like Snellen have had universal and accessible means of assessing visual acuity.
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