The most popular poster in history isn’t the Beatles at Abbey Road, a boy on a bicycle in E.T. or Einstein sticking out his tongue. It’s one we’ve all seen - more or less - at the doctor’s office: The Snellen chart.
The chart measuring visual acuity (clarity and sharpness of vision) was introduced by Herman Snellen in 1862. It was one of the first advances in the history of vision and eye health since spectacles were invented in the 1200s. It was part of a broader movement at the time that made ophthalmology - the study and treatment of disorders and diseases of the eye - the first organ-oriented specialty in medicine.1
Until the 1800s, measurements of visual acuity were vague. There are references in antiquity to gauging the sharpness of a person’s vision by their ability to distinguish double stars. In 1623, Benito Daza de Valdes, a notary in Seville, Spain, who served in the Inquisition, developed a forerunner to the Snellen chart based on mustard seeds. He measured the distance at which a person could not count a row of seeds, and then small letters. From those measurements, he determined how convex or concave to make lenses for glasses, and published a book on the topic.1
During that time, when accusations of heresy were rampant, many church members believed that eyeglasses, if not outright devil’s magic, were efforts to defeat God’s divine plan to inflict infirmities on the aged. Daza de Valdes defended spectacles against those who considered them a heresy:2
The next person to try to create a standardised eye test was Heinrich Küchler, a German ophthalmologist who developed an eye chart consisting of common symbols - farm implements, cannons, birds, frogs - which he glued to a sheet of paper in decreasing size. A few years later, he modified his chart to use letters of the alphabet in a graduated series. It was published in 1843, and nearly forgotten.1
In 1854, Edward Jaeger von Jaxtthal, a Viennese ophthalmologist, published a series of reading samples. Since Vienna was an international city, he did them in several languages, and they spread. Because of varying typefaces, the vision tests were not standardised, and they also couldn’t be used to test distance vision.1
Meanwhile Francis Cornelius Donders from the Netherlands, who had devoted his life to ophthalmology, realised there was a need for much better standardised visual acuity measurement and testing. Donders - along with colleagues Sir William Bowman, Herman von Helmholz and Albrecht von Graefe - created the specialty of ophthalmology. Donders was working on studies on refraction (the way eyes bend light for clear vision) and accommodation (the way eyes adjust to focus in distance or closeness). He created a formula to define the sharpness of vision by defining a “standard eye”, and then calculated the magnification each person needed to reach the same level of clear vision performance as the standard eye. The degree of magnification required was a measurement of visual acuity - for example, if a patient needed two times the standard, the visual acuity was calculated at half the standard, or 0.5.1
Donders turned to Herman Snellen, a physician co-worker also from the Netherlands, to find a measurement scale that would help implement his formula. Snellen began working on charts, first using abstract images and symbols. In 1862 he settled on a 5x5 grid using stylised letters and numbers, which he called “optotypes", as a more practical way to communicate between the patient and their doctor - and so the familiar eye chart test we all know was born.1
However, there were some challenges - the original Snellen chart was only based on letters used in the English alphabet which was not practical for those who could not read, or were not familiar with the English language. In 1868, Snellen devised a chart that could be used with people who were illiterate - the rotating “E”. That same year, Dr John Green of St. Louis, USA, who had previously worked with Snellen, improved the chart by making the progression in letter size more regular and suggesting the use of an easier to read font type.1
Thanks to all of these individuals, their collaboration and innovation, the Snellen eye chart has become one of the most common visual performance tests of the 20th century. Today it is accepted as the standard of accurate visual acuity measurement, making it one of the most revolutionary contributions to ophthalmology to date.