The most popular poster in history isn’t the Beatles at Abbey Road, Farrah Fawcett in a red swimsuit, Che Guevara, 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, introduced by Herman Snellen in 1862, was one of the first advances in optometry since spectacles were invented in the 1200s. It was part of a broader movement at the time that made ophthalmology the first organ-oriented specialty in medicine.
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 row of seeds could not be counted, and also small letters. From those measurements, he determined how convex or concave to make lenses for glasses, and published a book on the topic. During that time, when accusations of heresy were rampant, many church members believed that eyeglasses, if not outright devil’s magic, were audacious 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: “The invention of eyeglasses appears to have come from heaven, from where the eyes also came.”
The next person to try to create a standardized 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.
In 1864, 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 standardized, and they also couldn’t be used to test distance vision.
Meantime, Francis Cornelius Donders, from the Netherlands, who had devoted his life to ophthalmology, and privately founded an Eye Infirmary for the Indigent in London, realized there was a need for much better standardized visual acuity measurement and testing. Donders, along with colleagues Sir William Bowman, Herman von Helmholz (who invented the ophthalmoscope), and Albrecht von Graefe, had created the specialty of ophthalmology. Donders was working on studies on refraction and accommodation, and had created a formula to define sharpness of vision. He defined a “standard eye” and then compared the patient’s performance to that of a standard eye, and the magnification the patient needed to reach the same performance as the standard. The reciprocal of the magnification the patient needed was his measurement of visual acuity, so if the patient needed two times standard, his visual acuity was half standard, or .5.
Donders turned to Herman Snellen, a co-worker, to help measure the standard to implement his formula. Snellen was also a physician from the Netherlands. He began working on charts, first using abstract images, then settling on letters as more practical to communicate. He designed stylized letters on a 5x5 grid, which he called “optotypes.” Snellen used the standard of 20 feet to test distance. The familiar “20/20” ratio is the measurement of how many chart letters the patient could read at a specific number of feet. In regular terms, “20/20” means that at 20 feet, a patient can read a letter that most people should be able to read at 20 feet. He defined this reference standard more precisely as the ability to recognize one of his optotypes when it subtends an angle of 5 minutes of arc. “20/20” is not perfect vision, but standard, which explains why some people have “20/15” or less.
The original Snellen chart used the English alphabet, which only worked if people could read English. 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, who had done a fellowship with Snellen, improved the chart by making the progression in letter sizes more regular. In Snellen’s chart, they varied from 14 percent to 100 percent. Green created a uniform geometric progression of 25 percent for each line. He also suggested a non-serif font (clean type without curlicues or flags, like Helvetica) with proportional spacing. His non-serif suggestion was widely ignored until 1958, when Louise Sloan, of Johns Hopkins University, successfully removed the sarifs. She also created the term “M unit,” representing the denominator of a metric Snellen fraction. Instead of a measurement of how much a letter “subtends 5 minutes of arc at x meters,” the measurement is much simpler, it is x M units. In this system, a letter size of 1M means 20/20 when shown at 1 meter, 2M letter is 20/20 at 2 meters, etc.
While Snellen gets all the renown for the chart, he was actually implementing Donders’ formula, but Donders, a humble man, was never much interested in taking credit. By the time Snellen’s chart had come out, he’d moved on to studying the effects of age on visual acuity. `QED Mark`
Photo courtesy of the American Academy of Ophthalmology's Museum of Vision.