What is the Snellen Chart?

The Snellen chart is an eye chart that eye care professionals use to measure how clearly a person can see. The clarity of a person’s eyesight is known as visual acuity and it can provide doctors with useful information. For example, the progression of certain eye conditions can be determined by changes in visual acuity. Being able to accurately measure visual acuity is also important in other aspects of life, such as obtaining a driver’s license. The Snellen eye chart is a simple method of measuring vision that dates back to the early 19th century.

Q: When was it created?

A: The Snellen eye chart was originally created in 1862. Its creator was a Dutch ophthalmologist and professor by the name of Hermann Snellen.

Q: Who is the chart named after?

A: The Snellen chart, or Snellen eye chart, is named after the Dutch ophthalmologist, eye surgeon and professor Hermann Snellen. Snellen, who received his medical degree in 1857, created the chart along with the associated optotypes in the early 1860s.

Q: Who was Hermann Snellen?

A: Hermann Snellen was a Dutch ophthalmologist who was born on February 19, 1834. He was a former pupil and co-worker of ophthalmologist and scientist Franciscus Cornelis Donder, who himself made many advancements in the field of ophthalmology. Donder needed a standardized method of testing distance visual acuity in order to accurately determine refractive error. He encouraged Snellen to create a testing method that he could use. As a result, in 1862 Snellen created what was to become the Snellen eye chart. In addition to creating the chart, he also created the optotypes, or symbols, that are used for visual testing. In 1877 he became a professor of ophthalmology at Utrecht University. In 1884 he took over Donder’s position as director of the Netherlands Hospital for Eye Patients, where he worked until 1903. Snellen died in 1908.

  • History of Visual Acuity Measurement: A PDF that discusses visual acuity testing and its importance. Discusses key figures of the “Golden Age of Ophthalmology,” such as Franciscus Donder. Reviews Donder’s relationship with Snellen and his influence in creating the Snellen chart.

Q: What does it consist of?

A: A Snellen chart consists of dark, square-shaped capital letters on a white background. In modern eye practices, eye charts may be projected so that they are seen through a series of mirrors. They may also be in the form of wall-mounted or hanging charts. At the top of the chart there is typically a large “E.” Below that letter, there are rows of capital letters, with each row progressively smaller than the previous one. In some cases, there may be numbers or pictures in place of letters. Some charts may consist of the letter “E” in decreasing sizes, pointing in different directions. This is called the “tumbling E” chart and is used for people who have difficulty speaking, but can point to indicate which direction the “E” is facing.

  • The Examination Sequence: Discusses visual acuity assessments and charts used for testing. Reviews how the Snellen chart is composed.

Q: What is optotype?

A: An optotype is a numeric or alphabetical symbol that is used for measuring vision. These numbers are typically black and have a box-like shape that is five times larger than the strokes that form it. Optotypes were created by renowned ophthalmologist Herman Snellen (1834 – 1908), who also created the Snellen eye chart. Optotypes are not always numbers or letters. For children who are unable to recognize numbers and letters, optotypes may be easily identifiable shapes, such as a duck, horse or telephone.

  • Visual Acuity Standards: A PDF that summarizes the Snellen eye chart and optotypes. It also covers variations and changes to the chart.

Q:How does it measure vision?

A: The Snellen eye chart measures a person’s vision at a distance of 20 feet, one eye at a time. The patient is asked to sit or stand in front of the chart at a distance of 20 feet. In most optometery and ophthalmology practices, 20 feet is simulated by having the patient look through mirrors. The patient is then asked to cover one eye and to read the smallest line possible. The smallest line on the chart may represent 20/5 vision which is excellent vision, and the largest number represents 20/200 or 20/400, which is considered legally blind. When interpreting the numbers, the first number is the test distance of 20 feet. The second number is the distance that a person with “normal vision” can see the eye chart. For the purpose of this using this chart “normal vision” is equal to 20/20 vision. This means that if a patient can read the 20/30 line on a Snellen chart, a person with 20/20 “normal” vision can read the same line when standing further back at 30 feet.