A Snellen chart is an eye chart used to measure visual acuity by determining the level of visual detail that a person can discriminate [1]. Developed by Dutch ophthalmologist Herman Snellen in 1862, the chart consists of 11 rows of block letters, known as “optotypes,” which decrease in size as you move down the chart. The traditional Snellen chart begins with a large letter (typically E) at the top, followed by rows containing an increasing number of letters that become progressively smaller [2]. These optotypes aren’t ordinary typographic letters but specially designed characters constructed according to strict geometric rules on a 5×5 grid.
The standard Snellen chart uses only nine letters: C, D, E, F, L, O, P, T, and Z. Each letter is precisely constructed with specific proportions—the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter “C”. The height and width of each optotype are exactly five times the thickness of its line. This exacting design ensures consistency and standardisation in measuring visual acuity across different clinical settings.
Why is a Snellen Chart Used?
The primary purpose of the Snellen chart is to measure visual acuity—the clarity and sharpness of vision. This measurement helps eye care professionals identify vision problems such as myopia (short-sightedness), hyperopia (long-sightedness), and astigmatism. The standardised assessment provided by the chart serves as a crucial first step in determining whether a person requires corrective lenses or other interventions to improve their vision.
The Snellen chart became particularly valuable during the Industrial Revolution when proper vision was vital for operating machinery, and the ocular industry went through its own revolution. Before Snellen developed his standardised chart, every ophthalmologist used their own particular testing method, making consistent vision assessment impossible. Some charts included words to read, while others showed pictures of items like flowers, wagons, or houses in varying sizes. This lack of standardisation made it difficult to prescribe appropriate corrective lenses or track changes in visual function over time.
According to Precision Vision, Dr. Snellen’s contribution to vision testing was revolutionary, comparable to what the first automobile engineers did for the car industry5. His chart provided an objective, repeatable reference standard during a time when ophthalmology was becoming the first organ-based medical specialty, often referred to as “The Golden Age of Ophthalmology”. The standardisation provided by the Snellen chart allowed patients to carry consistent prescriptions from any eye care provider to any eyeglass maker, ensuring uniform results.
How a Snellen Chart Works
During a standard vision test using the Snellen chart, the patient is positioned at a distance of 6 metres (20 feet in the United States) from the chart. This specific distance is crucial for the test’s accuracy, as it determines the angular size of the optotypes relative to the viewer’s eye. The patient covers one eye and reads aloud the letters on each row, beginning at the top with the largest letter. The process continues down the chart to progressively smaller rows until the patient can no longer correctly identify the letters. The smallest row that can be read accurately indicates the visual acuity for that eye. The test is then repeated with the other eye to obtain a complete assessment.
The Snellen chart measures vision using a fraction such as 6/6 (or 20/20 in the U.S.), where the numerator represents the testing distance (6 metres) and the denominator represents the distance at which a person with normal vision could read that particular line. At exactly 6 metres from the patient, the letters on the 6/6 line should subtend 5 minutes of arc, with individual letter components subtending 1 minute of arc. This translates to these letters being precisely 8.73 mm tall, while the topmost (6/60) “E” should measure 87.3 mm in height [3].
Snellen defined “standard vision” as the ability to recognise one of his optotypes when it subtended 5 minutes of arc, meaning the observer could discriminate a spatial pattern separated by a visual angle of one minute of arc. This definition established the benchmark for normal visual acuity that remains the standard today, though research has shown that human eyes typically have higher acuity potential.
Understanding Your Results
When interpreting Snellen chart results, the fraction provided (such as 6/6 or 20/20) offers a comparative measurement of your vision against a standardised norm. For example, if your vision is measured as 6/12 (equivalent to 20/40 in the U.S. system), this means you need to stand at 6 metres to read what a person with standard vision could read from 12 metres away. In approximate terms, this could be described as having “half” the normal acuity of 6/6.
At the more severe end of the spectrum, 6/60 (or 20/200) vision is considered the threshold for legal blindness in many jurisdictions. A person with this level of acuity can only clearly see at 6 metres what someone with normal vision could see from 60 metres away.
It’s important to understand that a Snellen chart measurement is a screening tool, not a comprehensive diagnosis of eye health. As noted by researchers at StatPearls, while the Snellen chart serves as a portable tool to quickly assess monocular and binocular visual acuity, it should be part of a more comprehensive eye examination [4]. Various factors including lighting conditions, chart design, and individual fluctuations can influence test results. Always consult with an eye care professional for a definitive assessment of your vision and eye health.
Variations of Snellen Charts
Several variations of the Snellen chart have been developed to address specific testing needs and overcome certain limitations of the original design. Shortly after creating his standard chart, Snellen himself developed the “Tumbling E” chart, which features the letter E oriented in different directions. This alternative is particularly valuable for testing young children, illiterate individuals, or patients who cannot communicate verbally, as they can simply indicate the direction the E is facing using hand gestures rather than naming letters.
For international use, Snellen charts have been translated into dozens of different languages so the letters/characters match the patient’s native language. Alternative symbols that can be used for illiterate adults or in countries not familiar with the Roman alphabet include the Landolt C or broken ring test.
Modern adaptations include digital versions of the Snellen chart. Wall-mounted illuminated charts improve visibility and standardise lighting conditions, which is crucial for consistent testing4. According to British Standards Institution specifications (BS 4274-1:2003), “the luminance of the presentation shall be uniform and not less than 120 cd/m²” with variations across the test chart not exceeding 20%.
In 1976, Bailey and Lovie proposed a new layout, replacing the rectangular chart with a triangular one featuring five proportionally spaced letters on each line. In 1982, the National Eye Institute combined the logarithmic progression proposed by Dr. John Green in 1868 with specific letters and the Bailey-Lovie layout to create the ETDRS charts, now considered the gold standard for accurate visual acuity measurement in research settings.