Eye charts do not all use the same letters; many charts do not use letters at all. These charts are specifically designed tools to measure visual acuity, or the sharpness of vision, under standardized conditions. Measurement is achieved by presenting a series of symbols, or “optotypes,” that decrease in size. The selection of optotypes depends on the intended use, the patient’s age, and their literacy level.
The Classic: Snellen’s Limited Alphabet
The most recognized eye chart, developed by Dutch ophthalmologist Hermann Snellen in 1862, uses a limited set of letters from the Roman alphabet. The traditional Snellen chart uses only nine specific letters: C, D, E, F, L, O, P, T, and Z. These letters, known as optotypes, are constructed on a geometric scale based on angles of arc at a specified viewing distance.
This restricted alphabet was chosen because the characters were relatively easy to distinguish, even when blurred. The iconic “20/20” measurement relates to this chart, indicating the distance at which a person can read a line compared to what a person with normal vision can see at that same distance.
While the Snellen chart was the standard for decades due to its simplicity, it has limitations. The number of letters per row decreases toward the top, and the spacing between rows is not uniform. Furthermore, the legibility of the nine letters used is not perfectly equal, which introduces variability into the final visual acuity measurement.
The Modern Standard: The ETDRS Chart
The modern scientific standard for measuring visual acuity is the Early Treatment Diabetic Retinopathy Study (ETDRS) chart, also known as a LogMAR chart. Developed for clinical trials, this chart ensures more precise and consistent results than the Snellen chart. The ETDRS chart utilizes a set of 10 Sloan letters: C, D, H, K, N, O, R, S, V, and Z.
The design principles make the ETDRS chart the current gold standard. Every line contains five letters, ensuring a consistent number of characters for equal difficulty across all rows. The spacing between letters and rows is proportional to the size of the letters in a mathematically standardized logarithmic progression.
The 10 Sloan letters were specifically selected because they are considered “equally legible,” meaning they have similar difficulty when read at the threshold of vision. This standardization of letter difficulty and spacing reduces the variability of measurements, making the ETDRS chart more accurate for monitoring subtle changes in vision over time.
Testing Vision Without Letters
Eye care professionals use specialized symbol-based charts for patients who cannot read the Roman alphabet, such as young children or those with language barriers. One common method is the Tumbling E chart, which uses only the capital letter ‘E’ rotated in four directions. The patient points in the direction the ‘E’ is facing, transforming the test into a directional matching task rather than a literacy task.
Another technique is the HOTV chart, which limits the alphabet to four distinct letters: H, O, T, and V. This chart is often used for preliterate children, who match the letters on the wall chart to a response card they hold. This allows visual acuity to be measured without requiring the child to verbally name the letters.
For the youngest patients, Lea Symbols are frequently used, consisting of four simple, recognizable shapes: a circle, a square, a house, and an apple. These symbols are calibrated to blur equally, and children match them using a corresponding key card. This approach allows for an accurate assessment of a child’s visual acuity by eliminating the need for letter recognition or verbal communication.