The eye chart, most commonly the Snellen chart, is a standardized tool used by eye care professionals to measure visual acuity, which is the sharpness or clarity of vision at a distance. Developed in 1862 by Dutch ophthalmologist Herman Snellen, the chart consists of rows of block letters, or “optotypes,” that decrease in size. This progressive reduction allows a precise method for assessing how well a person can distinguish fine details.
Understanding the Visual Acuity Fraction
Visual acuity measurements are typically expressed as a fraction, such as 20/40 or 20/20, which is the core result of the Snellen test. In the United States, the numerator represents the standard distance at which the test is performed, which is usually 20 feet. This distance is used because at 20 feet, the light rays entering the eye are nearly parallel, minimizing the need for the eye’s focusing muscles to work.
The denominator of the fraction signifies the distance at which a person with “normal” visual acuity can clearly read that same line of letters. For instance, a result of 20/40 means that the patient can see at 20 feet what a person with normal vision could see from 40 feet away. This indicates that the patient needs the letters to be twice as large to read them at the standard distance.
Conversely, a fraction like 20/15 suggests vision that is sharper than the defined norm. This means the patient can read at 20 feet a line that a person with standard acuity could only read from 15 feet away. The fraction provides a clear, mathematical comparison between a patient’s sight and the established standard for distance vision.
Standardized Letter Scaling and Size
The physical size of the letters on the Snellen chart is determined by a precise mathematical and optical standard. Snellen designed his letters, known as optotypes, based on a 5×5 unit grid. For a letter to be recognized, its entire height and width must subtend a visual angle of five minutes of arc at the nodal point of the eye.
Furthermore, the thickness of the lines forming the letter, or the gaps between them, must subtend an angle of one minute of arc. This Minimum Angle of Resolution (MAR) is the smallest detail the human eye can resolve, and it is the foundational scientific principle behind the chart’s design. The letters are constructed so that the ability to recognize them depends only on the sharpness of vision, not on their specific shape.
For the line representing 20/20 visual acuity, the standard for normal vision, the entire letter must subtend five minutes of arc when viewed from 20 feet away. This geometric requirement translates to a letter height of approximately 8.87 millimeters (mm). The size of the letters changes geometrically; for example, the letters on the 20/40 line are exactly double the size of the 20/20 letters.
Different Types of Acuity Charts
While the Snellen chart is the most recognized tool for measuring distance visual acuity, other charts are used to accommodate different patient needs or for specialized testing. The LogMAR chart, which stands for Logarithm of the Minimum Angle of Resolution, is often considered the gold standard for clinical research. This chart features a standardized design with five letters on every line and a logarithmic progression of letter sizes, which provides a more statistically precise measurement of acuity.
For patients who are pre-literate, non-verbal, or unfamiliar with the Roman alphabet, alternative optotypes are necessary. The Tumbling E chart addresses this by using only the capital letter E, which is presented in four different orientations. The patient indicates the direction the “fingers” of the E are pointing, making the test a direction-matching task rather than a letter-recognition task.
For assessing a person’s ability to see objects up close, a near-vision chart is used, such as the Jaeger chart. This test uses paragraphs of text in progressively smaller font sizes to evaluate reading vision. It is typically held about 14 inches from the patient’s eyes.