The Snellen visual acuity test is a widely recognized method for measuring the sharpness of a person’s distance vision. Dutch ophthalmologist Herman Snellen created the chart in 1862, establishing a standardized way to assess how clearly an individual sees. This simple, non-invasive screening tool uses a chart with rows of letters, known as optotypes, to quickly identify potential vision problems. The test helps determine if a person may need corrective lenses.
Preparing the Testing Environment
The accuracy of the Snellen test depends significantly on maintaining a precise testing distance and consistent lighting. The standard required distance between the subject and the chart is 20 feet (or 6 meters). This fixed separation is the basis for the scoring system.
If a room is not long enough, a common method is to use a mirror to reflect the chart, effectively doubling the light’s travel distance. Alternatively, a proportionally scaled chart can be used at a shorter distance, such as 10 feet, provided it is designed for that reduced range. The chart should be placed on the wall at the subject’s eye level to prevent straining.
Proper illumination is required for reliable results, as dim or uneven light can make the letters appear blurred. The chart must be uniformly lit, ideally with a recommended luminance between 80 and 320 candelas per square meter (cd/m²). Ensure there is no distracting glare or shadows across the chart that could interfere with reading the characters.
Step-by-Step Administration
The administration begins by having the subject stand or sit at the 20-foot distance from the chart. The subject covers one eye, using an eye patch or a clean hand, taking care not to apply pressure to the eyeball. Testing is performed on one eye at a time, starting with the right eye, then the left, and finally with both eyes open.
The administrator asks the subject to read the letters aloud, starting at the top line and moving down to the smallest line they can clearly distinguish. Each row of letters is an optotype line and corresponds to a specific visual acuity score. The administrator monitors the subject to ensure they are not squinting or leaning forward.
The test for a specific eye stops when the subject misses more than half of the letters on a given line. The result is recorded as the fraction associated with the smallest line on which the subject correctly identified the majority of the letters. For example, if the subject reads the entire 20/40 line but misses three or more letters on the 20/30 line, the acuity is recorded as 20/40 for that eye.
Interpreting Visual Acuity Scores
The recorded score is expressed as a fraction, such as 20/40 or 20/200, representing the subject’s visual acuity. The numerator, typically 20 in the United States, represents the fixed distance in feet from the chart. The denominator indicates the distance in feet at which a person with standardized “normal” vision could read the same line of letters.
Vision designated as 20/20 is considered the standard for normal visual acuity. This means the subject can clearly read at 20 feet what a person with normal vision can also read at 20 feet. A score of 20/40 means the subject sees at 20 feet only what a person with normal vision could see clearly from 40 feet away. The larger the denominator, the less sharp the distance vision.
Results below 20/40 are often used as a benchmark indicating the need for professional follow-up with an eye care specialist. A significantly lower score, such as 20/200, indicates a greater reduction in visual sharpness and is often used as a definition for legal blindness in the United States. The Snellen score provides a quantifiable measure of visual sharpness that guides eye health care.