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An eye chart test is a type of vision test typically conducted by an optometrist and uses an eye chart or Snellen chart. This type of test is often utilized for situations in which general vision is being tested to determine how well a person can see. Other forms of tests are often used to indicate precise visual acuity, usually for determining a prescription for corrective lenses. An eye chart test consists of a person standing a set distance away from the eye chart and reading the letters on the chart. The smallest size he or she can read indicates how well he or she can see.
Often referred to as a vision test, an eye chart test primarily concerns the use of an eye chart to gauge how well a person can see. An eye chart, also called a Snellen chart after Hermann Snellen who created it, usually consists of a large white paper or poster with a series of black letters on it. These letters are arranged in rows from the top of the chart to the bottom. The top row consists of a single, very large letter, while the bottom row is made up of many much smaller letters.
Someone taking an eye chart test begins the test a set distance away from the chart itself, which is usually posted on a wall. In the US, this distance is 20 feet (about 6.1 m), while in the UK the distance is 6 meters (about 19.7 feet). When the required distance is not available, a chart that has each letter reversed can be used, and someone can look at the chart using a mirror to create the required distance in reflected space. The person taking the eye chart test then begins reading each letter, usually beginning at the top of the chart with the largest letter and moving down the chart reading each row.
An eye chart test indicates a person’s visual acuity based on the bottommost row that he or she can read. The lowest row a person with standard vision can clearly read is marked as 20/20, indicating that at a distance of 20 feet (about 6.1 m) he or she can read it. In the UK and other areas that use metric measurements, this line is marked 6/6 to indicate the same idea in meters. Those letters that are twice the size of this average row are marked 20/40. These letters can be read during an eye chart test by someone with average vision up to 40 feet (about 12.2 m) away, but someone with 20/40 vision must be closer, at 20 feet (about 6.1 m), to read them effectively.
Similarly, someone who takes an eye chart test and scores 20/100 would have very poor vision that requires a distance of no more than 20 feet (about 6.1 m) to read something that someone with average vision could read 100 feet (almost 30.5 m) away. A score of 20/200, or 6/60 in the UK, typically indicates that someone is legally blind. For someone to be considered legally blind, however, he or she would need to score this poorly while using corrective lenses.