## What is Visual Acuity?Visual Acuity (or VA) is a measure of the finest detail a person can see. While there are many different types of VA tests, the most commonly used clinical VA measure is letter reading on a Snellen chart. There are several different letter chart designs in use, but the principle is the same for all. The person being tested is asked to read progressively smaller letters until he or she can no longer correctly identify them. Unfortunately, while the concept of VA is simple, the terminology is extremely obscure and confusing. Someone who can just read the 6/6 line of letters, but not the smaller 6/4.5 line, is said to have "6/6 vision." 6/6 vision is the same as 20/20 vision. The difference in notation is due to the American use of imperial units (20 ft) and the rest of the world's metric preference (6 meters). ## A little mathematics (just a little!)The definition of a 6/6 letter is very specific: it is a letter which subtends an angle of 5 minutes of arc (5/60ths of a degree) at the eye, and whose smallest distinguishing detail subtends 1 minute of arc (1/60th of a degree) at the eye. Here "d" is the testing distance (from the patient to the chart), "h" is the height of the letter, and "a" is the angle subtended by the letter at the eye. From trigonometry we get the relationship between them as:
which can also be written as:
This means that for the "E" in the diagram above to be a 6/6 letter for a person viewing it 6 meters (20 ft) away, it must be (6 m)x(tan 5/60) = 0.00873 meters, or 8.73 millimeters, tall. Remember, though, that it's not just the size of the letter but the ## SummaryTherefore, a person with 6/12 vision has worse vision than the person who can read 6/6. He needs to be within 5 meters, for example, to be able to see what someone with 6/6 vision can see from 10 meters. Similarly, someone with 6/30 vision needs letters five times larger than 6/6 size to see them. Someone with 6/3 vision can make out letters which are only half the size of the 6/6 letters from the same distance. Got it? |