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Visual Acuity

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.

VAdiagram

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:

 VAeq1 

which can also be written as:

 VAq2

What does it all mean?

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 fineness of the detail within the letter which tests VA. In the "E" above, the horizontal pieces of the letter and the gaps between them are all exactly one fifth of the total height, so that, if this is a 6/6 letter, the gaps (or the legs) subtend 1 minute of arc at the eye. If the patient can't see the gaps he probably will not be able to tell that this is an E, since the lines will blur together. The set of letters used in letter charts, called the optotype, is carefully designed to have elements and gaps which are just the right proportionality and size. A 6/12 line consists of letters which are twice as big as 6/6 letters, subtending 10 minutes of arc at the eye at 6 meters. 

Summary

Therefore, 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?