You people may have known about that when you visit eye hospitals or eye centers for an eye examination. Eye health professionals will check your vision, Right?? If anyone has a normal vision, that will be denoted by 6/6 or 20/20. What actually is that 6/6 or 20/20 ??
To make you clear about 6/6 and 20/20 that both terms are equivalent. 6/6 is often used in countries that use metric units while 20/20 is used in countries that use imperial units. (6m~20feet)
These numbers are actually measurements of visual acuity which determines how clear and accurate your vision is. It’s quite a simple test that, you should stand at a certain distance (6m) away from vision charts with different letters/numbers. Sometimes a 6-meter room may not be available, therefore people may arrange a mirror to reflect the letters in the vision chart to stimulate 6m distance.
There are numbers of vision charts available, and among which “Snellen” vision chart is in common use. The topmost letter in the chart is boldly followed by progressively smaller ones. This chart was developed by Dutch Ophthalmologist Dr. Hermann Snellen in 1862.
The question may arise that why visual acuity always measured from 6 meters or 20 feet distance ??
We assume that parallel rays of light coming from infinity enters our eyes and focus on the retina . (Theoretical aspect)
Furthermore, for practical aspects, we assume that rays of light are emerging from a distance of 6 meters.
Our accommodation and vergence (the converging and diverging ability of our eye) are zero at 6 m distance. Therefore, vision measured from 6 m.
In the vision chart, we can see the letter or number at the top is bold and followed by progressively smaller ones. The figure below will make you understand.
There, in the visual chart, you can see certain numbers written in fraction patterns like 6/60,6/36,… 6/6 are visual measurements. The number written in the numerator indicates at which distance you are looking that letter. Or the normal distance between the patient and vision chart. Similarly, the number in the denominator indicates that from which distance you need to see that letter.
If your vision recorded as 6/60, then you need to understand that you have to see that letter from a 60-meter distance but you are able to see that letter from a 6-meter distance only.
Similarly, if your vision recorded as 6/12, then you need to get that, the number which you are reading at a distance of 6 meters, the same number can be read by a normal person from a 12-meter distance.
Furthermore, Vision 6/6 denotes, you are reading that letter from a 6-meter distance which you have to see/read from 6 meters as well.