Eye Charts - How Do They Work August 22, 2007
Posted by SonjaHelga in : Vision Therapy , trackback
My son is 5 years old and has just had his first school eyesight test. We have been advised to take him to see an optician - which of course we will do but I just want to check what the following means.
The school test said that on the Snellen scale he is 6/6 (20/20) in his left eye and 6/9 (20/30) in his right eye. (This is in metric)
Can someone explain what this means and if it is anything to seriously worry about.
Herman Snellen, a Dutch ophthalmologist, designed the first eye chart, which measures visual acuity. Referred to as the Snellen Chart, it was developed in 1862 and is still used today. He set the baseline of measurement at 20 feet, the approximate distance at which light rays entering the eye are parallel and do not require our eyes to bend the rays to focus them upon the retina.Snellen set the standard of 20/20 (6/6 Metric) vision to represent “normal sight,” or what a person with normal vision could see at a distance of 20 feet. (Weber, Humphrey, and Silver, 1997)
This means that a person with 20/40 (6/12 Metric) vision must be 20 feet away to read what he or she should be able to read at 40 feet; a person with 20/85 vision must be 20 feet away to read what he or she should be able to read at 85 feet away, and so on.
This standard does not measure visual acuity at close distances. Similar charts were devised with smaller letters to read at close range for the measurement of near vision. Read the whole article here
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