Visual Acuity and Snellen Chart
By gc on Jan 17, 2008
In China, the Snellen chart used to measure visual acuity (VA) are full of tumbling E, having the opening pointing to all four directions, up, down, left and right. This is reasonable, as ordinary Chinese people do not know English decades ago. I believe this is better than the one used at US, because not all of Latin characters are equally recognizable. It is obvious that there's much more difference between an H and an O than an O and a D.
Not just the chart itself, the scales of VA are different. China uses decimal number instead of 20/20 fractional system. I tried to find the way to convert the VA scale used in China to the one in US like crazy. But the searching is so time consuming. Before searching on this, I always think the search engines are so convenient to find information like this. Maybe I didn't use the right keywords. Anyway, the following table of equivalent values in different VA scales is the complete table I find, which helps us to translate any kind of chart which we may encounter.
|Row||metric||English||decimal||Jaeger (Near vision)||% visual efficiency||% of loss||Size of E||Function|
|1||6/60||20/200||0.1||J8||20%||80%||87mm||Much handicapped but still can get around on|
one's own, can can recognize people and objects
|4||6/30||20/100||0.2||J6||50%||50%||45mm||Can do routine work and look at movies. Car|
license plates are blurred. Dangerous to drive
|6||6/20||20/66.6||0.3||J4||65%||35%||30mm||Can enjoy TV, but outlines of faces and letters|
are not sharp. Can still drive a car but needs great caution
|7||6/10||20/33.3||0.6||J3||90%||10%||15mm||Can pass driving licence or seamen certificate |
Actually you don't need this table, converting the VA from 20/20 system to decimal system is easy.
20/20 should be treated as a fraction 1.0 and 20/200 = 0.1
Nevertheless, I still couldn't get all my doubts cleared. In China, the normal eye sight (standard acuity) is 1.5, not 1.0 as converted from 20/20. I don't know whether Chinese ophthalmologists have a higher standard of eye sight, or simply 1.5 scale is totally different from the decimal scale.