The Eye Test Chart - also known as the Snellen Chart
Professor David Gartry explains
We often measure the standard of vision achieved with or without glasses using the familiar eye test (Snellen) chart, which is viewed from a distance of 6 metres (20 feet). It should be noted that this is not a particularly accurate way of recording visual standards – other methods are used in laboratory and research settings. The high contrast lettering on a Snellen chart is also unrepresentative of more difficult visual tasks, such as driving at night or in conditions of poor visibility.
The top line on the chart is designated 6/60. The second line up from the bottom (on most charts) is 6/6 and represents ‘normal’ sight – commonly known as 20/20 vision, particularly in the USA, as 6 metres is equivalent to 20 feet. In the UK, the minimum standard of vision permissible for driving is between 6/9 and 6/12, equating to the ability to read a number plate from a distance of 20 metres (20.5 metres for older-style plates). The Royal Air Force requires its pilots to have an unaided vision of 6/5 in each eye (usually the bottom line on the chart).