Snellen visual acuity

A universal measurement of visual acuity, the Snellen chart consists of text lines of diminishing size, these labelled as ‘60’, ‘36’, ‘24’, ‘18’, ‘12’, ‘9’, ‘6’, and ‘5’, the number indicating the distance in metres at which the letters would always appear to be the same size – i.e, at 6 meters the ‘6’ line would appear the same size as the ‘60’ line read at 60 metres. By convention the measurement is taken at 6 metres (20 feet in the US), and the smallest line of letters is recorded. Acuity is recorded as a/b, with ‘a’ being the distance from the chart (6 metres), and ‘b’ indicating the smallest (labelled) line that can be read. In order of increasing visual acuity, these measurements are: 6/60 (at which only the large single top letter of the chart is seen), 6/36 (the top two lines can be seen), 6/24, 6/18, 6/12, 6/9, 6/6 and 6/5. A patient with normal vision would see 6/6 or better, and, if positioned closer to the cart, would see smaller lines of text.