The Big E
Anyone who has been into an optometrist’s practice is familiar with, and has probably experienced, the chart with the ‘big E’, which has become synonymous with vision and visual assessment. This is the Snellen chart, which was produced in 1862 by a Dutch ophthalmologist, Dr Herman Snellen, at a time when visual acuity was assessed using whatever reading material was available.
Dr Snellen wanted to be able to test his patients against an objective standard which could be repeated over time. To provide a standard, he designed seven rows of stylised letters which were printed progressively smaller as one moved down the chart. There were variations of this, and it took a century for the development of a generally accepted measurement of visual acuity. Today, as one recognises the contribution Snellen made to visual testing, the Snellen chart is universally accepted and used, although there are many variants of it.
The traditional Snellen chart is printed with eleven lines of block letters. The first line has one very large letter, which may be one of several letters, for example E, H, or N. All the rows that follow have increasing numbers of letters that progressively decrease in size. Different charts may have a different number of lines and may vary in the size progression. The patient taking the test has one eye covered while he reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that eye.
Using Snellen’s basic principles, the traditional Snellen chart has been adapted to accommodate a number of different situations. For patients who do not read English, the chart has been printed in languages including Russian, Arabic, Hebrew and others. Children who have not yet learnt to read or patients who are illiterate are tested on charts which require them to indicate the direction certain letters (e.g. E, C) are facing, or which have an arrangement of dots, or charts with silhouettes of animals, all of which get progressively smaller as one moves down the chart.
Perhaps the most creative adaptation of the Snellen chart was reported by a South African optometrist over 50 years ago. While conducting a clinic in a remote rural area, this optometrist came across a man of advanced age with extremely poor vision due to cataracts. Because of his age, surgery was out of the question, but the optometrist wanted to improve the patient’s vision as much as was possible.
The challenge was how to test an aged illiterate man who spoke and understood no English, but a solution was found! An assistant was sent to the fruit vendor across the road and instructed to hold up various pieces of fruit for the patient to identify. He moved further and further away until an estimated measure of visual acuity was established. After this “trial by fruit”, the old man was given a pair of glasses which helped him to see better, and the optometrist was able to continue with more conventional testing! One wonders how Dr Snellen would have reacted to this innovative modification of his chart.