You will be asked to read the lowest line of letters you can see clearly. This helps the optometrist find out how well you see, and also – if you need glasses – how they will work best for you.
The chart will either hang on the wall opposite where you sit, or behind you so that you look at its reflection in a mirror. It may be a fixed chart or it may change to vary the letters shown. It may be on a computer screen or on a screen where the letters are illuminated.
You may also come across charts which use only one letter presented in multiple directions, usually a C, an E or an F.
The standard distance between the patient and the chart is six metres, or 20 feet. However, if space is limited, optometrists can create that distance by reflecting the letters in a mirror.
Letter chart history
The inventor of the modern test chart was Dr Herman Snellen (1834-1908), a Dutch physician working in Utrecht. Before he set to work on the problem the patient was usually asked to read from a passage of text held in his or her hand, or held by the doctor or optician at a greater distance away.
Snellen recognised that this was unsatisfactory, because there was no standardised size or design to the lettering that might be used. Passages using complicated vocabulary or sentence structure could also give a misleading result, or the patient could guess the next word from the context of the sentence.
So in 1862 Snellen published his volume of ‘Optotypes’, which, for the first time, included characters that he thought were suitable for use on testing charts. It had single letters or numbers in addition to the more familiar reading tests.
The patient had to name each character individually and letters that might be easy to confuse were deliberately placed near to each other on the chart. The same charts could be used at various distances, but there was no provision for standard illumination, which in any case would have been impossible in the 1860s. Snellen’s work was translated into many languages within a few years of its introduction.
Snellen’s Optotypes are not identical to the test letters used today. They were printed in an ‘Egyptian Paragon’ font and were of a serif style: that is they included ornamental cross strokes at the ends of each limb. Today you will normally have non-serif letters which are considered to be clearer.
The first transparent test chart was devised in 1893 and was intended to be attached to the surface of a window in good daylight. It was reflected into a mirror to avoid glare. The first proper text chart with its own built-in light was designed in 1897. Lit by a gas light, it had a short working life as an instrument, because the light source bleached the chart and turned other parts of it yellow!
In more recent years there has been a move to create electronic charts including the British-designed Test Chart 2000, which was the world’s first Windows-based computerised test chart. It overcame several difficult issues such as screen contrast, and offered the chance to change the letters often, so people don’t ‘learn’ them.