The Snellen chart

This chart is named after a Dutch ophthalmologist (1834-1908). It consists of a series of letters of diminishing size as in Figure 19.1a. Each letter has an overall size five times the thickness of the lines composing the letter.

The results of the test are expressed as a fraction, in which the numerator is the testing distance (6 m) and the denominator is the distance at which a normal observer would be able to read the letter. Other test types, which are designed to be applicable to any nationality and to illiterate subjects, as in Figure 19.1b & c, follow the same principle.

The test types should be clearly printed, legible and uniformly illuminated. The patient should be sat at six metres, or at three metres using a reverse test type placed above the patient's head and observed as a reflection in a mirror hung on the opposite wall.

A new design of visual acuity chart has recently emerged: an example, the Bailey-Lovie chart (Bailey & Lovie 1976), is shown in Figure 19.2. This improved design has several advantages. First, the letters used are of approximately equal detectability, whereas earlier charts had some letters that were more legible than others. Second, each line has an equal

Figure 19.2 The Bailey-Lovie Chart

number of letters, as compared with earlier charts with only one or two letters for the 6/60 and 6/36 visual acuity lines, and a large number of letters for the 6/6 and smaller visual acuity lines. Third, the spacing between letters is proportional to the letter size, while the older acuity charts had unequal spacing between letters. Finally, the change in visual acuity from one line to another is in equal logarithmic steps, where there were very small changes for different lines at the small-letter end and rather large changes for the big-letter end of the older charts. This new eye chart permits more precise definition of visual acuity, especially at levels of diminished visual acuity. It is thus used in the current UK national study of photodynamic therapy in age related macular degeneration (TAP study group 1999; Bames et al. 2004). In addition, new methods of scoring responses to this type of visual acuity chart can provide greater sensitivity and reliability of measure. The ETDRS visual acuity chart is based on the design of this new eye chart.

One of the most important factors in testing visual acuity is the human interface. The tester must be competent at ensuring full and precise communication of what the subject can see. Patients should always be tested with their most updated glasses or with a pin hole.

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