Snellen Chart Errors

Although widely available in EDs, the Snellen chart method of acuity measurement is prone to error. One survey found that the chart distance and illumination varied enormously [8].

It is important that the Snellen chart is separately illuminated as, for those with poor vision, increasing illumination may improve accuracy by up to one line [9]. The recommended illumination of 480 lux [10] can be achieved by directing a spotlight at the chart.

Also, technical problems occur due to the progressive increase in the number of letters down the chart. This leaves patients with poor acuity fewer letters to identify and a ‘visual crowding’ of letters lower down the
chart [11].

These problems have been addressed by the development of newer charts, e.g. LogMAR, with identical numbers of letters on each line and greater accuracy of measurement [12].

LogMAR chart

Learning bite

When measuring visual acuity it is vital that the chart is the correct distance away from the patient and independently illuminated.