Snellen Chart

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. [9]

Ensuring proper lighting for both the room and the Snellen chart is essential. The recommended chart luminance should be 80 to 320 cd/m². [8]

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. [10]

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. [11]

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.