Few clinical tools are still in use after 150 years – but the Snellen chart is one that endures. The chart itself, designed by Dutch ophthalmologist Herman Snellen in 1862 for the clinical measurement of visual acuity (VA) (see In brief: visual acuity charts, below), has long since been synonymous with the “eye test” in public consciousness.

Instantly recognisable, with 11 lines of block optotypes in descending size, the chart uses the line assignment scoring method, and VA is expressed as a fraction, where the numerator represents the viewing distance and the denominator denotes the smallest readable line. This format remains widely used in both everyday language and legal definitions of visual standards.

The standard distance used for the chart is 20 feet or six metres, at which distance the rays of light are almost parallel and the patient does not usually accommodate to see. The numbers obtained from all VA tests are based on “normal” visual clarity at this distance. The recommended chart luminance is 80 to 320cd/m², with a usual luminance of 160cd/m² (Caltrider et al, 2024).