Snellen's test type

The letters on the familiar eye chart used to measure how well you see at various distances.

The eye chart, known as Snellen's chart, is imprinted with block letters that line-by-line decrease in size, corresponding to the distance at which that line of letters is normally visible.

The block letters, Snellen's test type, are quite scientific in design (so that at the appropriate distance each letter subtends a visual angle of 5 degrees and each component part subtends an angle of 1 minute).

The chart and the letters are named for a 19th-century Dutch ophthalmologist Hermann Snellen (1834-1908) who came up with them as a test of visual acuity.

Visual acuity refers to the clarity or clearness of the vision, a measure of how well a person sees.

The word 'acuity' comes from the Latin 'acuitas' = sharpness.