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Since Snellen first presented his test types in 1862, numerous attempts have been made to improve them. His figures subtended an angle of 5 minutes, each element subtending an angle of 1 minute. Oblong letters, 5 minutes in height and 4 in breadth, have been suggested. Easily recognized confusion and difficult letters have been used. Lines, dots and concentric circles have had their trial. The recognition of the capital E has been used, especially for illiterates. In order to exclude the large psychologic factor, Landolt devised the broken circle as a measure of two point separability. This has subsequently been accepted as the international standard for tests of visual acuity. Broken squares of different sizes have also been used.
Time and experience have left only two types for routine office and clinic practice, the Snellen figures and Arabic numerals of a similar size.
It was felt that a combination of
Waugh DD. A COMBINATION OF THE SNELLEN AND THE LANDOLT TEST TYPES. Arch Ophthalmol. 1939;21(4):671–672. doi:10.1001/archopht.1939.00860040109011