Sloan letters
Sloan letters, designed by Louise Sloan in 1959, are a set of optotypes used to test visual acuity generally used in Snellen charts and logMAR charts.
This set of optotypes consists of ten specially formed "letters", C, D, H, K, N, O, R, S, V and Z. These letters, unlike the ones used in older Snellen charts, are designed to give acuity testing results that are comparable to tests made using Landolt broken rings.