Sloan letters
Encyclopedia
Sloan letters, designed by Louise Sloan in 1959, are a set of optotype
Optotype
An optotype is a standardized symbol for testing vision. Optotypes can be specially shaped letters, numbers, or geometric symbols. For instance, to determine visual acuity, optotypes of different sizes are presented to a person and the smallest size is determined at which the person can reliably...

s used to test visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....

 generally used in Snellen chart
Snellen chart
A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart during 1862...

s and logMAR chart
LogMAR chart
LogMAR charts are used to assess an individual's visual acuity . LogMAR means the logarithm of the Minimum Angle of Resolution. It is recommended that whenever research on visual acuity has to be done, the LogMAR chart is to be used...

s.

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
Landolt C
A Landolt C, also known as a Japanese Vision Test, Landolt ring or Landolt broken ring, is an optotype, i.e. a standardized symbol used for testing vision. It was developed by the Swiss-born ophthalmologist Edmund Landolt....

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Computer fonts for Macintosh and Windows operating systems are available for research purposes. The fonts are based on Louise Sloan’s design, which has been designated the US standard for acuity testing by the National Academy of Sciences, National Research Council, Committee on Vision (1980, Adv Ophthalmol, 41, 103-148).
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