Two smaller groups 20/25 to 20/12.5 for repetitive eye testing.
Can be wall-mounted.
Sloan optotypes were developed from the idea of Snellen optotypes. However, Sloan optotypes do not feature any serifs, making distinguishing optotypes slightly easier. Also, Sloan optotypes were chosen for their equal difficulty along with the amount of space they occupy in the Snellen 5×5 grid. Sloan letters adhere to the principles set for Snellen optotypes, but also take in new principles of total equality and difficulty.