The Snellen Chart which has not changed since it was designed in 1862 is the only visual screening test used in practically all schools. The chart consists of rows of letters that vary in size. The largest are at the top with each succeeding row containing letters that are measurably smaller. beside each row is a number indicating the distance from the chart that the average eye can identify letters of that size.
Schubert, D. G. (1985). Out Damned Chart! Out, I Say!. Reading Horizons: A Journal of Literacy and Language Arts, 25 (4). Retrieved from https://scholarworks.wmich.edu/reading_horizons/vol25/iss4/3