The Eye Chart, or Snellen Eye Chart, is a chart used to measure visual acuity to detect refractive errors and amblyopia. While Snellen Charts often contain English alphabets, our charts use block numbers instead. This is because often times, Thai students under the age of 8 often are not yet literate in English. The numbers in Eye Charts become progressively smaller down the rows. In order to qualify for normal vision, a student must be able to read at least half the numbers in every row of the chart until reaching the 20/30 line.