The Snellen eye chart was invented by Dr. Herman Snellen in 1862. Snellen's test became the standard for vision testing because he created an original font with lines all of the same thickness and letters relatively equal in size and shape.
In the mid-1800s, optometrists used eye charts of various designs to test their patients' vision. These tests were never standardized and frequently inaccurate. The fonts used in these tests featured letters of different sizes and shapes with thick and thin lines. Such variation made it easier for patients to guess what letters were on the board, rather than providing an accurate assessment of a person's vision acuity.
Dr. Herman Snellen developed his own vision acuity chart in 1862, using block letters with lines of all the same thickness, removing the patient's ability to guess at letters. Snellen also used fractions based on the minutes of arc to determine the appropriate size of the letters on each row. The fraction of the arc is based on the patient standing a specific distance from the chart when reading it. This made it possible for the chart to be reproduced and provide accurate results anywhere, and the Snellen chart quickly became standardized.