The type used in the Snellen eye chart is not a font, but a typeface designed by Herman Snellen. There have been other adaptations of the chart, but they are also not a font.
When Snellen invented his chart, he called the black letters on a white background "optotypes." They are slightly square in shape with the lines of the figures are designed as five times thicker than the strokes that form it. Snellen designed his optotypes using serifs, but sans-serif charts and charts that do not use the Roman alphabet are also popular. One of these charts is known as the "tumbling E" chart and is meant for people who might be illiterate or unfamiliar with the Roman alphabet.