What font is in an Eye Chart?
This "font" caught my eye (pun fully intended with no apologies).
The real name of the chart that I had pictured is a Snellen Chart. This concept was invented in 1862 by a Dutch Opthamologist, Herman Snellen.
The letters in these charts are not referred to as a font, but as an optotype. The difference lies in the application of the letters - these letters (or any set of symbols, for that matter) are interpreted in such a way as to assess the visual acuity of the subject. A font can be derived from such optotypes.
There are specific design values (or medical requirements, if you will) for these letters and optotypes. From wikipedia again:
They have a particular, simple geometry in which:
the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
the height and width of the optotype (letter) is five times the thickness of the line.
These charts (and the optotypes used) have evolved over the 150 years since invention, with the latest iteration of the traditional letter-type chart using optotypes named "Sloan letters".