User:Zzjlamb/sandbox

Size of Letters Required for Visibility as a Function of Viewing Distance and Observer Visual Acuity. U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards by Gerald L. Howett. NBS TECHNICAL NOTE 1180. July 1983. https://www.gpo.gov/.../GOVPUB-C13-ff8dc22d75e66f29ebdb2bb2085ee683.pdf

https://archive.org/details/sizeoflettersreq1180howe

formula is page 6

The formula is $$w=2d \, {\tan{{\theta} \over {2}}}$$ where $$w$$ is the optotype height or width (which are the same due to the optotype being on a square grid), $$d$$ is the distance from eye to chart, and $$\theta$$ is the angle subtended by the optotype, which is 5 arcminutes as specified by Snellen.