Reading at a Distance

[John Carmichael]

Hello all,   Here is a simple but a very practical question that I'm sure many sundial designers have asked themselves.  I've never heard anybody discuss this topic before.  Since I've been designing more wall dials lately, this question often comes up:   How big should a sundial's hour numerals be so that a person with normal vision can read them at a known distance?   I am wondering if there might exist a table of letter readability based on distances and letter size.  Wouldn't sign makers need to know this?   For example, if you are designing a sundial for a tall tower and you know that it must be readable from a distance of 100 meters, how tall should the numerals be?   I suppose one could do experiments to come up with a table of numeral size based on distance, but I bet that such a table exists somewhere.    Does anybody have any thoughts on this or knows if this information exists?   thanks,   John