>Do you know of any kind of formula to estimate the proper proportions for >seeing high or distant dial lines, letters etc. clearly? I suspect >architects, engineers and advertisers (billboards) must have some way of >knowing what will be clear to a given eye at a given distance.
The best the average corrected eye can do is about a minute of arc. That's with very high contrast like truly black lines on a truly white background. For low contrast objects, you'd want more like 3 minutes of arc. I'm talking about spacing of lines here where the line width and the space width are equal. One example which might help would be the Snellen eye test chart. The lines in the letter E for 20-20 vision (when it's the correct distance away - no cheating) are 1 minute and the spaces are 1 minute so this has a 2 minute spacing. With corrected vision, that letter E should be easy. So all in all, I think I would try a minimum of 3 or 4 minute spacing because not everyone's been to his optometrist or ophthalmologist recently. Of course if you have the space, by all means do increase the size for more legibility. Also to make it faster to read - no one has to puzzle over it. As far as isolated lines, that's another thing. Here you just need high contrast and some width. As an example of very high contrast, think of power lines against a bright sky. You can see them for miles even though the angle they subtend is well below what your eye is "diffraction limited" at. So seeing an isolated line is strongly a matter of contrast as well as angle subtended. I'd still go for a minute or two of angle even with high contrast, though. John B
