Check out the standard instrument approach (IFR) runway identify lights, 
VASI (Visual approach slope indicator), etc.  construction and 
brightness. You should be able to find videos of planes making 
approaches from some distance and altitude out. At around 5 miles out 
height above ground level might be 2200'. About a 3 degree angle if I 
remember correctly. You don't say what the application is, but from 
5000', even with visual flight conditions (VFR) you might not see any 
light from even a bright light.

Padu Merloti wrote:
> My problem is simple: given a certain LED spec, how far will that LED be
> visible without any other relevant obstacles (fog, pollution, etc) or
> obfuscations (think dark night). Assume the LED is being observed at its
> most bright angle.
>
>  
>
> Does it depend solely on its brightness? I know our eyes have a certain
> resolution limitation and a regular dot the size of a LED would surely not
> be visible at great distances (1-3 miles for example), but I'm not sure the
> same principle applies if the dot irradiates light.
>
>  
>
> I'm designing a sign to be visible from an airplane that could be anywhere
> from 5000 to 15000 ft. The idea is to make an array of LED's, but I wanted
> to make some calculations before actually doing any trial and error.
> Disregarding proportions, the array would look like this, where LED columns
> would be 1 meter apart and rows about 3 inches from each other:
>
>  
>
> .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
>
> .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
>
> .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .
>
>  
>
>  
>
> Cheers!
>
>  
>
> Padu
>
>   
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