On Wed, Dec 16, 2009 at 5:33 PM, Padu Merloti wrote: > 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 sure it doesn't, given you're not trying to resolve the LED, but see the light it gives off, which will spread out quite nicely over the distances you're talking of, even assuming a tight optic. On which subject, if you just want it to be visible from a certain direction, then a good optic will vastly increase the brightness in a given direction. I'd imagine your basic calculation would first determine what brightness level you need to be visible in an absolute sense. Then determine the brightness of your LED at 1m and apply the inverse squared law, along with an allowance for atmospheric attenuation, which you will have some of even assuming good clear conditions - there must be standard rules of thumb for that. Chris -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist