Perhaps some of my readers can help me - what optical frequency is likely to be best for forward scatter off air molecules and dust particles? My tests to date have been over non line-of sight paths up to about 9km at 481THz (red light), but IR should work in daylight with filtered PIN diodes on RX, but I don't know whether infra-red frequencies scatter more easily or worse than red light? With red light, the RX can easily be de-sensitised by bright sunlight, and this should be (?) less of an issue with filtered IR detectors, I think.
As the atmosphere scatters blue light very well (giving it a blue colour) one might expect IR to scatter less well than shorter wavelengths like visible red or ultra-violet? See http://en.wikipedia.org/wiki/Atmosphere_of_Earth .