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 .
13 Sept 2013
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4 comments:
Hello, Roger!
I have connected an IR photodiode to a uA galvanometer and an visible ligth photodiode too.
If I remember well both diodes gived similar current with day ligth.
73 from Thierry.
Scattering of light by molecules & particles (also known as Rayleigh Scattering) is inversely proportional to lambda to the 4th. Theoretically I would expect IR to be scattered less than red light as the wavelength is longer. Never tried it practically though!
Pete
Distant white lights don't appear to have much of a colour shift, slight "yellowing" perhaps indicating blue loss over 20-30Km paths, an experiment waiting to be made.
Red certainly worked well according to the latest Radcom.
Enjoy your blog Roger!
Alan G8LCO
Why not try scattering from the higher metallic layers?
The Sodium (589nm)layer is the most known, but because of the use of sodium vapour lights, may be least useful to you.. However, there are Iron (386nm), Potassium (770nm) and Calcium+ layers (393.4nm) too (all from ablated meteorites) around 60 miles height. The Iron and Calcium ions would seem to be perhaps the most promising.. you can google for
"metallic mesosphere layers" for many papers on this subject, G6AIG
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