[Laser] Voice communication using 940nm IR LED in full daylight

[Chuck Hast]

Oh yes, to get directly to the point regarding 940 nm vs 850 nm. 940 falls
right in the middle of some nasty water lines. If you had a very stable source
you MIGHT be able to slip in between them but I would either go above
1000 nm where it gets very good again or down to 850 which is also very
good (and cheap). According to the pdf "Optical sensors for the free space
optical link" they settled on a 850nm link due to cost and availability. The
attenuation through clear atmostphere is 0.9 db/km. If you can scrounge
some 1550 nm hardware it drops to 0.35 db/km. Your diurnal background
radiation will be a lot higher at about 850 and 940, it really drops down
nicely at 1550 nm. Here is the link to the pdf again.

lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol7003/70031M.pdf


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Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."
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