13 Sep 2013

Optical forward scatter and frequency?

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 .

5 comments:

TiƩgui said...

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.

Anonymous said...

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

Alan said...

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

Anonymous said...

Here goes, this post is link-rich and will probably get tossed into the spam pile...

There are several factors involved with clear-sky atmospheric optical scattering and/or absorbtion:

Atmospheric scattering and/or absorbtion (Rayleigh scattering).

Scattering and/or absorption due to water vapor.

Scattering and/or absorbtion due to particulates.

Atmospheric scattering has been addressed by another commenter.

Scattering due to particulates depends on the mean properties of the particules such as size and shape (which define effective aperature), how efficiently the, material color, and material surface properties reflect light (which define aperture efficiency) and the wavelength of the incident light.

At different wavelengths, all else being equal particulate scattering gain is proportional to 1/lambda^2 in absolute units (10*log for dBi). So as all else being equal, you get more scattering at higher frequencies (lower wavelengths). But atmospheric absorption changes with frequency as well. The Devil is in the details.

I do remember seeing some work dome on optimizing emitter beamwidth for particulate scattering efficiency. This had something to do with the statistical distribution of the particles and particle types. So emitter beamwidth may be another factor.

I don't have anything off the top of my head on water vapor. But there may be more on all these topics in the International Telecommunications Union regualtions and publications, many of which can be freely downloaded for a limited time here:

www.itu.int

Here is a list of some ITU document's I use from time to time. This list is by no means comprehensive. There is more out there.

ITU-R P-Series, "Radiowave Propagation" Index:

http://www.itu.int/rec/R-REC-P/en

Recommendations likely to apply to C-band and Ku-band Earth-space communications links:

11 Recommendations, 104 pages...

ITU-R P.525-2 (1994-08, 3-pgs): Calculation of free-space attenuation

http://www.itu.int/rec/R-REC-P.525-2-199408-I/en

ITU-R P.581-2 (1990-06, 1-pg): The concept of "worst month"

http://www.itu.int/rec/R-REC-P.581-2-199006-I/en

ITU-R P.676-9 (2012-02, 24-pgs): Attenuation by atmospheric gases

http://www.itu.int/rec/R-REC-P.676-9-201202-I/en

ITU-R P.835-5 (2012-02, 12-pgs): Reference Standard Atmospheres

http://www.itu.int/rec/R-REC-P.835-5-201202-I/en

ITU-R P.836-4 (2009-10, 16-pgs): Water vapour: surface density and total columnar content

http://www.itu.int/rec/R-REC-P.836-4-200910-I

ITU-R P.837-6 (2012-02, 14-pgs): Characteristics of precipitation for propagation modelling

http://www.itu.int/rec/R-REC-P.837-6-201202-I

ITU-R P.838-3 (2005-03, 8-pgs): Specific attenuation model for rain for use in prediction methods

http://www.itu.int/rec/R-REC-P.838-3-200503-I/en

ITU-R P.839-3 (2001-02, 2-pgs): Rain height model for prediction methods

http://www.itu.int/rec/R-REC-P.839-3-200102-I/en

ITU-R P.840-5 (2012-02, 10-pgs): Attenuation due to clouds and fog

http://www.itu.int/rec/R-REC-P.840-5-201202-I/en

ITU-R P.841-4 (2005-03, 6-pgs): Conversion of annual statistics to worst-month statistics

http://www.itu.int/rec/R-REC-P.841-4-200503-I/en

ITU-R P.1815-1 (2009-10, 10-pgs): Differential rain attenuation

http://www.itu.int/rec/R-REC-P.1815-1-200910-I/en

73's Roger, David WB4ONA

Anonymous said...

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