# LW Infrared Trasmissivity of Air

1. Mar 10, 2014

### ALm865

Hi All,

I am after an equation/table of that describes the Transmissivity of Air in the 7.5-13.5um region.

I understand that Beer–Lambert's law says that the relative intensities of a beam in a medium varies exponentially with distance and particle density, but you need to know the attenuation coefficient use those equations.

The attenuation coefficient appears to be made up of a scattering and absorption component. I've read (correct me if I'm wrong) that the scattering component can be ignored in air and the absorption is then equal to emissivity due to Kirchhoff's Law.

I also understand that emissivity changes at different wavelengths and may have a complex component (which I think I can ignore because I think it relates to scattering/dispersion?)

How would I go about this problem? There are computer packages such as MODTRAN but they seem way too complex for what I am after and I haven't the faintest idea on how to use them.

All I'm after is an equation, or a look-up table to calculate the Transmissivity of air under certain conditions (i.e. Humidity, Distance, Air Temperature/Density)

In addition, how do I account for relative humidity, and increased percentages of other gasses such as CO2?

2. Mar 10, 2014

### Andy Resnick

You do realize that you are describing MODTRAN/LOTRAN/HITRAN?

http://www.astro.virginia.edu/~mfs4n/ir/atmtrans.html
http://gs.flir.com/uploads/file/tech-notes/tech note13 - ir spectral bands.pdf

3. Mar 10, 2014

### ALm865

Ah Damn, i was hoping the problem would be more simple than diving into something MODTRAN. I was hoping since I was choosing a specific wavelength I could get away with a few constants.

I've been looking into FLIR's GEV Demo code here: http://support.flir.com/SwDownload/Assets/ThermoVision/PvSample.zip [Broken] and they do compensate for humidity and distance in their temperature calculations (see PvSimpleUISampleDlg.cpp and functions doCalcAtmTao, doCalcK2, and doCalcK1) but they use what looks to be a polynomial to describe relative humidity. It looks to me like a curve of best fit that they've calculated somehow (probably using MODTRAN or equivalent).

Can anyone suggest any free and easy to use alternatives to MODTRAN? I've gone through a few on the list but I haven't got the faintest idea on whether they are only relevant for satellites or also can be used at sea level? A lot of them talk about typical atmosphere from satellites to the ground and include factors such as cloud cover, and gasses. I'm only after sea level calcs. http://en.wikipedia.org/wiki/List_of_atmospheric_radiative_transfer_codes

Last edited by a moderator: May 6, 2017
4. Mar 10, 2014

### ALm865

Also, the reason I'm trying to find an alternative to MODTRAN is because you have to buy a license to use MODTRAN.