LW Infrared Trasmissivity of Air

[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?

 

[Andy Resnick]

<snip>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?

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

In any case, google is your friend:

http://www.astro.virginia.edu/~mfs4n/ir/atmtrans.html
http://books.google.com/books/about/LWIR_Long_Wavelentth_Infra_Red_Atmospher.html?id=Iaa7tgAACAAJ
http://gs.flir.com/uploads/file/tech-notes/tech note13 - ir spectral bands.pdf

 

[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 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

Thanks in advance

 

[ALm865]

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

 

[sardar]

Hello, thank you for your question. The transmissivity of air in the 7.5-13.5um region can be calculated using the following equation:

T = e^-αL

where T is the transmissivity, α is the attenuation coefficient, and L is the distance the beam travels through the air.

The attenuation coefficient can be calculated using the following equation:

α = αs + αa

where αs is the scattering component and αa is the absorption component.

As you mentioned, the scattering component can be ignored in air, so the absorption component is equal to the emissivity of the air due to Kirchhoff's Law. The emissivity of air can be calculated using the following equation:

ε = 1 - R

where R is the reflectivity of air, which can be assumed to be very low in the infrared region.

To account for relative humidity, you can use the following equation to calculate the absorption component:

αa = f(ε, RH)

where f is a function that takes into account the emissivity of air and relative humidity.

To account for increased percentages of other gases such as CO2, you can use the following equation:

αa = f(ε, RH, CO2)

where f is a function that takes into account the emissivity of air, relative humidity, and the concentration of CO2.

There are also look-up tables available for the transmissivity of air under different conditions, such as humidity and temperature. These tables can be found in various references and publications.

I hope this helps answer your question. Let me know if you need any further clarification or assistance.