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?