Atmospheric Absorption

Just as sunlight is scattered by smoke, infrared energy is absorbed by various chemical compounds found in the atmosphere. However, there are several "windows" in the spectrum where this effect is minimized. The graph below demonstrates why the most common bands of interest are the 3 to 5 and 8 to 12 µm regions. These two bands have the least atmospheric absorption under standard atmospheric conditions. The presence of high humidity , clouds, fog or smoke can dramatically influence IR absorption. 

Transmission of Infrared Energy at S.L.
Path Length = 6000 ft.