Atmosphere and IR windows

The atmosphere and infrared windows

The earth’s atmosphere is thankfully transparent in the visible region but this is an exception because gases and water vapour strongly absorb at different wavelengths. The graph below shows the atmospheric windows in the visible and infrared. The dotted lines are the thermal emission spectra for bodies at 5700°C and 20°C. The atmosphere divides the spectra into windows with names as shown. The solar data corresponds to a bright sunlit day of approximately 17500 lux. This was chosen because it roughly matched the photon flux from a 20°C body. Absorption is mostly by water vapour so the windows can get pinched in when the atmosphere is moist. This graph is for 1km of atmosphere and 1 cm of precipitated water.

The near-infrared, NIR, is sometimes used for short-range wildlife filming. An artificial source of NIR is used to covertly illuminate the subject and a special camera captures the images. The thermal infrared starts with the medium waveband (MWIR) between 3.2 and 5.0 µm with a carbon dioxide notch around 4.3 µm. Most high performance cameras used in wildlife documentaries use this window because the contrast is high. For each degree of temperature the photon flux rises by about 4%. The only other window used by thermal imaging cameras is the long waveband (LWIR) from 7.7 to 14 µm. There are high performance cameras for this window but they are more difficult to make and therefore more expensive. Most cameras operating in the LWIR are so-called uncooled cameras which rely on the heating effect of the infrared energy on a membrane (very much like a pit viper). These cameras need to operate where the energy is at a maximum in the LWIR. They also need to use fast optics (typically F1.2) and that limits the range for wildlife imaging. However most infrared adaptations are tuned to the LWIR so uncooled cameras are very useful for studying thermo-regulation and infrared sensing in mammals and birds.