Atmospheric transmission at infrared wavelengths

Traditionally the infrared region of the spectrum was considered to be wavelengths just longer than the reddest colours visible to the human eye, typically about 7000Å or (0.7$\mu$m). However, in modern astronomical usage wavelengths up to about 10,000Å (or 1$\mu$m) tend to be described as `optical' and longer wavelengths as `infrared'. This terminology reflects both a change in the type of detector used and the behaviour of the terrestrial atmosphere at about 10,000Å. It is cumbersome to quote infrared wavelengths in Ångström and they are usually given in micron. The infrared region stretches to about 350$\mu$m; longer wavelengths are referred to as the sub-millimetre part of the spectrum.

The terrestrial atmosphere is largely opaque at most infrared wavelengths longer than 1$\mu$m, largely due to absorption by water vapour and carbon dioxide. However, there are a series of wavelength ranges or windows where the atmosphere is mostly transparent (see Figure ). Ground-based infrared observations must be made in these windows. Each window corresponds to a band in the $JHKLM$ system (see Section ) and the corresponding band and window have the same name.

Figure: Atmospheric transmission at infrared wavelengths. The windows where the atmosphere is largely transparent are marked. The short-wavelength windows correspond to bands in the $JHKLM$ photometric system. Note the additional $N$, $Q$ and $Z$ windows at longer wavelengths. Adapted from Allen[2]

The width, maximum transmission and, to an extent, central wavelength of the windows vary with geographical location and in particular with altitude. They also vary with the current meteorological conditions. The windows are particularly sensitive to the amount of water vapour, which is why infrared observatories tend to be at high, dry sites. The variation in the infrared windows at different locations is the underlying reason for different versions of the $JHKLM$ system being used at different observatories, as mentioned in Section .