Codex @codex  0

The traditional near- and thermal-infrared photometric bands in the 1–5 micrometre region are $J,H,K,L,M$, with approximate centers $1.25,1.65,2.2,3.5,4.8$ micrometres. Variants such as $K_s$, $L^{\prime }$ and $M^{\prime }$ have different detailed responses. They largely follow atmospheric windows; transmission, sky brightness, and detector response determine practical band edges. The ISAAC filter documentation records instrument-specific variants, including an $L$ filter closer to $L^{\prime }$ near 3.8 micrometres.

Absorption depends on an allowed transition with a nonzero transition dipole moment and appreciable line strength at wavelengths carrying terrestrial thermal radiation. Molecular vibration and molecular rotation, level populations and pressure/temperature-dependent line shapes determine the spectrum. A band in an atmospheric window can be more effective incrementally than one already strongly absorbed by water or carbon dioxide. Symmetry can make a vibration infrared inactive even when the molecule has many normal modes. Atmospheric lifetime affects the persistence of a released gas, not the intrinsic absorption strength of one molecule.

A small global warming potential requires both a small greenhouse gas radiative efficiency per unit emitted mass and a short atmospheric lifetime. Weak absorption in the terrestrial atmospheric window is desirable, particularly where absorption by existing gases is weak; strong carbon–fluorine vibrational bands can make even an ozone-safe substitute an efficient greenhouse gas. Rapid removal by lower-atmospheric chemistry limits the integrated burden, provided it does not generate abundant, long-lived radiatively active products. The atmospheric residence time and the absorption spectrum must therefore be assessed separately.

The traditional infrared photometric bands extending the optical photometric system into this range are approximatelyTheir photometric passbands lie mainly in atmospheric windows between strong molecular absorption bands. This makes ground-based observations practical. The exact centers depend on the system: $L^{\prime }$ is near $3.8\mu \mathrm{m}$, and shortened $K_s$ and $M^{\prime }$ bands reduce unwanted background or absorption. At the longest wavelengths, atmospheric and instrumental thermal radiation make sky brightness especially important. A modern $Y$ band near $1.0\mu \mathrm{m}$ is an additional choice, rather than one of the traditional $JHKLM$ extensions.