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An atmospheric window is a wavelength interval of relatively high atmospheric transmission. <Infrared photometric bands> are largely chosen to lie in such windows.

Atmospheric window

The traditional <infrared photometric bands> extending the optical <photometric system> into this range are approximately
$$
\boxed{J:1.25\,\mu{\rm m},\quad H:1.65\,\mu{\rm m},\quad K:2.2\,\mu{\rm m},\quad L:3.5\,\mu{\rm m},\quad M:4.8\,\mu{\rm m}.}
$$
Their <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: $L'$ is near $3.8\,\mu{\rm m}$, and shortened $K_s$ and $M'$ 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 $1.0\,\mu{\rm m}$ is an additional choice, rather than one of the traditional $JHKLM$ extensions.

Solution

Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 338 / 1 / c / ii / Solution

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 $K_s$, $L'$ and $M'$ have different detailed responses. They largely follow <atmospheric windows>; transmission, <sky brightness>, and detector response determine practical band edges. The https://www.eso.org/instruments/isaac/doc/VLT-MAN-ESO-14100-0841_v76.pdf[ISAAC filter documentation] records instrument-specific variants, including an $L$ filter closer to $L'$ near 3.8 micrometres.

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