J band

Within astronomy, the term J band refers to an atmospheric window within the near-infrared range centered on 1.22 microns (1220 nm) or 240 THz, and a photometric system band aimed at this range, e.g., with a full width at half maximum of 230 nm. In addition to infrared-specific telescopes, many optical telescopes can observe in this range. The letter is also used to indicate the magnitude of the signal observed through the filter, e.g., citing "J = 1.5". It is incorporated into the 2MASS JHK photometric system, as well as other infrared photometric systems. Note that there is a little bit of variation in such bands regarding the central wavelength and the FWHM between different systems and surveys, but versions of the J band are generally designed to observe through the J atmospheric window.

The term J band also has other meanings outside astronomy, one being a radio communications band, which is nowhere near the above infrared band.