An Etymological Dictionary of Astronomy and Astrophysics
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1) Three → atmospheric windows in the → near infrared portion of the → electromagnetic spectrum at 1.25 (J), 1.65 (H), and 2.20 (K) μm.
2) An extension of Johnson's → photometric system into near infrared using filters corresponding to the atmospheric windows J (1.25 μm), H (1.65 μm), and K (2.20 μm), with → bandwidths 0.3 μm, 0.4 μm, and 0.6 μm respectively.

Letters of alphabet, used conventionally; → system.