U

The letter U (or U band) is commonly used to indicate a passband located in the ultraviolet range (U for ultraviolet), typically for one located not far from the visible light range. The letter is also often used for a filter for the band (or more explicitly, a U filter), and to indicate a magnitude of a signal observed through such a filter, e.g., citing "U = 1.5". In the conventional UBV photometric system and its offshoots, the U band is centered around wavelength 365 nm, with a width of 66 nm (full width at half maximum). Other systems may have different specifications, in some cases, similar.