B

The letter B is commonly used to designate a blue passband or a corresponding filter. Without further qualification, B is most often intended to mean the blue passband/filter of the UBV photometric system, in which it is centered around wavelength 445 nm, and has a width of 94 nm (full width at half maximum). This falls in the blue range of the spectrum, its width ranging from violet to green. Other systems may have different specifications, in some cases, similar. The letter is also used to indicate the magnitude of the signal observed through the filter, e.g., citing "B = 1.5".