passband

A passband (or just band) is a specification of the range and strength of wavelengths (and in effect, frequencies) allowed through a filter that cuts off both high and low wavelengths, i.e., a specification of the wavelengths allowed to pass through the filter, i.e., the filter's specified sensitivity function. The term is also sometimes used to indicate the physical filter. Filter and passband are common terms in optical telescopes (infrared through ultraviolet) while the word band is also used in radio astronomy to refer to the wavelength-sensitivity of a radio receiver and as a general term for a wavelength or frequency range.

The term is used in engineering disciplines such as communications engineering as well as astronomy. In the latter, a photometric system, e.g., the UBV photometric system, is a conventional set of such filter specifications.

Passbands and their associated filters are generally indicated by letters, but the same letter may have differing specifications under different photometric systems or different portions of the electromagnetic spectrum. Within visible light, sometimes the letters are chosen that correspond with their associated color, e.g., B for blue, R for red, etc. Some common passband letters in astronomy:

• U for ultraviolet.
• B for blue.
• V for visual.
• G for green.
• R for red.
• I for infrared.

Infrared observations from the ground are confined to atmospheric windows, the commonly-used infrared bands match a window, and the relevant band letters are also used to label the window itself. Near infrared includes bands lettered Y, J, H, K, and midinfrared, M, N, and Q. The letter Z is also used for infrared bands, but some systems use it for a near-infrared band and some for a far-infrared band.

The above passbands are commonly used, and considered basic to photometry, but for particular uses, narrower bands are used. Among the descriptive terms based upon bandwidth:

• broad band - passband width on the order of 100 nm (e.g., for a broad band filter, which applies the above-mentioned examples).
• intermediate band - on the order of 20 nm.
• narrow band - on the order of 5 nm (e.g., for a narrow band filter).

The term wide band also means a large width, but its exact usage varies.