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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:
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:
The term wide band also means a large width, but its exact usage (i.e., the criteria for declaring it "wide") varies.
The passbands of filters are sufficiently consistent to produce scientifically-useful observation data, such as filtered magnitudes consistent within a tenth of a magnitude. However, their characterization by a mid-frequency (or wavelength) and a FWHM only loosely describes a sensitivity function, and furthermore, actual filters only loosely match either of these: it is a challenge to manufacture filters consistently: they (slightly) vary according to manufacturing process, and even by manufacturing run. Furthermore, specifications of the passbands themselves evolve with time: an improved manufacturing process that produces more consistent filters certainly produces some differences in the sensitivity function and may even affect the mid-frequency and the FWHM. There can be scientific reasons to adjust particular values as well. The letter names generally give an overfall idea of where the passband falls, but these may vary according to particular photometric systems, and also according to the actual filter used. (The response of sensors, such as CCDs, now also affects the resulting observation data, as has the response of photographic film or plates.) In some cases, differences associated with observations using (slightly) differing filters can be accommodated through application of methods to calculate magnitudes of one passband from those of another; SDSS has used such conversions to enhance consistency of its data products.