filter

In astronomy, optical filters are used with telescopes to limit the spectrum reaching a sensor, by which an image over a more-limited range of electromagnetic radiation can be produced. A sharp cutoff at some precise wavelength is typically unachievable, so their sensitivity function generally shows a hill-like (or somewhat bell-curve-like) peak. They are used for visible light and adjacent regions of the electromagnetic spectrum (within the infrared and ultraviolet bands).

The term passband can be thought of as the specification of such a filter, but the two terms are often treated almost as synonyms, and often the letter identifying the band is used to refer to the filter or to the intensity of an object (such as a star) seen through the filter. The definition and standardization of passbands (photometric systems) similarly applies to filters for those passbands.

There are also filters for higher energy EMR, e.g., X-rays.