A narrowband filter is either a mechanical or an electronic device used to limit the wavelength of the electromagnetic spectrum a given piece of equipment will process or display. For example, without an electronic narrowband filter, also called a band-pass filter, a radio would not be able to play only one station at a time. It would instead, play all the stations it is capable of receiving simultaneously. Narrowband filters not only see heavy use in electronics; astronomers use them to limit the light spectrum received by telescopes for the study of various heavenly bodies.
Exactly what is, or is not, a narrowband filter is often nothing more than a matter of perspective. For example, a television is only capable of processing radio waves in the band of frequencies on which television signals are broadcast. When compared to the entire range of radio frequencies available, the television frequency band is very narrow. As such, the television receiver itself is a fixed narrowband filter, which only allows the processing of frequencies in the narrow range assigned for television signals.
There are, however, many different frequency bands that a specific television signal, or station, may broadcast on in that range — 68, to be exact. Inside their receivers, televisions have adjustable narrowband filters. These filters limit the reception capabilities of the receiver to a band only as wide as used by a single television station. In relation to the entire range of frequencies the television can process, this is a narrow band of frequencies.
Further, in digital television signals, a series of fixed narrowband filters isolate the different segments of the television transmission. Signals such as audio, display, and sub-channel transmissions, which may have their own audio and display frequencies, must be filtered and isolated in order to process and display. Without electronic narrowband filters, the practical use of most electronic devices simply would not be possible.
Astronomers and photographers use mechanical narrowband filters heavily. Filters of this type are usually a colored film or lens that blocks a significant portion of the light wave spectrum. This is in contrast to devices such as sunglasses, which block a portion of all light frequencies simultaneously and relatively evenly.
When applied to cameras or telescopes, these filters allow only the desired light frequencies to enter the viewing apparatus. In this way, astronomers are able to study specific attributes of stars, planets, and nebulae that would otherwise be obscured by brighter light wave frequencies. Likewise, photographers also use these types of narrowband filters to accentuate or diminish certain frequencies and colors of light in their compositions, much in the way a painter may choose to accentuate or omit a specific color from his work.