• In optical astronomy, a device for selecting a wavelength of radiation, e.g. to allow only yellow light or only light of a particular spectral line to be observed. Optical filters can be made to allow light from a range of just a few Ã…ngstroms to be passed, although this means losing almost all the light received, so that solar astronomers, who have lots of energy to play with, are the main users of these very narrow filters. In radio astronomy, filter effects are obtained by tuning receivers to record a wider or narrower range of radiation.
  • That which serves to selectively allow matter and/or energy to pass, remain, be blocked, or get absorbed.
  • An electric circuit or device which selectively transmits or rejects signals in one or more intervals of frequencies. The transmitted intervals are called passbands, and the rejected intervals are called stopbands. When a filter incorporates active components, such as transistors, it is an active filter, if not, it is passive. A capacitor, for instance, may serve as a passive filter, because it blocks DC. Filters may be classified as falling within one of the following four categories: low-pass, high-pass, bandpass, and bandstop. There are many examples of filters, including comb, ripple, Butter-worth, and loop filters. Also called electrical filter, or electrical-wave filter.
  • A device which blocks or absorbs sounds of certain frequencies while leaving others unaffected. Also called acoustic filter, or sound filter.
  • An element or device, such as a disk or plate of plastic or glass, which selectively blocks or absorbs one or more intervals of frequencies of electromagnetic radiation, such as light. The optical properties of the element or device determine which frequencies pass, and which are blocked or absorbed. Also called optical filter, or radiation filter (2).
  • In computers, a program, function, or process which transforms data from one format to another. For instance, such a filter may convert a document from the format of one word processing application to that of another.
  • In computers, a program, function, or process which selectively passes or separates data or items. For example, an email filter.

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