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Filter

1. A porous substance through which a liquid or gas is passed in order to separate it from contained particulate matter or impurities.

Synonym: filtrum.

2. To use or to subject to the action of a filter.

3. In diagnostic or therapeutic radiology, a plate made of one or more metals such as aluminum and copper that, placed in the x-or gamma-ray beam, permits passage of a greater proportion of higher energy radiation and attenuation of lower and less desirable energy radiation, raising the average energy or hardening the beam.

4. A device used in spectrophotometric analysis to isolate a segment of the spectrum.

5. A mathematical algorithm applied to image data for the purpose of enhancing image quality, usually by suppression of high spatial frequency noise.

Origin: Mediev. L. Filtro, pp. -atus, to strain through felt, fr. Filtrum, felt remove by passing through a filter; filter out the impurities.