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filter

 [fil´ter]
1. a device for eliminating or separating certain elements, as (a) particles of certain size from a solution, or (b) rays of certain wavelength from a stream of radiant energy.
2. to cause such separation or elimination.
membrane filter a filter made up of a thin film of collodion, cellulose acetate, or other material, available in a wide range of defined pore sizes, the smaller ones being capable of retaining all the known viruses.
Millipore filter trademark for a device used to filter nutrient solutions as they are administered intravenously.
vena cava filter (vena caval filter) a filter used in the inferior vena cava to prevent pulmonary embolism.
Wood's filter a nickel-oxide filter that holds back all but a few violet rays and passes ultraviolet rays of about 365 nm; see also Wood's light.

fil·ter

(fil'tĕr),
1. A porous substance through which a liquid or gas is passed to separate it from contained particulate matter or impurities to sterilize. Synonym(s): 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-energy and less desirable 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 imaging data for the purpose of enhancing image quality, usually by suppression or enhancement of high spatial frequencies.
6. A passive electronic circuit or device that selectively permits the passage of certain electrical signals.
7. A device placed in the inferior vena cava to prevent pulmonary embolism from low extremity clot. There are many variants.
[Mediev. L. filtro, pp. -atus, to strain through felt, fr. filtrum, felt]

filter

/fil·ter/ (fil´ter)
1. a device for eliminating or separating certain elements, as (1) particles of certain size from a solution, or (2) rays of certain wavelength from a stream of radiant energy.
2. to cause such separation or elimination.

membrane filter  a filter made up of a thin film of collodion, cellulose acetate, or other material, available in a wide range of pore sizes.
Millipore filter  trademark for any of a variety of membrane filters.

filter

(fĭl′tər)
n.
1.
a. A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particulate matter.
b. A device containing such a material, especially one used to extract impurities from air or water.
2.
a. Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiations of certain frequencies while allowing others to pass.
b. A colored glass or other transparent material used to select the wavelengths of light allowed to reach a photosensitive material.
3. Computers A program or device that blocks e-mail or restricts website access when specific criteria are met.
v. fil·tered, fil·tering, fil·ters
v.tr.
1. To pass (a liquid or gas) through a filter.
2. To remove by passing through a filter: filter out impurities.
3. Computers To use a filter to block or restrict access to: a program that filters spam.
v.intr.
1. To pass through or as if through a filter: Light filtered through the blinds.
2. To come or go gradually and in small groups: The audience filtered back into the hall.

fil′ter·er n.
fil′ter·less adj.

filter

Etymology: Fr, filtrer, to strain
1 a device or material through which a gas or liquid is passed to separate out unwanted matter.
2 (in radiology) a device added to radiographic equipment that selectively removes low-energy x-rays that have no chance of reaching the film. Examples include bow-tie, compensating, and conic filters.

filter

Imaging A layer of absorbing material, usually a metal–eg, Al, Cu, Pb, Sn that increases the ratio of hard X-rays to soft X-rays, the latter of which are of greater diagnostic value, given their ability to penetrate the imaged tissues. See Radiation Medtalk A device used to separate one material from another. See Absolute filter, Adhesion filter, Blood filter, Inferior vena cava filter, Microaggregate filter, Red-free filter, Standard filter, Water filter.

fil·ter

(fil'tĕr)
1. A porous substance through which a liquid or gas is passed to separate it from contained particulate matter or impurities.
Synonym(s): filtrum.
2. To use or to subject to the action of a filter.
3. radiology A device, used in both diagnostic and therapeutic radiology, which permits passage of useful x-rays and absorbs those with a lower and less desirable energy.
4. A device used in spectrophotometric analysis to isolate a segment of the spectrum.
5. A mathematic algorithm applied to image data for the purpose of enhancing image quality, usually by suppression of high spatial frequency noise.
6. A passive electronic circuit or device that selectively permits the passage of certain electrical signals.
7. A device placed in the inferior vena cava to prevent pulmonary embolism from lower limb clot.
8. radiation physics Material placed in an x-ray beam that is used to improve the beam's quality by removing low-energy beams.
[Mediev. L. filtro, pp. -atus, to strain through felt, fr. filtrum, felt]

filter

Material or device used to absorb or transmit light of all wavelengths equally (neutral density filter which is abbreviated ND filter) or selectively, such as the coloured filters (blue filter transmits only blue light, green filter transmits only green light, etc.). See optical density; absorptive lens; neutral density filter test; optical wedge.
bandpass filter A filter that allows the passage of radiations only within a narrow band of wavelengths around a central wavelength. This is done by multilayer coating, which produces destructive interference. See coating; coated lens.
green filter A filter which transmits only green light. It may be used in ophthalmoscopy to increase the contrast of the blood vessels to the background facilitating the visibility of retinal circulation defects, haemorrhages and microaneurysms and the distinction between retinal and choroidal lesions. However, ophthalmoscopes actually use a filter that transmits a certain amount of red light, as otherwise the observation would be so dark as to make it extremely difficult. Syn. red-free filter.
interference filter A coloured filter consisting of five layers, two outside glass, two intermediate evaporated metal films and one central evaporated layer of transparent material. These filters act not by absorption of light but by destructive interference for all except a very narrow band of wavelengths, which are transmitted. Syn. coloured filter.
neutral density filter  See filter.
red filter A filter that transmits only red light. It may be used in ophthalmoscopy to facilitate viewing the yellow macular pigment, but other structures are seen with less contrast. It also produces a larger pupil allowing observation of a larger fundus area.
red-free filter See green filter.
Wood's filter See Wood's light.

fil·ter

(fil'tĕr)
1. A porous substance through which a liquid or gas is passed to separate it from contained particulate matter or impurities to sterilize.
2. 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-energy and less desirable radiation.
[Mediev. L. filtro, pp. -atus, to strain through felt, fr. filtrum, felt]

filter

a device for eliminating certain elements, as (1) particles of certain size from a solution, (2) bacteria and fungi from suspensions of virus, or (3) rays of certain wavelength from a stream of radiant energy.

Wood's filter
a nickel-oxide filter that holds back all but a few violet rays and passes ultraviolet rays of about 365 nm. See also wood's light.
x-ray filter
see grid (1).
References in periodicals archive ?
For observing planets in the daytime, an orange or red filter dims the sky's blue light while dimming the planet a bit less, improving contrast a trace.
However, using my 10-inch Newtonian scope on Saturn, I don't go to longer wavelengths than a normal red filter (610 nanometers); otherwise the images become too grainy for the high-contrast processing I apply to reveal weak spots.
Parker of Coral Gables, Florida, often uses a red filter to cut down the sky background.
Since a standard red filter in an RGB set will pass both the S II and H[alpha] lines, it can be difficult to see contrast between sulfur and hydrogen regions in a conventional color image.
Measuring only 5 1/2 inches across, the maps show so many faint stars that I consider them of little use, especially on a dark night lit by a flashlight covered with a red filter.
3 nanometers (January issue, page 129) falls entirely within the transmission curve of the red filter.
As mentioned previously, H[alpha] filters are very dark red and transmit much less light than a normal red filter.
Such a sooty atmosphere acts like a red filter, almost completely absorbing the blue and violet portions of the star's spectrum; see the May 1998 issue, page 90.
With a 2x Barlow and a red filter, the limit is about 8th magnitude.
My image of the Cone Nebula on page 133 is an example of how a red filter can help increase the contrast and produce a quality image of an object that is notoriously dim and difficult to record.
Two-frame mosaic with each frame assembled from LRGB images using a 60-minute exposure with a red filter as the luminance component and RGB exposures of 10, 10, and 20 minutes, respectively.
Richard Kendrick, who is running Rhyl's Poppy Appeal, is helping businesses join the commemorations by handing out red filters.