(fil′tĕr)

[L. filtrum, felt (for straining)]

1. To pass a liquid through any porous substance that prevents particles larger than a certain size from passing through.
2. A device for filtering liquids, light rays, or radiations.
SEE: absorption; SEE: osmosis
3. Material, such as aluminum or molybdenum, inserted between the radiation source and the patient to absorb low-level radiation that would increase the dose.
4. In audiology, to separate sounds according to their frequencies.

Berkefeld filter

SEE: Berkefeld filter

compensating filter

In radiography, a filter that shields less dense areas to produce a more nearly uniform radiographic image.

high efficiency particulate air filter

ABBR: HEPA filter An air filter capable of removing 99.7% of particles greater than 0.3 μm in diameter.

infrared filter

A filter that permits passage of only infrared waves of a certain wavelength.

low-pass filter

A filter that removes high-frequency waves from a sample, allowing only those below a specified frequency to pass.

membrane filter

A filter made from biologically inert cellulose esters, polyethylene, or other porous materials.

Millipore filter

Trademark name of a filter usually composed of cellulose acetate with controlled pore size that separates particles above specific sizes from the solutions that flow through.

optical filter

A device that passes only a portion of the visible light spectrum. Absorption filters absorb the unwanted wavelengths. Interference filters employ the wave effects of constructive and destructive superposition to pass or inhibit appropriate wavelengths.

Pasteur-Chamberland filter

An unglazed porcelain filter capable of retaining bacteria and some viruses. Either pressure or suction is required to force or draw the liquid through the filter.

umbrella filter

A filter placed in a blood vessel to prevent emboli from passing that point. It has been used in the vena cava to prevent emboli reaching the lungs via the chambers of the right heart and pulmonary arteries.
Complications of filter use include perforation of the vein, migration of the filter, or fragmentation and embolization of its parts.

vena cava filter

A wire apparatus inserted through a catheter into the inferior vena cava to prevent pulmonary emboli.

wedge filter

A filter used in radiography and radiation therapy to vary the intensity of the x-ray beam. This compensates for differences in the thicknesses of the parts being exposed to radiation.

Wood's filter

An ultraviolet light source used to diagnose some fungal and bacterial skin diseases.