filter

In electronics, a circuit that transmits a signal of some frequencies better than others. A low-pass filter transmits signals of low frequency and also direct current; a high-pass filter transmits high-frequency signals; a band-pass filter transmits signals in a band of frequencies.

filter

In optics, a device that absorbs some parts of the visible spectrum and transmits others. A beam of white light can be made into a beam of coloured light by placing a transparent colour filter in the path of the beam. For example, a green filter will absorb or block all colours of the spectrum except green, which it allows to pass through. A yellow filter absorbs only light at the blue and violet end of the spectrum, transmitting red, orange, green, and yellow light.

filter

In chemistry, a porous substance, such as blotting paper, through which a mixture can be passed to separate out its solid constituents.

filter

In computing, a program that transforms data. Filters are often used when data output from one application program is input into a different program, which requires a different data format. For example files transferred between two different word-processing programs are run through either an output filter supplied with the first program or an input filter supplied with the second program.

Filters are also used to expand coding structures, which have been simplified for keyboard input, into the often more verbose form required by such standards as SGML (Standard Generalized Markup Language).