In photography, a plate of glass or gelatin of a particular color placed between an original and the lens of a camera, used to separate out certain colors, thus allowing only certain other colors to register on the film. Filters—also called color filters—are used to make color separations, or four negatives that will be used to print the four process colors of cyan, magenta, yellow, and black.The original is thus photographed four times using different color filters. The choice of colors for the filters is based on additive color theory; a blue filter is used to produce the yellow printer; a green filter is used to produce the magenta printer; a red filter is used to produce the cyan printer; and a combination of all three filters is used to generate the black printer. See also dichroic filter.
In digital imaging, a filter is a software function (available as a native part of the image-processing application or as a plug-in) that alters an image in a particular way, changing either the color balance, or adding special effects such as a "ripple" effect, a motion blur, etc.