A porous or permeable material or device which prevents some things from passing through it, and allows others passage. Filters are used to separate or refine some particles, liquids, gases, or light. Most light filters, such as might be used in photography or in light projection are tinted, but some are polarizing. Acting like seives, some filters are screens, or act like them. Particles of one size or smaller will pass through a screen with openings of a consistent size, while particles of any larger size cannot. Such a filtering screen is useful in removing impurities from paints, for instance, or in separating sizes of sand.Also see clean up, day for night, lens, moir?, plaster trap, and trit.