filter

—filterer, n.

/fil"teuhr/, n.

1. any substance, as cloth, paper, porous porcelain, or a layer of charcoal or sand, through which liquid or gas is passed to remove suspended impurities or to recover solids.

2. any device, as a tank or tube, containing such a substance for filtering.

3. any of various analogous devices, as for removing dust from air or impurities from tobacco smoke, or for eliminating certain kinds of light rays.

4. Informal. a filter-tipped cigarette or cigar.

5. Photog. a lens screen of dyed gelatin or glass for controlling the rendering of color or for diminishing the intensity of light.

6. Electronics, Physics. a circuit or device that passes certain frequencies and blocks others.

7. Math. a collection of subsets of a topological space, having the properties that the intersection of two subsets in the collection is a subset in the collection and that any set containing a subset in the collection is in the collection.

v.t.

8. to remove by the action of a filter.

9. to act as a filter for; to slow or partially obstruct the passage of: The thick leaves filtered the sunlight.

10. to pass through or as through a filter.

v.i.

11. to pass or slip through slowly, as through an obstruction or a filter: Enemy agents managed to filter into the embattled country.

[1375-1425; late ME filtre < ML filtrum felt, piece of felt used to strain liquids < Gmc; see FELT²]

Syn. 11. penetrate, sift, seep, trickle, leak.

* * *

▪ optics and photography

      in photography, device used to selectively modify the component wavelengths of mixed (e.g., white) light before it strikes the film. Filters may be made of coloured glass, plastic, gelatin, or sometimes a coloured liquid in a glass cell. They are most often placed over the camera lens but can in some cases be placed over the light source with the same effect.

      Black-and-white films are imperfect in their colour sensitivity, and coloured filters are used to modify the light and translate the subject into gray tones that correspond to the tones seen by the human eye. Coloured filters can also brighten or darken the reproduction of coloured objects, permitting local contrast controls at the point of exposure. In colour photography, coloured filters are used to alter the colour quality of the light to match the colour sensitivity of the film.

      Some light filters are used in both colour and black-and-white photography. Neutral density filters decrease the intensity of the light without affecting its colour and are used when the light intensity is too great for the correct exposure. Polarizing filters enhance colour vividness by reducing glare from the reflecting surfaces of such substances as glass and water. Colour filters are also used for colour correction in the printing process and for selecting contrast scales of multicontrast black-and-white enlarging paper.

      Since filters absorb some of the light that passes through them, an increase in the calculated exposure is usually required. This increase is known as the filter factor. Modern cameras with built-in meters measure the light after the filtration and thus take the decrease in intensity into account.

* * *