filter

n.

1. 1. A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particulate matter.
   2. A device containing such a material, especially one used to extract impurities from air or water.
2. 1. Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiations of certain frequencies while allowing others to pass.
   2. A colored glass or other transparent material used to select the wavelengths of light allowed to reach a photosensitive material.
3. Computer Science. A program or routine that blocks access to data that meet a particular criterion: a Web filter that screens out vulgar sites.

v., -tered, -ter·ing, -ters.

v.tr.

1. To pass (a liquid or gas) through a filter.
2. To remove by passing through a filter: filter out impurities.
3. Computer Science. To use a filter to block access to (a website or Web content).

v.intr.

1. To pass through or as if through a filter: Light filtered through the blinds.
2. To come or go gradually and in small groups: The audience filtered back into the hall.

[Middle English filtre, from Old French, from Medieval Latin filtrum, of Germanic origin.]

filterer fil'ter·er n.
filterless fil'ter·less adj.

A procedure that converts one time series into another. A simple example is a moving average, which, because it is a linear combination of the terms in the time series, is described as a linear filter. A filter that removes short-term random fluctuations is called a low-pass filter, and one that removes long-term fluctuations is called a high-pass filter.

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A device for separating solid particles from a liquid or gas. The simplest laboratory filter for liquids is a funnel in which a cone of paper (filter paper) is placed. Special containers with a porous base of sintered glass are also used. See also Gooch crucible.

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To strain a liquid through a porous paper, fine cloth, etc., to remove small particles, as in making coffee or clarifying fruit juice to make jelly. Water may be filtered through charcoal to remove unpleasant flavours and colours; bacterial filters for water have pores fine enough to remove bacteria.

To strain through a paper filter or several layers of cheesecloth.

v

Definition: separate to refine; seep through
Antonyms: collect, combine, mix

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[very common; orig. Unix] A program that processes an input data stream into an output data stream in some well-defined way, and does no I/O to anywhere else except possibly on error conditions; one designed to be used as a stage in a pipeline (see plumbing). Compare sponge.

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n

A material placed in the useful beam to absorb preferentially the less energetic (less penetrating) radiations. See also filtration.

1. A device to separate solids, such as dust, from air.
2. A device to separate solids from liquids.
3. A charcoal filter.
4. A layer or combination of layers of pervious materials designed and installed in such a manner as to provide drainage, yet prevent the movement of soil particles due to flowing water.
5. See heat filter. 6. See light filter.

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Some photographers always use filters; others never do. Claims by either side to a greater purity of vision are spurious.

In monochrome, the classic ‘sky’ filters (yellow, orange, red, in increasing order of effect) not only darken blue skies, but alter other tonal relationships and cut through haze. Deep yellow and orange filters are often reckoned to give good tonal relationships for the majority of outdoor shots: it is instructive to go through any Ansel Adams book and see how often he used such filters.

Other filters for monochrome are used to lighten their own colour, or darken complementary colours. For example, deep red will lighten a red rose, making it easier to hand-colour; weak red will lighten a florid complexion; green will lighten foliage and darken red brick; and so forth.

Until the advent of orthochromatic films, ‘sky’ filters were all but useless, and with ortho films, the effect of even a yellow filter is greater than with panchromatic. Filter factors, the extent to which exposure must be increased to compensate for the light absorbed by the filter, vary from film to film and from subject to subject. With experience, the photographer may well halve or double the ‘average’ exposure correction factor.

Neutral-density (ND) filters are equally useful in both colour and monochrome, when a long exposure is desired and for whatever reason it is not practicable to reduce the aperture below a certain level. They may be described in stops; in filter factors; or in terms of log density. Thus a weak ND filter may be 1 stop, 2×, D = 0.3, while a stronger one may be three stops, 8×, D = 0.9. The importance of knowing which description is used is obvious. Cheap NDs sometimes introduce colour casts, usually green.

For shooting colour out of doors, especially transparencies where the final picture is a camera original, the most useful filter is often a polarizer. As well as suppressing obvious reflections, these can darken blue skies and (by suppressing white-light reflections) increase overall colour saturation.

The other filter most often used in professional outdoor colour photography is the ‘grey grad’ or graduated filter, clear at one end, and neutral grey at the other. The density of the dark side, and the abruptness of the transition, can vary widely: some photographers carry several grey grads to achieve different effects. Coloured ‘effects’ grads such as the tobacco grad are all too easy to overuse.

In the studio, weak colour-correction (CC) filters are often employed. These are available in a wide range of (numbered) strengths, in both additive and subtractive primary colours: red, green, blue (RGB) and cyan, magenta, yellow (CMY). Thus CC05R is a weak red, CC50G a strong green, and CC20Y a middling yellow. A special case for colour correction is for daylight-balanced films under tungsten lighting (for which a blue filter is required) or for tungsten-balanced films under daylight (amber). These are often used with CC filters to get the best possible result. Balancing any film for fluorescents is difficult, but a CC30M is often a good starting point.

Most modern glass filters are dyed in the mass, so the colour goes all through the filter, but optical resin filters are often dip coated with colour (grads have to be) and a few filters, especially polarizers, are sandwiched in glass. Gelatin filters (literally of dyed gelatin) and their rather tougher acetate cousins are widely used for studio photography or for highly specialized filtration such as tri-cut colour, or infrared.

Traditionalists distinguish between filters (which change the colour of the light) and screens (which do not), such as polarizers, soft-focus attachments, and neutral density.

Any filter will slightly reduce lens resolution, but except for the very worst filters, this varies from the negligible to the imperceptible: even optical resin filters, often derided by the ignorant, are extremely unlikely to have any significant adverse effect on image quality. This can be confirmed by photographing a test chart with and without the filter, rather than relying on what ‘everyone knows’. Clean gels have no perceptible effect, and the effect of glass is measurable only in the laboratory.

High-quality filters are more often coated than cheaper ones, though the importance of this can be overstated; they are likely to be mechanically stronger; and if they are in brass threaded mounts, as the best are, then there is less danger of their binding in the filter threads of lenses.

In digital photography, filtration effects can be applied by means of image-processing software like Adobe Photoshop, or in-camera.

— Roger W. Hicks

Bibliography

• Life Library of Photography: Light and Film (1971)

A device for eliminating certain elements, as (1) particles of certain size from a solution, (2) bacteria and fungi from suspensions of virus, or (3) rays of certain wavelength from a stream of radiant energy.

• bacteria-proof f. — see berkefeld's filter, pasteur–chamberland filter, millipore filter.
• Wood's f. — a nickel-oxide filter that holds back all but a few violet rays and passes ultraviolet rays of about 365 nm. See also wood's light.
• x-ray f. — see grid (1).

Network consisting of capacitors, resistors and/or inductors used to pass certain frequencies and block others.

(DOD, NATO) In electronics, a device which transmits only part of the incident energy and may thereby change the spectral distribution of energy: a. High pass filters transmit energy above a certain frequency; b. Low pass filters transmit energy below a certain frequency; c. Band pass filters transmit energy of a certain bandwidth; d. Band stop filters transmit energy outside a specific frequency band.

IN BRIEF: n. - Device that removes something from whatever passes through it; An electrical device that alters the frequency spectrum of signals passing through it.

Tutor's tip: The wizard told the gnome to "filter" (to strain out impurities) juice from a magical fruit to make a "philter" (a magic potion, a love charm) that would turn him into a handsome prince.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - filter, filtrerapparat
v. tr. - filtrere, filtrere fra
v. intr. - sive, filtrere

idioms:

• filter out    filtrere fra
• filter tip    filtercigaret

Nederlands (Dutch)
filter, (uit)filtreren, druppelen (figuurlijk), voorsorteren

Français (French)
n. - filtre, (Tech) filtre, (Audio, Phot, Télécom) filtre, (Cosmét) filtre (solaire), (GB, Transp) voie (de stockage)
v. tr. - filtrer, épurer
v. intr. - filtrer, suinter, (GB, Transp) passer sur la voie de gauche pour tourner, pénétrer dans (une zone)

idioms:

• filter out    filtrer, éliminer par filtrage
• filter tip    filtre, bout filtré

Deutsch (German)
n. - Filter
v. - filtern, durchsickern, sich einordnen

idioms:

• filter out    herausfiltern
• filter tip    Filter

Ελληνική (Greek)
n. - φίλτρο
v. - διυλίζω, φιλτράρω/-ομαι, (μτφ.) διεισδύω, διαρρέω

idioms:

• filter out    φιλτράρω, διαχωρίζω, διαρρέω
• filter tip    τσιγάρο με φίλτρο, φίλτρο τσιγάρου, επιστόμιο φίλτρου

Italiano (Italian)
colare, trapelare, filtro

idioms:

• filter out    trapelare

Português (Portuguese)
n. - filtro (m)
v. - filtrar

idioms:

• filter out    filtrar
• filter tip    filtro (m) do cigarro

Русский (Russian)
процеживать, фильтровать, фильтр, цедилка, светофильтр

idioms:

• filter out    просочиться, постепенно выходить
• filter tip    сигарета с фильтром

Español (Spanish)
n. - filtro
v. tr. - filtrar, colar, infiltrarse
v. intr. - filtrar, colar, infiltrarse

idioms:

• filter out    filtrarse
• filter tip    cigarrillo con filtro, boquilla con filtro

Svenska (Swedish)
n. - filter, grön pil för svängande trafik
v. - filtrera, filtreras, svänga av från stillastående fil (trafik)

中文（简体）(Chinese (Simplified))
滤波器, 滤光器, 过滤器, 过滤, 走漏, 渗透, 滤过, 渗入

idioms:

• filter out    滤除, 渗漏
• filter tip    香烟的滤嘴

中文（繁體）(Chinese (Traditional))
n. - 濾波器, 濾光器, 篩檢程式
v. tr. - 過濾, 走漏, 滲透
v. intr. - 濾過, 走漏, 滲入

idioms:

• filter out    濾除, 滲漏
• filter tip    香煙的濾嘴

한국어 (Korean)
n. - 여과기, 필터
v. tr. - 거르다, 여과하다
v. intr. - 여과하다, 스며들다

idioms:

• filter out    (빛, 소문 등이) 새어 나오다

日本語 (Japanese)
n. - ろ過器, ろ過用材料, フィルター
v. - ろ過する, 漏れる, しみとおる, ゆっくり進む

idioms:

• filter out    ろ過して取り除く
• filter tip    フィルター, フィルター付きタバコ

العربيه (Arabic)
‏(الاسم) فلتر , مصفاة , مرشحه (فعل) يصفي , يرشح‏

עברית (Hebrew)
n. - ‮מסנן, פילטר‬
v. tr. - ‮סינן‬
v. intr. - ‮הסתנן, חדר‬

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filter	Glare Filter
Water Filter	8171414 Filter