filter

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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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An accessory used with an optical instrument or detector of electromagnetic radiation to either narrow down the wavelength band or to reduce the total intensity passing into the instrument. Filters are often valuable in improving contrast and delineating detail that would otherwise be less visible.

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

[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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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)

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.

i. A device to trap and hold all impurities or solids beyond a specific thickness. The thicknesses of impurities are measured in microns. See fuel filter. See also micron (i).
ii. To study all information and discard what is irrelevant.
iii. A device for discriminating between currents of different frequencies, selecting some frequency bands and rejecting interference from other frequencies. A band-pass filter has a single transmission band. A high-pass filter passes currents with frequencies higher than a nominal cut-off frequency, and it highly attenuates those with frequencies below. A low-pass filter works in the opposite manner. See also bandpass.

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1. any piece or layer of material used or useful for (completely or partially) removing selected components, especially in suspension, from liquid or gaseous mixtures, e.g. a sheet of paper, a stratum of sand (or similar material), or a piece of porous ceramic or plastic. See also membrane filter, ultrafilter.
2. a device incorporating such a piece or layer of material.
3. a block or sheet of material for reducing the intensity of particulate radiation.
4. a screen for selectively absorbing or attenuating electromagnetic radiation or sound waves of some particular or all frequencies.
5. a passive electronic circuit or device that selectively attenuates or allows the passage of certain frequencies of an electrical signal.
6. to remove or separate (suspended particles, macromolecular solutes, selected frequencies of electrical signal or electromagnetic radiation, etc.) from (a heterogeneous or composite fluid, beam, signal, etc.) by means of a filter; to pass (something) through a filter.
7. (as modifier) containing or using a filter or filters, or used for filtering, as in a filter fluorometer or filter photometer.

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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).

n

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

For a list of words related to filter, see:

• Building and Machine Parts - filter: device that separates particles from air or liquid
• Electricity and Electronics - filter: device that selectively damps or suppresses certain oscillations and frequencies while not affecting others
• Working Parts - filter: device that removes foreign substances from air, oil, gasoline, or water
• Sound Reproduction Technology - filter: device for suppressing or minimizing certain frequencies
• Tools and Techniques - filter: glass or plastic gel or disc that screens certain light frequencies or colors from film exposure, also used in printing to alter contrast
• Smoking and Tobacco - filter: porous substance used to remove impurities from smoke

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See words rhyming with "filter."

  See crossword solutions for the clue Filter.

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

Some good "filter" pages on the web: