Optical Bandpass Filter
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An optical filter is a device that selectively transmits
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
of different
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s, usually implemented as a glass plane or
plastic Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...
device in the
optical path Optical path (OP) is the trajectory that a light ray follows as it propagates through an optical medium. The geometrical optical-path length or simply geometrical path length (GPD) is the length of a segment in a given OP, i.e., the Euclidean dis ...
, which are either
dyed Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular ch ...
in the bulk or have
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
coatings. The
optical properties The optical properties of a material define how it interacts with light. The optical properties of matter are studied in optical physics (a subfield of optics) and applied in materials science. The optical properties of matter include: *Refractiv ...
of filters are completely described by their
frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and Phase (waves), phase of the output as a function of input frequency. The frequency response is widely used in the design and ...
, which specifies how the magnitude and phase of each frequency component of an incoming signal is modified by the filter. Filters mostly belong to one of two categories. The simplest, physically, is the absorptive filter; then there are
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
or
dichroic filters An interference filter, dichroic filter, or thin-film filter is an optical filter that reflects some wavelengths (colors) of light and transmits others, with almost no absorption for all wavelengths of interest. An interference filter may be ...
. Many optical filters are used for
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
imaging and are manufactured to be transparent; some used for
light source Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
s can be
translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable light scattering by particles, scattering of light. On a macroscopic scale ...
. Optical filters selectively transmit light in a particular range of
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s, that is,
colours Color (or colour in Commonwealth English; see spelling differences) is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorpt ...
, while absorbing the remainder. They can usually pass long wavelengths only (longpass), short wavelengths only (shortpass), or a band of wavelengths, blocking both longer and shorter wavelengths (bandpass). The passband may be narrower or wider; the transition or cutoff between maximal and minimal transmission can be sharp or gradual. There are filters with more complex transmission characteristic, for example with two peaks rather than a single band;Transmission curves of many filters for monochrome photography, Schneider
See Redhancer 491 for a very complex curve with many peaks (
PDF Portable document format (PDF), standardized as ISO 32000, is a file format developed by Adobe Inc., Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, computer hardware, ...
)
these are more usually older designs traditionally used for photography; filters with more regular characteristics are used for scientific and technical work. Optical filters are commonly used in photography (where some special effect filters are occasionally used as well as absorptive filters), in many
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
instruments, and to colour
stage lighting Stage lighting is the craft of lighting as it applies to the production of theater, dance, opera, and other performance arts.
. In
astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
optical filters are used to restrict light passed to the spectral band of interest, e.g., to study infrared radiation without visible light which would affect film or sensors and overwhelm the desired infrared. Optical filters are also essential in fluorescence applications such as
fluorescence microscopy A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. A fluorescence micro ...
and
fluorescence spectroscopy Fluorescence spectroscopy (also known as fluorimetry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electro ...
.
Photographic filters In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a gl ...
are a particular case of optical filters, and much of the material here applies. Photographic filters do not need the accurately controlled optical properties and precisely defined
transmission curve The transmission curve or transmission characteristic is the mathematical function or graph that describes the transmission fraction of an optical or electronic filter as a function of frequency or wavelength. It is an instance of a transfer func ...
s of filters designed for scientific work, and sell in larger quantities at correspondingly lower prices than many laboratory filters. Some photographic effect filters, such as star effect filters, are not relevant to scientific work.


Measurement

In general, a given optical filter transmits a certain percentage of the incoming light as the wavelength changes. This is
measured Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared to ...
by a spectrophotometer. As a linear material, the absorption for each wavelength is independent of the presence of other wavelengths. A very few materials are
non-linear In mathematics and science, a nonlinear system (or a non-linear system) is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathe ...
, and the
transmittance Electromagnetic radiation can be affected in several ways by the medium in which it propagates.  It can be Scattering, scattered, Absorption (electromagnetic radiation), absorbed, and Fresnel equations, reflected and refracted at discontinui ...
depends on the intensity and the combination of wavelengths of the incident light. Transparent
fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...
materials can work as an optical filter, with an
absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which su ...
spectrum, and also as a
light source Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
, with an
emission spectrum The emission spectrum of a chemical element or chemical compound is the Spectrum (physical sciences), spectrum of frequencies of electromagnetic radiation emitted due to electrons making a atomic electron transition, transition from a high energ ...
. Also in general, light which is not transmitted is absorbed; for intense light, that can cause significant heating of the filter. However, the optical term
absorbance Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...
refers to the
attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...
of the incident light, regardless of the mechanism by which it is attenuated. Some filters, like
mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
s, interference filters, or metal meshes, reflect or scatter much of the non-transmitted light. The (
dimensionless Dimensionless quantities, or quantities of dimension one, are quantities implicitly defined in a manner that prevents their aggregation into units of measurement. ISBN 978-92-822-2272-0. Typically expressed as ratios that align with another sy ...
)
Optical Density Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...
of a filter at a particular wavelength of light is defined as -\log_ T where is the (dimensionless)
transmittance Electromagnetic radiation can be affected in several ways by the medium in which it propagates.  It can be Scattering, scattered, Absorption (electromagnetic radiation), absorbed, and Fresnel equations, reflected and refracted at discontinui ...
of the filter at that wavelength.


Absorptive

Optical filtering was first done with liquid-filled, glass-walled cells; they are still used for special purposes. The widest range of color-selection is now available as colored-film filters, originally made from animal
gelatin Gelatin or gelatine () is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also be referred to as hydrolyzed collagen, coll ...
but now usually a thermoplastic such as
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic, or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...
, acrylic,
polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transp ...
, or
polyester Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include some natura ...
depending upon the application. They were standardized for
photographic Photography is the art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many ...
use by Wratten in the early 20th century, and also by
color gel A color gel or color filter ( Commonwealth spelling: colour gel or colour filter), also known as lighting gel or simply gel, is a transparent colored material that is used in theater, event production, photography, videography and cinematogr ...
manufacturers for
theater Theatre or theater is a collaborative form of performing art that uses live performers, usually actors to present experiences of a real or imagined event before a live audience in a specific place, often a stage. The performers may communi ...
use. There are now many absorptive filters made from
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
to which various
inorganic An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''. Inor ...
or
organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
have been added. Colored glass optical filters, although harder to make to precise transmittance specifications, are more durable and stable once manufactured.


Dichroic filter

Alternately,
dichroic filters An interference filter, dichroic filter, or thin-film filter is an optical filter that reflects some wavelengths (colors) of light and transmits others, with almost no absorption for all wavelengths of interest. An interference filter may be ...
(also called "reflective" or "thin film" or "interference" filters) can be made by coating a glass substrate with a series of
optical coating An optical coating is one or more thin-film optics, thin layers of material deposited on an optical component such as a lens (optics), lens, prism (optics), prism or mirror, which alters the way in which the optic reflection (physics), reflects a ...
s. Dichroic filters usually reflect the unwanted portion of the light and transmit the remainder. Dichroic filters use the principle of
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
. Their layers form a sequential series of reflective cavities that resonate with the desired wavelengths. Other wavelengths destructively cancel or reflect as the peaks and troughs of the waves overlap. Dichroic filters are particularly suited for precise scientific work, since their exact colour range can be controlled by the thickness and sequence of the coatings. They are usually much more expensive and delicate than absorption filters. They can be used in devices such as the
dichroic prism A dichroic prism is a prism (optics), prism that splits light into two beams of differing wavelength, wavelengths (colour). A trichroic prism assembly combines two dichroic prisms to split an image into 3 colours, typically as red, green and blue ...
of a
camera A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...
to separate a beam of light into different coloured components. The basic scientific instrument of this type is a
Fabry–Pérot interferometer In optics, a Fabry–Pérot interferometer (FPI) or etalon is an optical cavity made from two parallel reflecting surfaces (i.e.: thin mirrors). Optical waves can pass through the optical cavity only when they are in resonance with it. It is ...
. It uses two mirrors to establish a resonating cavity. It passes wavelengths that are a multiple of the cavity's resonance frequency. Etalons are another variation: transparent cubes or fibers whose polished ends form mirrors tuned to resonate with specific wavelengths. These are often used to separate channels in
telecommunications networks Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...
that use
wavelength division multiplexing In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This techni ...
on long-haul
optic fiber An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...
s.


Monochromatic

Monochromatic filters only allow a narrow range of wavelengths (essentially a single color) to pass.


Infrared

The term "infrared filter" can be ambiguous, as it may be applied to filters to pass infrared (blocking other wavelengths) or to block infrared (only). Infrared-passing filters are used to block visible light but pass infrared; they are used, for example, in
infrared photography In infrared photography, the photographic film or image sensor used is sensitive to infrared light. The part of the spectrum used is referred to as near-infrared to distinguish it from far-infrared, which is the domain of thermal imaging. Wav ...
. Infrared cut-off filters are designed to block or reflect infrared wavelengths but pass visible light. Mid-infrared filters are often used as heat-absorbing filters in devices with bright
incandescent light bulb An incandescent light bulb, also known as an incandescent lamp or incandescent light globe, is an electric light that produces illumination by Joule heating a #Filament, filament until it incandescence, glows. The filament is enclosed in a ...
s (such as
slide Slide or Slides may refer to: Places * Slide, California, former name of Fortuna, California Arts, entertainment, and media Music Albums * ''Slide'' (Lisa Germano album), 1998 * ''Slide'' (George Clanton album), 2018 *''Slide'', by Patrick Glee ...
and
overhead projector An overhead projector (often abbreviated to OHP), like a Movie projector, film or slide projector, uses light to Projector, project an enlarged image on a Projection screen, screen, allowing the view of a small document or picture to be shared ...
s) to prevent unwanted heating due to infrared radiation. There are also filters which are used in solid state video cameras to block IR due to the high sensitivity of many camera
sensors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
to unwanted near-infrared light.


Ultraviolet

Ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
(UV) filters block ultraviolet radiation, but let visible light through. Because photographic film and digital sensors are sensitive to ultraviolet (which is abundant in skylight) but the human eye is not, such light would, if not filtered out, make photographs look different from the scene visible to people, for example making images of distant mountains appear unnaturally hazy. An ultraviolet-blocking filter renders images closer to the visual appearance of the scene. As with infrared filters there is a potential ambiguity between UV-blocking and UV-passing filters; the latter are much less common, and more usually known explicitly as UV pass filters and UV bandpass filters.


Neutral density

Neutral density (ND) filters have a constant attenuation across the range of visible wavelengths, and are used to reduce the intensity of light by reflecting or absorbing a portion of it. They are specified by the
optical density Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...
(OD) of the filter, which is the negative of the
common logarithm In mathematics, the common logarithm (aka "standard logarithm") is the logarithm with base 10. It is also known as the decadic logarithm, the decimal logarithm and the Briggsian logarithm. The name "Briggsian logarithm" is in honor of the British ...
of the
transmission coefficient The transmission coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A transmission coefficient describes the amplitude, intensity, or total power of a transmitt ...
. They are useful for making photographic exposures longer. A practical example is making a waterfall look blurry when it is photographed in bright light. Alternatively, the photographer might want to use a larger aperture (so as to limit the
depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...
); adding an ND filter permits this. ND filters can be reflective (in which case they look like partially reflective mirrors) or absorptive (appearing grey or black).


Longpass

A longpass (LP) Filter is an optical interference or coloured glass filter that attenuates shorter wavelengths and transmits (passes) longer wavelengths over the active range of the target spectrum (ultraviolet, visible, or infrared). Longpass filters, which can have a very sharp slope (referred to as edge filters), are described by the cut-on wavelength at 50 percent of peak transmission. In fluorescence microscopy, longpass filters are frequently utilized in dichroic mirrors and barrier (emission) filters. Use of the older term 'low pass' to describe longpass filters has become uncommon; filters are usually described in terms of wavelength rather than frequency, and a "
low pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter d ...
", without qualification, would be understood to be an
electronic filter Electronic filters are a type of signal processing filter in the form of electrical circuits. This article covers those filters consisting of lumped-element model, lumped electronic components, as opposed to distributed-element filters. That ...
.


Band-pass

Band-pass filters only transmit a certain wavelength band, and block others. The width of such a filter is expressed in the wavelength range it lets through and can be anything from much less than an
Ångström The angstrom (; ) is a unit of length equal to m; that is, one ten-billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–18 ...
to a few hundred nanometers. Such a filter can be made by combining an LP- and an SP filter. Examples of band-pass filters are the
Lyot filter A Lyot filter (polarization-interference monochromator, birefringent filter), named for its inventor and French astronomer Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths. L ...
and the
Fabry–Pérot interferometer In optics, a Fabry–Pérot interferometer (FPI) or etalon is an optical cavity made from two parallel reflecting surfaces (i.e.: thin mirrors). Optical waves can pass through the optical cavity only when they are in resonance with it. It is ...
. Both of these filters can also be made tunable, such that the central wavelength can be chosen by the user. Band-pass filters are often used in astronomy when one wants to observe a certain process with specific associated
spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...
s. The Dutch Open Telescope and
Swedish Solar Telescope The Swedish 1-m Solar Telescope (or SST) is a refracting solar telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. It is run by the Institute for Solar Physics of Stockholm University. The primary element is a single F ...
are examples where Lyot and Fabry–Pérot filters are being used.


Shortpass

A shortpass (SP) Filter is an optical interference or coloured glass filter that attenuates longer wavelengths and transmits (passes) shorter wavelengths over the active range of the target spectrum (usually the ultraviolet and visible region). In fluorescence microscopy, shortpass filters are frequently employed in dichromatic mirrors and excitation filters.


Guided-mode resonance filters

A relatively new class of filters introduced around 1990. These filters are normally filters in reflection, that is they are
notch filter In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the inverse of a ''band-pass filter''. A notch filter is ...
s in transmission. They consist in their most basic form of a substrate waveguide and a subwavelength
grating A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicu ...
or 2D hole array. Such filters are normally transparent, but when a leaky guided mode of the waveguide is excited they become highly reflective (a record of over 99% experimentally) for a particular
polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
, angular orientations, and wavelength range. The parameters of the filters are designed by proper choice of the grating parameters. The advantage of such filters are the few layers needed for ultra-narrow bandwidth filters (in contrast to dichroic filters), and the potential decoupling between spectral bandwidth and angular tolerance when more than 1 mode is excited.


Metal mesh filters

Filters for sub-millimeter and near infrared wavelengths in astronomy are metal mesh grids that are stacked together to form LP, BP, and SP filters for these wavelengths.


Polarizer

Another kind of optical filter is a
polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization (waves), polarization pass through while attenuation, blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed ...
or polarization filter, which blocks or transmits light according to its
polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
. They are often made of materials such as Polaroid and are used for
sunglasses Sunglasses or sun glasses (informally called shades or sunnies; more names Sunglasses#Other names, below) are a form of Eye protection, protective eyewear designed primarily to prevent bright sunlight and high-energy visible light from damagin ...
and
photography Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...
. Reflections, especially from water and wet road surfaces, are partially polarized, and polarized sunglasses will block some of this reflected light, allowing an
angler Angler may refer to: * A fisherman who uses the fishing technique of angling * Angler (video game), ''Angler'' (video game) * Angler (restaurant), a seafood restaurant in San Francisco, California * The angler, ''Lophius piscatorius'', a monkfish ...
to better view below the surface of the water and better vision for a driver. Light from a clear blue sky is also polarized, and adjustable filters are used in colour photography to darken the appearance of the sky without introducing colours to other objects, and in both colour and
black-and-white photography Monochrome photography is photography where each position on an image can record and show a different ''amount'' of light ( value), but not a different color ( hue). The majority of monochrome photographs produced today are black-and-white, ei ...
to control
specular reflection Specular reflection, or regular reflection, is the mirror-like reflection (physics), reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray (optics), ray of light emerges from the reflecting surf ...
s from objects and water. Much older than g.m.r.f (just above) these first (and some still) use fine mesh integrated in the lens. Polarized filters are also used to view certain types of
stereograms Stereoscopy, also called stereoscopics or stereo imaging, is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...
, so that each eye will see a distinct image from a single source.


Arc welding

An arc source puts out visible,
infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
and
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
light that may be harmful to human eyes. Therefore, optical filters on
welding helmet A welding helmet is a piece of personal protective equipment used by welders to protect the user from concentrated light and flying particles. Different welding processes need stronger lens shades with auto-darkening filters, while goggles suffice ...
s must meet ANSI Z87:1 (a safety glasses specification) in order to protect human vision. Some examples of filters that would provide this kind of filtering would be earth elements embedded or coated on glass, but practically speaking it is not possible to do perfect filtering. A perfect filter would remove particular wavelengths and leave plenty of light so a worker can see what he/she is working on.


Wedge filter

A wedge filter is an
optical filter An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...
so constructed that its thickness varies continuously or in steps in the shape of a wedge. The filter is used to modify the
intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...
distribution in a radiation beam. It is also known as linearly variable filter (LVF). It is used in various optical sensors where wavelength separation is required e.g. in hyperspectral sensors.


See also

*
Anti-aliasing filter An anti-aliasing filter (AAF) is a filter used before a signal sampler to restrict the bandwidth of a signal to satisfy the Nyquist–Shannon sampling theorem over the band of interest. Since the theorem states that unambiguous reconstructi ...
*
Astronomical filter An astronomical filter is a telescope accessory consisting of an optical filter used by amateur astronomers to improve the details and contrast of celestial objects, either for viewing or for photography. Research astronomers, on the other han ...
*
Atomic line filter An atomic line filter (ALF) is a more effective optical band-pass filter used in the physical sciences for filtering electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption ...
*
Dichroic prism A dichroic prism is a prism (optics), prism that splits light into two beams of differing wavelength, wavelengths (colour). A trichroic prism assembly combines two dichroic prisms to split an image into 3 colours, typically as red, green and blue ...
*
Filter (signal processing) In signal processing, a filter is a device or process that removes some unwanted components or features from a Signal (electronics), signal. Filtering is a class of signal processing, the defining feature of filters being the complete or partial s ...
* Filter fluorometer *
Lyot filter A Lyot filter (polarization-interference monochromator, birefringent filter), named for its inventor and French astronomer Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths. L ...
*
Photographic filter In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a ...
*
Photometric system In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric s ...
* Rugate filter * Warm filter


References

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