Oct. 28, 2024
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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 optical properties of filters are completely described by their frequency response, which specifies how the magnitude and phase of each frequency component of an incoming signal is modified by the filter.[1]
Filters mostly belong to one of two categories. The simplest, physically, is the absorptive filter; then there are interference or dichroic filters. Many optical filters are used for optical imaging and are manufactured to be transparent; some used for light sources can be translucent.
Optical filters selectively transmit light in a particular range of wavelengths, that is, colours, 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;[2] these are more usually older designs traditionally used for photography; filters with more regular characteristics are used for scientific and technical work.[3]
Optical filters are commonly used in photography (where some special effect filters are occasionally used as well as absorptive filters), in many optical instruments, and to colour stage lighting. In astronomy 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 and fluorescence spectroscopy.
Photographic filters 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 curves 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.
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In general, a given optical filter transmits a certain percentage of the incoming light as the wavelength changes. This is measured 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, and the transmittance depends on the intensity and the combination of wavelengths of the incident light. Transparent fluorescent materials can work as an optical filter, with an absorption spectrum, and also as a light source, with an emission spectrum.
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 refers to the attenuation of the incident light, regardless of the mechanism by which it is attenuated. Some filters, like mirrors, interference filters, or metal meshes, reflect or scatter much of the non-transmitted light.
The (dimensionless) Optical Density of a filter at a particular wavelength of light is defined as &#; log 10 &#; T {\displaystyle -\log _{10}T} where T is the (dimensionless) transmittance of the filter at that wavelength.
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Optical filtering was first done with liquid-filled, glass-walled cells;[citation needed] 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 but now usually a thermoplastic such as acetate, acrylic, polycarbonate, or polyester depending upon the application. They were standardized for photographic use by Wratten in the early 20th century, and also by color gel manufacturers for theater use.
There are now many absorptive filters made from glass to which various inorganic or organic compounds[citation needed] have been added. Colored glass optical filters, although harder to make to precise transmittance specifications, are more durable and stable once manufactured.[citation needed]
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Alternately, dichroic filters (also called "reflective" or "thin film" or "interference" filters) can be made by coating a glass substrate with a series of optical coatings. Dichroic filters usually reflect the unwanted portion of the light and transmit the remainder.
Dichroic filters use the principle of interference. 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 of a camera to separate a beam of light into different coloured components.
The basic scientific instrument of this type is a Fabry&#;Pérot interferometer. 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 that use wavelength division multiplexing on long-haul optic fibers.
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Monochromatic filters only allow a narrow range of wavelengths (essentially a single colour) to pass.
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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.
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 bulbs (such as slide and overhead projectors) 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 to unwanted near-infrared light.
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Ultraviolet (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.[4]
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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 (OD) of the filter, which is the negative of the common logarithm of the transmission coefficient. 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); 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).
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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", without qualification, would be understood to be an electronic filter.
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 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 and the Fabry&#;Pérot interferometer. 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 lines. The Dutch Open Telescope[5] and Swedish Solar Telescope[6] are examples where Lyot and Fabry&#;Pérot filters are being used.
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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.
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A relatively new class of filters introduced around . These filters are normally filters in reflection, that is they are notch filters in transmission. They consist in their most basic form of a substrate waveguide and a subwavelength grating 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, 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.
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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.
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Another kind of optical filter is a polarizer or polarization filter, which blocks or transmits light according to its polarization. They are often made of materials such as Polaroid and are used for sunglasses and photography. Reflections, especially from water and wet road surfaces, are partially polarized, and polarized sunglasses will block some of this reflected light, allowing an angler 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 to control specular reflections 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, so that each eye will see a distinct image from a single source.
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An arc source puts out visible, infrared and ultraviolet light that may be harmful to human eyes. Therefore, optical filters on welding helmets 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.
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A wedge filter is an optical filter so constructed that its thickness varies continuously or in steps in the shape of a wedge. The filter is used to modify the intensity 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.[7]
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Optical filters are devices designed to modify the spectral properties of light. They are transmitting specific wavelengths while blocking others. Depending on the desired application, they are composed of materials that exhibit specific optical properties, such as absorption, reflection, or transmission, Here&#;s an overview of the different kinds of optical filters and their respective purposes.
An optical filter is a tool that lets through specific colors of light while stopping others. It&#;s like sunglasses for light. These filters can be crafted from different materials like glass or plastic.
They&#;re used in many things like cameras, microscopes, and even phones to improve pictures and study tiny things. Scientists and engineers use optical filters to understand light better and create remarkable technologies.
Optical filters are primarily categorized as absorptive and dichroic filters. However, they can also be classified based on various factors. Such as applications, the wavelengths they transmit, and the materials.
Bandpass filters are a type of optical filter. A specific range of wavelengths of light pass through while blocking other wavelengths. It&#;s like a gate that only allows particular light colors to go through while stopping others.
These filters are used in many devices, like cameras and sensors, to capture or detect light only within a specific range of colors. They&#;re helpful in various applications. Such as photography, telecommunications, and scientific instruments. Where isolating particular wavelengths of light is essential.
Click here to learn more about this article What is bandpass filter
Longpass filters are optical filters that transmit light with longer wavelengths&#;and block shorter wavelengths. They&#;re commonly used in various applications such as photography, spectroscopy, and fluorescence microscopy.
Longpass filters let in longer light waves. They only allow specific colors to go through, keeping out the ones we don&#;t want. This helps to pick out particular colors or eliminate the ones we don&#;t need from the light.
Shortpass filters are optical filters designed to transmit light within a specific wavelength range while blocking shorter wavelengths. They are commonly used in various applications, such as photography, spectroscopy, and fluorescence microscopy.
Allow particular wavelengths of light to pass through while attenuating others. Shortpass filters are characterized by their cut-on wavelength, which marks the point at which transmission decreases rapidly.
Notch filters are another type of optical filter. But instead of letting light through within a specific range, they do the opposite. They block a very narrow band of wavelengths while transmitting all others.
They&#;re used where you want to remove or suppress a particular frequency or color of light. Telecommunications are used to eliminate specific noise or interference from signals.
Dichroic filters are special filters that let some colors of light through while blocking others. They work like magic by using thin layers of different materials. These layers make specific light colors bounce off while letting others pass through.
People use dichroic filters in cameras, lights, microscopes, and other cool gadgets to make things look colorful or separate different light colors. So, they&#;re like the superheroes of optics, helping scientists and photographers see the world in all its bright glory.
A neutral-density filter is like sunglasses for cameras. It reduces the amount of light entering the lens without changing its color. This helps photographers control exposure and achieve effects like blurring motion or wider apertures in bright conditions.
Think of it as a tool that lets photographers adjust how much light reaches the camera&#;s sensor, giving them more creative control over their shots.
It plays a crucial role in photography, astronomy, telecommunications, and spectroscopy. The materials and fabrication methods used to achieve specific optical properties. Such as transmission, reflection, and absorption characteristics across particular wavelengths of light.
Traditional optical filters are often made from glass. Different types of glass can be used to achieve specific optical properties. Such as soda-lime glass for visible light filters and fused silica for UV applications.
Thin film coatings deposited onto glass or other substrates are standard in modern. Materials like silicon dioxide (SiO2), titanium dioxide (TiO2), and tantalum oxide (Ta2O5) are used in thin film interference filters.
Semiconductor materials like silicon and gallium. Create tunable filters, where the properties can be controlled by applying an electric field or changing temperature.
Organic polymers can create flexible and lightweight optical filters suitable for applications where weight and flexibility are essential, such as in wearable devices or flexible displays.
Thin film optical filters are made by depositing thin layers of material onto a surface. This is done using physical vapor deposition (PVD) or chemical vapor deposition (CVD).
These techniques let us control how thick each layer is and what it&#;s made of. This is needed for things like cameras, eyewear, or scientific instruments.
This technique is used in semiconductor fabrication. Create intricate patterns on substrates for specialized optical filters.
Grinding and polishing are used to ensure the desired thickness and surface quality. This process is crucial for guaranteeing optical clarity and minimizing light scattering.
MBE is a technique used to grow thin films with atomic-level precision. It is often employed in the fabrication of semiconductor-based optical filters.
Spin coating is used to apply uniform, thin polymer films onto substrates. It is commonly used in the fabrication of polymer-based optical filters.
These applications highlight the diverse roles of optical filters. These filters play various scientific, industrial, and consumer fields, enabling manipulation and analysis of light for many purposes. Here are some typical applications:
Optical filters are used in cameras and imaging systems. Enhance image quality, improve contrast, and eliminate unwanted wavelengths. For example, in photography, filters control the amount of light entering the camera, adjust color balance, and reduce glare.
Optical filters are utilized in security and forensic applications for analyzing documents. They identify counterfeit and detect trace amounts of substances such as drugs or explosives. They enable the selective visualization of features that may be invisible under normal illumination.
In medical tools, filters are like helpers. They do jobs such as making it easier to see tiny things in cells and tissues. For example, in a special microscope called a fluorescence microscope. Filters help scientists and doctors see specific parts of cells.
They also help in flow cytometry, which counts and sorts cells, and DNA sequencing, which reads genetic information. So, filters are essential for doctors and scientists to look at things inside our bodies.
Astronomy enhances contrast and isolates specific wavelengths of light from celestial objects. They help astronomers observe faint astronomical phenomena. Study the composition and properties of stars, galaxies, and planetary atmospheres.
It plays a vital role in spectroscopy, like fluorescence, Raman, and absorption spectroscopy. They are used to isolate specific wavelengths of light. It allows scientists to analyze the composition of substances based on their spectral characteristics.
Fibre optic communication systems manipulate light signals. They are used in wavelength division multiplexing (WDM) systems to separate and combine optical channels. Carry data at different wavelengths, enabling high-speed data transmission over long distances.
In satellite and aerial imaging systems for remote sensing applications such as environmental monitoring, agriculture, and geology. By filtering out unwanted wavelengths, these systems can capture and analyze specific features of the Earth&#;s surface or atmosphere.
Optical filters are employed in display technologies such as liquid crystal displays (LCDs) and organic light-emitting diode (OLED) screens to control the colors and improve the contrast ratio. Polarizing filters are commonly used to enhance display visibility and reduce glare.
Optical filters are devices that send light of specific wavelengths while blocking others. Future trends and developments are expected to focus on several key areas:
&#; Advances in nanotechnology are enabling the development of nanostructured materials. These nanostructured filters offer precise control over the transmission of light.
&#; Scientists are making unique materials called metamaterials. These materials don&#;t exist and can do extraordinary things like bending light in strange ways, even making it go backward. They&#;re using these metamaterials to build small and super good filters for light. This helps scientists control light better for all sorts of important jobs.
&#; People want special filters that can change their work depending on their needs. These filters can quickly change the colors of light they let through. They&#;re super helpful in things like fancy cameras, talking through light, and looking at unique colors in science.
&#; In the future, filters will do more than one job at a time. They might change colors and twist light uniquely in one filter. This makes things like cameras and other gadgets smaller and work better.
&#; Nowadays, filters can join forces with tiny circuits called photonic integrated circuits (PICs). These special filters are part of the circuit, like a team working together. They&#;re small, light, and don&#;t use much power. This makes them great for gadgets that need to be small and work well.
Optical filters are super crucial for controlling. How light moves in different things we use every day. They make images more transparent and help signals work better. Let scientists study things better. Even though they have some limits, intelligent people are constantly working to make them even better for the future.
Imagine light as an orchestra, with every color a different instrument. Optical filters act like conductors, controlling which instruments play and which stay silent.
They can block unwanted colors (like sunglasses), isolate specific ones (for analysis in science), or adjust the volume (in photography). This magic of light control finds uses everywhere. Vibrant stage shows study distant stars, making our world a more colorful and informative place.
Optical filters let some types of light pass through while blocking others. They&#;re like super picky bouncers at a party. Letting in the guests they like while keeping out the ones they don&#;t. This helps control the colors and brightness of the light. Making things like pictures more explicit or signals stronger.
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