Dichroic filter

A dichroic filter, thin-film filter, or interference filter is a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the color(s) of light that they reflect, rather than the color(s) they pass. (See dichroism for the etymology of the term.)

Dichroic filters can filter light from a white light source to produce light that is perceived by humans to be highly saturated (intense) in color. Although costly, such filters are popular in architectural[1] and theatrical applications.

Dichroic reflectors are commonly used behind a light source to reflect visible light forward while allowing the invisible infrared light (radiated heat) to pass out of the rear of the fixture, resulting in a beam of light that is literally cooler (of lower thermal temperature). Such an arrangement allows a given light to dramatically increase its forward intensity while allowing the heat generated by the backward-facing part of the fixture to escape. Many quartz halogen lamps have an integrated dichroic reflector for this purpose, being originally designed for use in slide projectors to avoid melting the slides, but now widely used for interior home and commercial lighting. This improves whiteness by removing excess red; however, it poses a serious fire hazard if used in recessed or enclosed luminaires by allowing infrared radiation into those luminaires. For these applications non cool beam (ALU or Silverback) lamps must be used.

Dichroic filters
Dichroic filters

Theory

Dichroic filters use the principle of thin-film interference, and produce colors in the same way as oil films on water. When light strikes an oil film at an angle, some of the light is reflected from the top surface of the oil, and some is reflected from the bottom surface where it is in contact with the water. Because the light reflecting from the bottom travels a slightly longer path, some light wavelengths are reinforced by this delay, while others tend to be canceled, producing the colors seen.

In a dichroic mirror or filter, instead of using an oil film to produce the interference, alternating layers of optical coatings with different refractive indices are built up upon a glass substrate. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually added by vacuum deposition. By controlling the thickness and number of the layers, the frequency (wavelength) of the passband of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and so do not become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband). (See Fabry–Pérot interferometer for a mathematical description of the effect.)

Where white light is being deliberately separated into various color bands (for example, within a color video projector or color television camera), the similar dichroic prism is used instead. For cameras, however it is now more common to have an absorption filter array to filter individual pixels on a single CCD array.

Applications

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IEC 60598 No Cool Beam symbol

Recessed or enclosed luminaires that are unsuitable for use with dichroic reflector lights can be identified by the IEC 60598 No Cool Beam symbol.

In fluorescence microscopy, dichroic filters are used as beam splitters to direct illumination of an excitation frequency toward the sample and then at an analyzer to reject that same excitation frequency but pass a particular emission frequency.

Some LCD projectors use dichroic filters instead of prisms to split the white light from the lamp into the three colours before passing it through the three LCD units.

Dichroic filter from a DLP projector color wheel
Six-segment dichroic color wheel from a DLP projector. Segments transmit red, green and blue, and therefore reflect cyan, magenta, and yellow.

Older DLP projectors typically transmit a white light source through a color wheel which uses dichroic filters to rapidly switch colors sent through the (monochrome) Digital micromirror device. Newer projectors may use laser or LED light sources to directly emit the desired light wavelengths.

They are used as laser harmonic separators. They separate the various harmonic components of frequency doubled laser systems by selective spectral reflection and transmission.

Dichroic filters are also used to create gobos for high-power lighting products. Pictures are made by overlapping up to four colored dichroic filters.

Photographic enlarger color heads use dichroic filters to adjust the color balance in the print.

Advantages

  • Much better filtering characteristics than conventional filters
  • Ability to easily fabricate a filter to pass any passband frequency and block a selected amount of the stopband frequencies (saturation)
  • Because light in the stopband is reflected rather than absorbed, there is much less heating of the dichroic filter than with conventional filters
  • Much longer life than conventional filters; the color is intrinsic in the construction of the hard microscopic layers and cannot "bleach out" over the lifetime of the filter (unlike for example, gel filters)
  • Filter will not melt or deform except at very high temperatures (many hundreds of degrees Celsius)
  • Capable of achieving extremely high laser damage thresholds (dichroics are used for all the mirrors on the world's most powerful laser, the National Ignition Facility)

Other uses

Artistic glass jewelry is occasionally fabricated to behave as a dichroic filter. Because the wavelength of light selected by the filter varies with the angle of incidence of the light, such jewelry often has an iridescent effect, changing color as the (for example) earrings swing. Another interesting application of dichroic filters is spatial filtering.[2]

With a technique licensed from Infitec, Dolby Labs uses dichroic filters for screening 3D movies. The left lens of the Dolby 3D glasses transmits specific narrow bands of red, green and blue frequencies, while the right lens transmits a different set of red, green and blue frequencies. The projector uses matching filters to display the images meant for the left and right eyes.[3]

Long-pass dichroic filters applied to ordinary lighting can prevent it from attracting insects. In some cases, such filters can prevent attraction of other wildlife, reducing adverse environmental impact.[4]

See also

References

  1. ^ The Copenhagen Opera House
  2. ^ Optics Letters
  3. ^ Shankland, Stephen (2007-10-09). "Dolby Stakes Its Claim in 3D Movie Tech". CNET. CBS Interactive. Archived from the original on 2012-02-24. Retrieved 2016-12-08.
  4. ^ Witherington, Blair E.; Martin, R. Erik (2003). "Understanding, Assessing, and Resolving Light-Pollution Problems on Sea Turtle Nesting Beaches" (PDF). Florida Marine Research Institute Technical Report TR-2 (3rd ed.). Florida Fish and Wildlife Conservation Commission: 23. ISSN 1092-194X.

Further reading

3LCD

3LCD is the name and brand of a major LCD projection color image generation technology used in modern digital projectors. 3LCD technology was developed and refined by Japanese imaging company Epson in the 1980s and was first licensed for use in projectors in 1988. In January 1989, Epson launched its first 3LCD projector, the VPJ-700.Although Epson still owns 3LCD technology, it is marketed by an affiliated organization simply named after the technology:"3LCD". The organization is a consortium of projector manufacturers that have licensed 3LCD technology to be used in their products. To date, about 40 different projector brands worldwide have adopted 3LCD technology.

According to electronics industry research company Pacific Media Associates, projectors using 3LCD technology comprised about 51% of the world's digital projector market in 2009.3LCD technology gets its name from the three LCD panel chips used in its image generation engine.

Clementine (spacecraft)

Clementine (officially called the Deep Space Program Science Experiment (DSPSE)) was a joint space project between the Ballistic Missile Defense Organization (BMDO, previously the Strategic Defense Initiative Organization, or SDIO) and NASA. Launched on January 25, 1994, the objective of the mission was to test sensors and spacecraft components under extended exposure to the space environment and to make scientific observations of the Moon and the near-Earth asteroid 1620 Geographos. The Geographos observations were not made due to a malfunction in the spacecraft.

The lunar observations made included imaging at various wavelengths in the visible as well as in ultraviolet and infrared, laser ranging altimetry, gravimetry, and charged particle measurements. These observations were for the purposes of obtaining multi-spectral imaging of the entire lunar surface, assessing the surface mineralogy of the Moon, obtaining altimetry from 60N to 60S latitude, and obtaining gravity data for the near side. There were also plans to image and determine the size, shape, rotational characteristics, surface properties, and cratering statistics of Geographos.

Clementine carried seven distinct experiments on-board: a UV/Visible Camera, a Near Infrared Camera, a Long Wavelength Infrared Camera, a High Resolution Camera, two Star Tracker Cameras, a Laser Altimeter, and a Charged Particle Telescope. The S-band transponder was used for communications, tracking, and the gravimetry experiment. The project was named Clementine after the song "Oh My Darling, Clementine" as the spacecraft would be "lost and gone forever" following its mission.

Cold mirror

A cold mirror is a specialized dielectric mirror, a dichroic filter, that reflects the entire visible light spectrum while very efficiently transmitting infrared wavelengths. Similar to hot mirrors, cold mirrors can be designed for an incidence angle ranging between zero and 45 degrees, and are constructed with multi-layer dielectric coatings, in a manner similar to interference filters. Cold mirrors can be employed as dichroic beamsplitters with laser systems to reflect visible light wavelengths while transmitting infrared.

Color filter array

In photography, a color filter array (CFA), or color filter mosaic (CFM), is a mosaic of tiny color filters placed over the pixel sensors of an image sensor to capture color information.

Dielectric mirror

A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified reflectivity at different wavelengths of light. Dielectric mirrors are also used to produce ultra-high reflectivity mirrors: values of 99.999% or better over a narrow range of wavelengths can be produced using special techniques. Alternatively, they can be made to reflect a broad spectrum of light, such as the entire visible range or the spectrum of the Ti-sapphire laser. Mirrors of this type are very common in optics experiments, due to improved techniques that allow inexpensive manufacture of high-quality mirrors. Examples of their applications include laser cavity end mirrors, hot and cold mirrors, thin-film beamsplitters, and the coatings on modern mirrorshades.

Fovea centralis

The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.The fovea is responsible for sharp central vision (also called foveal vision), which is necessary in humans for activities where visual detail is of primary importance, such as reading and driving. The fovea is surrounded by the parafovea belt, and the perifovea outer region. The parafovea is the intermediate belt, where the ganglion cell layer is composed of more than five rows of cells, as well as the highest density of cones; the perifovea is the outermost region where the ganglion cell layer contains two to four rows of cells, and is where visual acuity is below the optimum. The perifovea contains an even more diminished density of cones, having 12 per 100 micrometres versus 50 per 100 micrometres in the most central fovea. This, in turn, is surrounded by a larger peripheral area that delivers highly compressed information of low resolution following the pattern of compression in foveated imaging.Approximately half of the nerve fibers in the optic nerve carry information from the fovea, while the remaining half carry information from the rest of the retina. The parafovea extends to a radius of 1.25 mm from the central fovea, and the perifovea is found at a 2.75 mm radius from the fovea centralis.The term fovea comes from the from Latin foves, meaning 'pit'.

H-alpha

H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha light is important to astronomers as it is emitted by many emission nebulae and can be used to observe features in the Sun's atmosphere, including solar prominences and the chromosphere.

Hot mirror

A hot mirror is a specialized dielectric mirror, a dichroic filter, often employed to protect optical systems by reflecting infrared light back into a light source, while allowing visible light to pass. Hot mirrors can be designed to be inserted into the optical system at an incidence angle varying between zero and 45 degrees, and are useful in a variety of applications where the buildup of waste heat can damage components or adversely affect spectral characteristics of the illumination source. Wavelengths reflected by an infrared hot mirror range from about 750 to 1250 nanometers. By transmitting visible light wavelengths while reflecting infrared, hot mirrors can also serve as dichromatic beam splitters for specialized applications in fluorescence microscopy or optical eye tracking.

Some early digital cameras designed for visible light capture, such as the Associated Press NC2000 and Nikon Coolpix 950, were unusually sensitive to infrared radiation, and tended to produce colours that were contaminated with infrared. This was particularly problematic with scenes that contained strong sources of infrared, such as fires, although the effect could be moderated by inserting a photographic hot mirror filter into the imaging pathway. Conversely, these cameras could be used for infrared photography by inserting a cold mirror filter into the imaging pathway, most commonly by mounting the filter on the front of the lens.New incandescent bulbs incorporate hot mirrors, increasing efficiency by redirecting unwanted infrared frequencies back to the filament.

Index of optics articles

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

Index of physics articles (D)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Interference filter

An interference filter or dichroic filter is an optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection.

An interference filter consists of multiple thin layers of dielectric material having different refractive indices. There also may be metallic layers. In its broadest meaning, interference filters comprise also etalons that could be implemented as tunable interference filters. Interference filters are wavelength-selective by virtue of the interference effects that take place between the incident and reflected waves at the thin-film boundaries. The important characteristic of the filter is the form of the leaving signal. It is considered that the best form is a rectangle.

Bandpass filters are normally designed for normal incidence. However, when the angle of incidence of the incoming light is increased from zero, the central wavelength of the filter decreases, resulting in partial tunability. The transmission band widens and the maximum transmission decreases. If λc is the central wavelength, λ0 is the central wavelength at normal incidence, and n* is the filter effective index of refraction, then:

For example, for λ0=1550 nm, n*=1.5, Δλ = λ0−λc=32 nm, the rotation angle is θ = 17.7°. This corresponds to C-band or L-band in 1550 nm fiber-optic communications window. Equipped with a stepper motor and electronics, a tunable optical filter that tunes center transmission wavelength over C-band or L-band by remote control can be achieved. See diagram below for its working principle and tunable optical filter devices.

Iridescence

Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change colour as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings and seashells, as well as certain minerals. It is often created by structural coloration (microstructures that interfere with light).

Pearlescence is a related effect where some or all of the reflected light is white, where iridescent effects produce only other colours. The term pearlescent is used to describe certain paint finishes, usually in the automotive industry, which actually produce iridescent effects.

List of infrared articles

This is a list of infrared topics.

List of things named after James Clerk Maxwell

This is a list of things named for James Clerk Maxwell.

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 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.Filters mostly belong to one of two categories. The simplest, physically, is the absorptive filter; then there are interference or dichroic filters.

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; 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 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.

Thin-film optics

Thin-film optics is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin-film optics, the thickness of the layers of material must be on the order of the wavelengths of visible light (about 500 nm). Layers at this scale can have remarkable reflective properties due to light wave interference and the difference in refractive index between the layers, the air, and the substrate. These effects alter the way the optic reflects and transmits light. This effect, known as thin-film interference, is observable in soap bubbles and oil slicks.

More general periodic structures, not limited to planar layers, are known as photonic crystals.

In manufacturing, thin film layers can be achieved through the deposition of one or more thin layers of material onto a substrate (usually glass). This is most often done using a physical vapor deposition process, such as evaporation or sputter deposition, or a chemical process such as chemical vapor deposition.

Thin films are used to create optical coatings. Examples include low emissivity panes of glass for houses and cars, anti-reflective coatings on glasses, reflective baffles on car headlights, and for high precision optical filters and mirrors. Another application of these coatings is spatial filtering.

Three-CCD camera

A three-CCD (3CCD) camera is a camera whose imaging system uses three separate charge-coupled devices (CCDs), each one receiving filtered red, green, or blue color ranges. Light coming in from the lens is split by a complex prism into three beams, which are then filtered to produce colored light in three color ranges or "bands". The system is employed by high quality still cameras, telecine systems, professional video cameras and some prosumer video cameras.

Wood's glass

Wood's glass is an optical filter glass invented in 1903 by American physicist Robert Williams Wood (1868–1955), which allows ultraviolet and infrared light to pass through, while blocking most visible light.

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