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An optical filter is usually meant to be a component with a wavelength-dependent (actually frequency-dependent) transmittance or reflectance, although there are also filters where the dependence is on polarization or spatial distribution, or some uniform level of attenuation is provided. Filters with particularly weak wavelength dependence of the transmittance are called neutral density filters.
There are many different types of optical filters, based on different physical principles:
Absorbing glass filters, dye filters, and color filters are based on intrinsic or extrinsic wavelength-dependent absorption in some material such as e.g. a glass, a polymer material or a semiconductor. For example, one may exploit the intrinsic short-wavelength absorption of a semiconductor, or extrinsic absorption caused by certain ionic impurities or by semiconductor nanoparticles in a glass. As the absorbed light is converted into heat, such filters are usually not suitable for high-power optical radiation.
Various kinds of optical filters are based on interference effects, combined with wavelength-dependent phase shifts during propagation. Such filters &#; called interference filters &#; exhibit wavelength-dependent reflection and transmission, and the light which is filtered out can be sent to some beam dump, which can tolerate high optical powers.
An important class of interference-based filters contains dielectric coatings. Such coatings are used in dielectric mirrors (including dichroic mirrors), but also in thin-film polarizers, and in polarizing and non-polarizing beam splitters. Via thin-film design it is possible to realize edge filters, low-pass, high-pass and band-pass filters, notch filters, etc.
The same physical principle is used in fiber Bragg gratings and other optical Bragg gratings such as volume Bragg gratings.
Apart from step-index structures, there are also gradient-index filters, called rugate filters. That approach allows one to make high-quality notch filters, for example.
Fabry&#;Pérot interferometers, etalons and arrayed waveguide gratings are also based on interference effects, but sometimes exploiting substantially larger path length differences than monolithic devices. Therefore, they can have sharper spectral features.
Lyot filters are based on wavelength-dependent polarization changes. Similar devices are used as birefringent tuners in tunable lasers.
Other filters are based on wavelength-dependent refraction in prisms (or prism pairs) or on wavelength-dependent diffraction at gratings, combined with an aperture.
There are acousto-optic tunable filters, where it is exploited that Bragg reflection at an acoustic wave works only within a narrow frequency range.
While most types of optical filters exhibit fixed optical characteristics, some types are tunable, i.e., their optical characteristics can be actively modified. Some examples:
See the article on tunable optical filters for more details.
Concerning the shape of the transmission curve, there are
Of course, a wide range of filter shapes can also be realized, particularly with interference filters.
Figure 1:
Wavelength-dependent reflectance of a dielectric edge filter with high transmittance below 980 nm and high reflectance above nm. Starting from an analytically formulated design, the performance has been further optimized numerically (using the software RP Coating).Such a filter can be used for injecting pump light into the ytterbium-doped crystal of a laser.
Some examples of the many applications of optical filters are:
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Light is central to our existence, from the process of photosynthesis which generates much of our food to illuminating our homes. But some of its most fascinating applications are in industrial and scientific applications. Various methods of modifying the spectral properties of light drive everything from commercial photography and security to weapons guidance, space exploration, spectroscopy, and more.
Optical filters represent one of the key components used in altering the properties of light. Using glass (or sometimes plastic) as a substrate, optical filters are made by assembling thin films of materials that affect light &#; how much is absorbed, reflected or transmitted, its polarization, or otherwise changing it for its intended purpose. If you&#;re not sure which optical filter best suits the needs of your business, look no further. Here&#;s a rundown of the different types of optical filters featuring a wide range of coatings available on substrates like glass, plastic, metal and ceramic.
Without the full understanding of its purpose, an optical filter simply appears as an inconspicuous piece of glass or plastic. The design and treatment determine how it will alter the light resulting in a range of industrial, scientific, and consumer applications.
Also called interference filters, bandpass filters allow only a specific wavelength band to pass through. Physicists Charles Fabry and Jean-Baptiste Alfred Perot discovered the application of bandpass filters in astronomy as noted in the Astrophysical Journal. The Fabry-Perot interferometer earned its name due to their discovery.
Modern interference filters have their central wavelength tuned to address specific requirements in spectroscopy, photometry, clinical chemistry, or laser separation among others. An examination of the use of bandpass filters for astronomical photometry can be found in an article on filter profiles and zero points from the Astronomical Society of the Pacific.
Long-pass filters transmit longer wavelengths of light while blocking shorter wavelengths. Short-pass filters serve the opposite purpose &#; transmitting shorter wave bands and blocking longer wavelengths. Fluorescence spectroscopy use such filters in, for example, cell biology or DNA studies, separating excitation energy from the fluorescence band.
Lasers often emit radiation that is not totally monochromatic, and users rarely want other wavelengths to interfere with their primary purpose. A laser line filter eliminates this unfavorable radiation and allows engineers, doctors, and scientists to perform surgical procedures, enable fiber-optic communications, sequence DNA, create eye-popping laser light shows for theatrical productions, and much more.
Neutral density filters reduce the intensity of light evenly across wide wavelength bands. For photographers wishing to alter shutter speed and aperture and thus create unique, experimental images that aren&#;t overexposed, neutral-density filters are essential, even for exposure lengths of 10 minutes or more at high F-stops.
Elaborate theatrical light shows and retail displays require the gamut of precise colors to produce stunning visual effects. Dichroic filters, essentially color separation filters, provide the means to control and project specific colors brightly and efficiently.
It is necessary to ensure precise color integrity in photographic, stage lighting, medical and industrial applications. Color temperature correction filters control light-wavelength transmission and reflection as needed for the accurate appearance of even the most esoteric color blends. These coatings tolerate temperatures up to 650º C (1,202º F), making them ideal for high-intensity lighting.
UV-blocking filters protect objects from the depredations of ultraviolet light, such as delicate, sensitive and expensive works of art in an art or history museum. Optivex&#; UV blocking filters ensure reliable UV protection without distorting colors in the visible-light region or absorbing excess heat.
Whether you seek non-standard optical filters or want a particular material coating blend to meet your technical specs, our optical experts bring decades of experience and deep expertise in optics to help you design filters that match your requirements. Contact us today to learn more about custom options or request a quote.
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