Optical filters are passive devices that allow the transmission of a specific wavelength or set of wavelengths of light. There are two classes of optical filters that have different mechanisms of operation: absorptive filters and dichroic filters.
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Absorptive filters have a coating of different organic and inorganic materials that absorb certain wavelengths of light, thus allowing the desired wavelengths to pass through. Since they absorb light energy, the temperature of these filters increases during operation. They are simple filters and can be added to plastics to make less costly filters than their glass-based counterparts. The operation of these filters does not depend on the angle of the incident light but on the properties of the material that makes up the filters. As a result, they are good filters to use when reflected light of the unwanted wavelength can cause noise in optical signal.
Dichroic filters are more complicated in their operation. They consist of a series of optical coatings with precise thicknesses that are designed to reflect unwanted wavelengths and transmit the desired wavelength range. This is achieved by causing the desired wavelengths to interfere constructively on the transmission side of the filter, while other wavelengths interfere constructively on the reflection side of the filter.
There are three types of optical filters: shortpass filters, longpass filters, and bandpass filters. A shortpass filter allows shorter wavelengths than the cut-off wavelength to pass through, while it attenuates longer wavelengths. Conversely, a longpass filter transmits longer wavelengths than the cut-on wavelength while it blocks shorter wavelengths. A bandpass filter is a filter that lets a particular range, or &#;band&#;, of wavelengths to go through, but attenuates all wavelengths around the band. A monochromatic filter is an extreme case of a bandpass filter, which transmits only a very narrow range of wavelengths.
Optical filters are passive optical devices that consist of specialized optical coatings applied onto a substrate. The coatings modify the refractive index of the substrate, enabling them to reflect, transmit, or absorb incoming light depending on its wavelength. This quality is useful for various optical tools and systems, such as chemical analysis units and microscopes.
Optical filters are available in many variations, each of which possesses distinct characteristics that make it suitable for particular applications. Below, we provide an overview of some of the different types available.
Absorptive filters have coatings made from organic and inorganic materials. These materials enable the filter to absorb the undesirable wavelengths and transmit the desirable wavelengths. This design ensures that no energy is reflected back toward the light source.
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In contrast to absorptive filters, dichroic filters&#;also called thin-film filters or interference filters&#;have coatings that enable them to reflect the undesirable wavelengths and transmit the desirable wavelengths. The thickness and properties of the coatings determine which wavelengths are reflected and which wavelengths are transmitted. These types of optical filters are highly accurate, enabling users to target a small range of wavelengths.
Notch filters&#;also called band-stop filters or band-reject filters&#;are designed to block a specific frequency band (i.e., the stopband frequency range). Any wavelengths above or below this range are allowed to pass through freely. These types of optical filters are ideal for applications involving the combination of two or more signals since they can help isolate out interference.
In contrast to notch filters, bandpass filters are designed to block every frequency except for a small range. They are a combination of shortpass filters and longpass filters&#;filtering out any wavelengths that are too short or too long. This cutoff range can be lengthened or narrowed by adjusting the number of layers in the filter.
Shortpass filters are designed to transmit wavelengths below a set length determined by the optical coating and substrate. Any wavelengths that are longer than that point are blocked. These types of optical filters are commonly used to isolate specific higher regions of a broad spectrum and in conjunction with longpass filters for bandpass filtration applications. Typical applications include chemical analysis systems.
Longpass filters are designed to transmit wavelengths above a set length determined by the optical coating and substrate. Any wavelengths that are shorter than that point are blocked. Typical applications include fluorescent spectroscopy systems. Additionally, they are commonly used in conjunction with shortpass filters for bandpass filtration applications.
Want to learn more about optical filters and how to choose the right one for your optical needs? Turn to the experts at Evaporated Coatings! We specialize in the supply of high-precision optical coatings. By helping customers select the right coating and applying it to their substrates, we can make custom optical filters for virtually any application.
Check out our custom optical filters page to learn more about our thin-film coating capabilities. To discuss your optical filter requirements with one of our team members, contact us today.
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