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.
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.
Thin-Film Optical Filter Solutions From Evaporated Coatings, Inc.
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.