An Introduction to the Different Types of Optical Filters

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.

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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

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.

Dichroic Filters

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

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.

Bandpass Filters

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

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

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.

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.

What are optical filters? 

Optical filters are components that selectively transmit or block certain wavelengths or ranges of light while allowing others to pass through. These filters are commonly used in various optical systems and instruments to manipulate the spectral content of light. Optical filters play a crucial role in applications such as photography, microscopy, spectroscopy, telecommunications, and many other fields. There are different types of optical filters designed to perform specific functions: 

Color Filters: These filters are designed to transmit specific colors of light while absorbing or blocking others. In photography, for example, color filters are often used to enhance or modify the colors in a scene. 

Neutral Density Filters: These filters uniformly reduce the intensity of light across all wavelengths without affecting the color balance. They are often used in imaging systems to control the amount of light entering a camera, especially in situations with bright lighting conditions. 

Polarizing Filters: These filters selectively block or transmit light waves that are oriented in a particular direction. Polarizing filters are commonly used in photography to reduce glare and reflections from surfaces like water and glass. 

Interference Filters: These filters work based on the principle of interference and are designed to transmit specific wavelengths of light while blocking others. They are used in applications like spectroscopy and fluorescence microscopy. 

Bandpass Filters: These filters allow a specific range of wavelengths to pass through while blocking others. They are often used in applications where isolating a particular spectral band is crucial. 

Longpass and Shortpass Filters: Longpass filters transmit longer wavelengths while blocking shorter ones, and shortpass filters do the opposite. They are used in various applications, such as fluorescence microscopy and imaging. 

Infrared (IR) Filters: These filters block visible light and allow infrared light to pass through. They are used in applications like night vision and infrared photography. 

Ultraviolet (UV) Filters: UV filters block ultraviolet light and are often used in photography to reduce the haziness caused by UV radiation. 

Filter glass represents one option for manipulating light in diverse applications while another option uses a thin film coating. The choice between optical filter glass and thin film coatings depends on the specific requirements of the application. In some cases, the durability and fixed characteristics of glass filters are preferred, while in others, the flexibility and customization options offered by thin film coatings make them more suitable. Additionally, cost considerations, environmental factors, and the desired spectral performance all play roles in determining the most appropriate choice for a given application. 
 

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Filter Glass 

Thin Film Coatings 

Material Properties: Optical filter glass is made of specific types of glass that inherently possess the desired optical properties. The glass itself determines the filtering characteristics. 

Customization: Thin film coatings offer greater flexibility in tailoring the spectral characteristics of a filter. Multiple layers of thin films can be deposited on a substrate to achieve specific transmission and reflection properties. 

Spectral Range: The spectral characteristics of glass filters are determined by the inherent properties of the glass material, and they may not be as easily tailored to specific wavelength ranges as thin film coatings. 

Weight and Size: Thin film coatings are generally lighter and can be more compact than glass filters, making them suitable for applications where size and weight are critical factors. 

Applications: Glass filters are commonly used in situations where durability and a fixed spectral range are essential, such as in optical instruments where a specific wavelength or range of wavelengths needs to be selectively transmitted or blocked. 

Spectral Precision: Thin film coatings can achieve higher spectral precision, enabling the creation of narrow-band filters with well-defined wavelength characteristics. 

Esco Optics offers a complete selection of colored glass, neutral density, and bandpass filters. Our filter materials are manufactured from Schott, Hoya and Isuzu materials. If you cannot find the particular filter to satisfy your specific application, our sales engineers will be happy to assist you in selecting a custom filter to meet your needs. Please reach out to the Esco sales staff to discuss your needs to determine what is best for your project.  

Are you interested in learning more about Optical Glass Filters? Contact us today to secure an expert consultation!