What Is an Optical Filter and Its Types?

A filter is any device that allows some materials to pass through a material while stopping others.

When it comes to light, an optical filter is any device that selectively transmits a range of wavelengths while rejecting others. Optical filters are used for a range of applications, including photography, clinical chemistry, and microscopy, among others.

Additionally, there are various types of optical filters. Here is a look at each one of these optical filter types and their uses.

What Is an Optical Filter?

An optical filter is a device that selectively transmits a portion of the optical spectrum while rejecting the transmission of other wavelengths. Depending on the means by which filters block unwanted wavelengths, they can be classified as either dichroic or absorptive.

Dichroic filters, which are also referred to as interference filters, reflect all the undesired wavelengths and allow the desired wavelengths to pass through. On the other hand, absorptive filters do not reflect but absorb certain wavelengths and transmit the remaining wavelengths.

Types of Optical Filters

Optical filters are mainly classified as absorptive and dichroic filters.

However, there are other classifications of optical filters based on applications, the type of wavelengths they transmit, and their construction materials, as explained below.

Bandpass Filter

Bandpass filters block all wavelength frequencies except for a narrow range. The bandpass filter is a combination of the shortpass and longpass filter. These filters block all wavelengths that are either too long or too short than the cutoff range. The cutoff range can be manipulated to transmit a wider or narrower range of light by changing the filter’s layers.

Bandpass Filter

Shortpass Filter

A shortpass filter transmits a short wavelength of light that is set below a predetermined length. This length is determined by the optical coating and substrate. All other wavelengths that are longer than the set length are blocked. These types of filters are used together with longpass filters.

Shortpass Filter

Longpass Filters

As mentioned earlier, longpass filters are used with shortpass filters in bandpass filtration applications such as fluorescence spectroscopy and chemical analysis systems. These filters transmit wavelengths that are above a specified length which is determined by the optical coating and substrate. All other wavelengths that are shorter than the set point are blocked.

Longpass Filter

Infrared Filters

Infrared filters are heat-absorbing filters. These filters block the visible rays of light and pass the red light spectrum, also called infrared light. IR filters are often used in infrared photography to create unique and stunning infrared images.

Ultraviolet Filters

Unlike IR filters, ultraviolet filters cut off ultraviolet rays and transmit visible light. UV filters can be classified into inorganic and organic UV filters. The former works by reflecting the UV rays, while the latter absorbs the ultraviolet light. These filters are commonly used in film photography as they help reduce photo discoloration caused by UV light. UV filters are also used in digital cameras to protect the camera lens from scratches, dust, and other hazards.

Other Types of Optical Filters

To help you understand the similarities and differences between the varieties of optical filters available, here is a detailed look at other types of filters that haven’t been mentioned above.

•         Cold and hot mirrors – These two filters are types of dichroic filters that remove unwanted heat energy from light sources. Cold mirrors have high reflectivity in the visible light spectrum and allow the transmission of infrared wavelengths. Hot mirrors reflect infrared light and transmit visible light.
  

  Cold Mirror

•         Notch filters – Notch filters block selected bandwidth and transmit every other wavelength within the filter’s range. These filters remove a narrowband of the wavelength from the optical system, and they don’t affect the transmission of the other wavelengths.
  

  Notch Filter

•         Color filter substrate – Colored glass filters are made of slim glass substrates, and three types of color resist. The color resist forms a lattice pattern on the glass such that the display can express a wide variety of colors.
•         Neutral density filters – ND filters are used to reduce the intensity of the wavelength of light by filtering part of each wavelength. Neutral density filters are categorized into absorptive and reflective ND filters. The absorptive ND filter absorbs the light that isn’t transmitted through the filter, while the reflective ND filter reflects it back to the direction from which it was incident. The ND filters are used to capture images accurately in bright light or to prevent what is referred to as blooming.
  

  Neutral Density Filter

•         Polarization filters – These filters transmit or block light depending on the polarization of the filter. For instance, the filter can transmit light that vibrates in one direction and reflect light vibrating from a perpendicular direction. Polarization filters, such as the ones found in sunglasses, create better visibility by blocking the light spectrum that causes reflection. Other polarization filters help darken bright components, such as the sky, in photography.

Conclusion

Hopefully, now you’re informed on what optical filters are and the different types available.

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