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How Do Optical Filters Work? What You Need to Know

Optical filters are primarily designed to transmit certain wavelengths of light — or part of the light spectrum — while blocking others. This forms the basis of their application. Below, we examine how optical filters work. Also, how their operation makes them useful in many different fields and industries where the science of optics is involved.

What are Optical Filters?

An optical filter, when used in the path of light, means an optical component that’s capable of isolating and blocking or transmitting certain wavelengths of light. In other words, it selectively transmits light and its different colors. Optical filters are also generally classified as long pass and short pass, normally based on the type light wavelengths they can block or transmit.

Long Pass Optical Filter

The long pass optical filter is named so for allowing longer wavelengths above a specified cutoff point. Put another way, these types of optical filters generally block or reject the shorter wavelengths of light. In terms of application, the long pass filter is often used in precision spectroscopy to separate wavelength bands, as blocking filters in photometric devices, and more.

Short Pass Optical Filter

Unlike the long pass type, the short pass optical filter allows short wavelengths while rejecting the longer ones. Because it blocks light on the lower end of the spectrum, this type of filter finds useful applications in imaging applications. These can be short pass filters that isolate and block infrared light in photography and microscopy or Raman spectroscopy devices.

Interference filter working principle
Interference filter working principle
Resource: https://www.researchgate.net

How Do Optical Filters Work?

The ability by optical filters to block or transmit light is based on two different working principles: filtering by absorption and filtering by interference. These are the light filtering methods that we’ll use to explain how optical filters work, in addition to comparing them.

Absorptive Filters

Absorptive filters, just as their name reveals, work by absorbing a specific color or wavelength range. These are mainly colored glass, although pigmented plastic or lacquered gelatin is also available, and usable in a broad range of environments. This is how optical filters work when using the absorption method.

  • The absorptive filter is made up of filtration layers deposited on the substrate, or it can be dyed glass.
  • Based on the absorptive filter’s design (thickness) and material composition, its atoms can absorb certain wavelengths of light than others.
  • This is what causes them to selectively transmit the photons of light.
  • It doesn’t matter the angle of incident light when using absorptive filters. Instead, it depends on the materials used to make them.

The working of these optical filter types makes them best in situations where reflected light (but that must be absorbed) is likely to cause optical noise. This light filtration method also offers reduced incidences of stray light. Absorption filters are also a low cost option, given their simple working principle and other features.

Optical Interference Filters

These types of optical filters are also commonly called dichroic filters. They are mostly an optical glass sheet several coatings of filtration layers. These coatings are meant to achieve the desired wavelength selectivity, using the principle of light interference. Let’s see how that works:

  • The interference filter working principle is based on the effects of constructive and destructive interference.
  • The multiple filtration layers or coatings possess different refractive indices.
  • As light passes through the layers, it splits into the parts that will pass and those that will be reflected away and scattered or rejected.
  • Successive splitting then causes the separated rays to get into each other’s path and interfere either constructively or destructively in the boundaries between the filtration layers.
  • Wanted wavelengths are allowed to pass through and amplified through constructive interference.
  • On the other hand, unwanted wavelengths are attenuated and their intensity reduced through destructive interference.
  • All this happens inside the filter layers, aided by the filtration coatings or layers of film.

Absorption optical filters are sensitive to angular light, unlike the absorptive types. Based on their working principle, the filters are also characterized by stray light. This light must be damped, or the filters performance will not be satisfactory.

Camera lens filter
Camera lens filter
Resource: https://www.youtube.com/watch?v=cpGo8J-dIgI

Uses of Optical Filters

The selective transmittance of optical filters makes them useful in a number of applications. Now that you understand how optical filters work, let’s see where they’re commonly used. The uses of optical filters cover a vast range of sectors or industries, with the component finding application in many different instruments and devices.

  • In the world of photography, camera filters isolate spectral regions of light, enabling the control of image colors and other aspects. The use of filters in photography is what makes infrared imaging possible, for example.
  • In the field of medicine and life sciences, optical filters are important components of diagnostic and analytical instruments. These include microscopes and other imaging equipment. It allows the visualization of cells and even the detection of diseases by spectroscopy or spectrophotometry.
  • In astronomy and space exploration, optical filters are vital components that help enhance the viewing of celestial bodies by isolating unwanted parts of the spectrum. Astronomical filters allow scientists to examine the light coming from space in order to determine the composition and motion aspects of distant objects.
  • The uses of optical filters also include industrial applications that involve the control of processes or object detection and material analysis. Examples include the monitoring of manufacturing steps, specifically when used alongside sensors and sensing systems.

Conclusion

The selective transmittance of optical filters makes them useful in a number of applications. In this article, we have seen how optical filter work and their most common uses in a range of industries. Based on the type of filter, its operation method, and intended use, one filter might suit an application better than another, and users must carefully choose the type of filter to use.

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