What is an Optical Filter?

Optical filters are passive optical devices that modify the refractive index of a substrate through specialized optical coatings. This modification enables the filters to reflect, absorb, or transmit incoming light, depending on its wavelength. These filters are usually made from plate glass or plastic and they have diverse applications, such as in microscopy, spectroscopy, chemical analysis and machine vision.

The fundamental purpose of an optical filter is to selectively allow the transmission of a specific portion of the optical spectrum while blocking the transmission of other portions. Typically, an optical filter has a wavelength-dependent transmittance or reflectance, although there are also filters that rely on polarization or spatial distribution. Filters with weak wavelength dependence are known as neutral density filters.

Filters can be broadly categorized based on their approach to preventing the transmission of unwanted wavelengths. Optical filters fall into two classes mainly: Absorptive Filter and Dichroic Filter, and each class operates differently to filter specific wavelengths or sets of wavelengths.

Absorptive Filters

Absorptive filters consist of coatings made from organic and inorganic materials that enable them to absorb unwanted wavelengths and transmit desirable ones, preventing any energy from being reflected back towards the light source. They tend to be made from glass with selected compounds added to absorb certain wavelengths while transmitting others. These filters are simple and can be added to plastics to create more affordable options.

Their operation depends on the material properties, not the angle of the incident light, making them effective in reducing noise from reflected light of unwanted wavelengths. But the filters' temperature tends to increase during operation due to the absorption of light energy. They are less precise than dichroic filters and are commonly used in applications that require the transmission of a broad range of wavelengths or the blockage of short wavelengths and transmission of longer ones.

Absorptive filters that absorb light are commonly fabricated from dyed glass or pigmented gelatin resins and are generally low-cost to produce and acquire. The filter's ability to attenuate light depends on the physical thickness and the amount of dye or pigmentation present. Even though they have advantages, such as low cost, stability, and insensitivity to illumination angle, absorptive filters have limitations, including inadequate performance in precision applications and poor long-term temperature resistance. Gelatin types are commonly used in optical microscopy and other applications.

Dichroic Filters

Dichroic filters, which are also referred to as thin-film or interference filters, are optical filters that selectively allow certain wavelengths to pass through while blocking all unwanted wavelengths. Unlike absorptive filters, dichroic filters are made up of a sequence of optical coatings with specific thicknesses that reflect unwanted wavelengths and transmit the desired wavelength range. This makes them extremely precise in targeting particular wavelengths. By producing constructive interference on the transmission side of the filter for desired wavelengths, while constructive interference happens on the reflection side of the filter for other wavelengths, dichroic filters are able to achieve this level of accuracy.

The term "dichroic" comes from the Greek word "díchros," meaning "of two colors." Thin film technology is employed to fabricate these filters by depositing multiple layers of dielectric film onto one side of a transparent glass that is optically flat. When light hits the coated side of the filter, the various layers of film amplify and transmit the desired wavelengths, while reflecting and diminishing the unwanted ones.

Dichroic filters are especially effective in the scientific field because of their precise wavelength targeting. They achieve their precise filtering by relying on a series of reflective cavities between the film layers. These cavities resonate with the desired frequencies of the wavelengths while eliminating or blocking all other frequencies, a phenomenon referred to as optical interference. Compared to absorptive types, interference filters are capable of significantly improved selectivity and are much more suitable for precision applications. Dichroic filters are also utilized in cameras to separate light wavelengths in accordance with the specifications of the film. While both absorptive and dichroic filters have numerous functions, dichroic filters are more delicate and expensive due to their superior precision.

Other types of Optical Filters

Notch Filters

Notch filters, also known as band-stop filters or band-reject filters, are used to obstruct a particular frequency band known as the stopband frequency range. Any wavelengths above or below this range are allowed to pass through without hindrance. These filters are particularly useful for applications that require the combination of two or more signals as they can help in isolating out any interference.

Bandpass Filters

Bandpass filters allow a specific frequency range to pass through while blocking all others. They are constructed by combining shortpass and longpass filters to filter out wavelengths that are either too short or too long. The cutoff range can be adjusted by varying the number of layers in the filter.

Shortpass Filters

Optical shortpass filters allow light waves below a certain threshold to pass through, while blocking those with wavelengths above that point. These filters are often utilized to isolate specific regions of a broad spectrum, working in conjunction with longpass filters to achieve bandpass filtration. Applications of shortpass filters include chemical analysis systems, where they are used to separate out specific regions of a spectrum. The cutoff wavelength of a shortpass filter is determined by its substrate and optical coating, and can be adjusted as needed.

Longpass Filters

Longpass filters are optical filters that allow wavelengths above a specified cutoff point to pass through while blocking shorter wavelengths. The cutoff point is determined by the properties of the optical coating and substrate. Longpass filters are often used in applications such as fluorescent spectroscopy systems. They can also be used in conjunction with shortpass filters to create bandpass filters for isolating specific wavelength ranges.

Neutral Density Filters

Neutral density filters, which are also called grey filters or ND filters, evenly decrease the transmission of light across a portion of the spectrum. While these filters are somewhat angle-sensitive, they are less so compared to interference filters. ND filter technology is common in consumer applications, especially in photography.

Color Filters

Color filters are essentially tinted glasses that are identified by their unique color wavelengths. They usually exhibit their color only when they filter within the visible light spectrum. A prefix is used to indicate the glass color and transmission properties of a color filter. Some examples are:

• UG: Ultraviolet (UV) transmitting black and blue glasses
• VG: Green glass
• GG: Infrared (IR) transmitting nearly colorless to yellow glasses
• RG: IR transmitting red and black glasses
• N-WG: Visible and IR transmitting colorless glasses

Polarization Filters

Polarization filters operate by selectively blocking or allowing the passage of light based on the filter's polarization. Surfaces with a high degree of polarization, such as water or shiny materials, are known for producing reflections that can hinder visibility. By filtering out a portion of the spectrum responsible for these reflections, polarized filters found in sunglasses and certain camera lenses can enhance visibility. Also, polarization filters can reduce the intensity of bright elements in color photography, like the sky which emits a significant amount of ultraviolet light. They can also minimize the appearance of reflections in the captured image.

Applications of Optical Filter

Photography is one of the most popular applications of optical filters. Filters such as polarizing filters and neutral density filters are widely used in photography. Polarizing filters can be used to reduce glare and reflections from non-metallic surfaces, and color correction filters are used to adjust the color temperature of light sources. 

Optical filters are also used in various medical instruments, including microscopes and endoscopes. They are used in spectroscopy to selectively filter specific wavelengths of light. Filters such as bandpass filters and longpass filters are used to selectively transmit specific wavelengths of light, which is essential in imaging and spectroscopic applications.

These filters are used in telecommunications systems to selectively transmit specific wavelengths of light. Optical filters are used in display technology to enhance the color and contrast of images. Color filters and polarizing filters are used in liquid crystal displays (LCDs) and other display technologies to improve image quality.

Optical filters are used in astronomy to isolate specific wavelengths of light. Some of the filters are used in telescopes to isolate specific spectral lines. They are also used in environmental monitoring applications. They are used to selectively transmit or block ultraviolet radiation, which is essential in monitoring the amount of UV radiation in the environment.