Optical filters are devices, typically comprised of plate glass or plastic, which selectively transmit light of different wavelengths.  Filters can be broadly classified as absorptive or dichroic, dependent upon their means of blocking unwanted wavelengths.

Absorptive Filters

As their name implies, absorptive filters transmit desired wavelengths by absorbing unwanted ones. These filters typically consist of dyed glass or pigmented gelatin resins, and are relatively inexpensive to manufacture and purchase. The ability of absorptive filters to attenuate light is based on the filter's physical thickness and the amount of dye or pigmentation present.

Absorptive filtration. Image credit: UC Davis

Absorptive filters are relatively less precise when compared with dichroic types. For this reason, they are often used in applications requiring the transmission of broad band of wavelengths, as well as applications involving the blockage of short wavelengths and transmission of longer ones.

Absorption filters have several inherent advantages and disadvantages. Their main strong points are their low cost, stability when used in a wide variety of environments, and insensitivity to illumination angle. Disadvantages include poor long-term temperature resistance and inadequate performance when used in precision applications. Absorptive filters, especially gelatin types, are suitable for a wide range of applications, including optical microscopy.

Dichroic (Interference) Filters

Dichroic filters, also known as interference filters, filter light by rejecting all undesired wavelengths, allowing selected wavelengths to pass through. The term "dichroic" is derived from the Greek word díchros, meaning "of two colors." These filters are constructed using thin film technology by depositing several layers of dielectric film on one side of an optically flat piece of transparent glass. When light strikes the coated side of the filter, the various layers of film magnify and transmit the desired wavelengths while reflecting and reducing the undesired ones.

Cross-section of a typical interference filter. Image credit: Florida State University

As shown in the image above, dichroic filters rely on the series of reflective cavities between the film layers to achieve their precise filtering. These cavities resonate with the desired wavelength frequencies while destroying or rejecting all others; this is known as optical interference.

Compared to absorptive types, interference filters are capable of significantly improved selectivity and are much more suitable to precision applications. This superior precision makes dichroic filters much more delicate and expensive than absorptive filters.

Image credit: Florida State University

The table below compares both filter technologies, noting their advantages, disadvantages, and common applications.

Types

Optical filters can be classified into different types, dependent upon their construction, applications, or type of light they filter.

Color Filters

Color filters are simply colored glasses distinguished by their specific color wavelengths. These filters typically only appear colored if their filter action is within the visible light spectrum. Color filters are typically specified by a prefix which determines glass color as well as transmission properties. Examples include:

• UG: black and blue glasses, ultraviolet (UV) transmitting
• VG: green glass
• GG: nearly colorless to yellow glasses, infrared (IR) transmitting
• RG: red and black glasses, IR transmitting
• N-WG: Colorless glasses, visible and IR transmitting

Hot and Cold Mirrors

Hot mirrors and cold mirrors are dichroic filters that selectively transmit certain wavelengths.

Hot mirrors transmit visible light wavelengths while reflecting near-infrared (near-IR) bands. Because near-IR wavelengths generate significant heat, hot mirrors are used to reflect this heat. Hot mirrors are typically light yellow in color.

Cold mirrors are similar to hot mirrors in that they separate IR and non-IR wavelengths. Cold mirrors, however, are designed to transmit IR bands while reflecting one or more non-IR bands, such as visible light. This helps isolate wavelengths needed for a particular application while eliminating the unneeded or potentially harmful bands. Cold mirrors are sometimes used in a "reversed" fashion, in that they reflect visible light to the application while transmitting unneeded IR light away from the application.

Cold mirror operation. Image credit: Harvard University

Hot and cold mirrors can be used in conjunction to protect optical fiber, which may be harmed by UV and IR wavelengths. In this application, the hot mirror would reflect the UV and IR light while transmitting visible light, and the cold mirror would reflect the desired visible light to the fiber while transmitting the IR wavelengths to a designated disposal location.

Long- and Shortpass Filters

Longpass and shortpass filters are named for the wavelengths they transmit and attenuate. Longpass filters transmit (or "pass") longer wavelengths and block shorter ones, while shortpass filters block longer wavelengths and pass shorter ones. Both are frequently used in fluorescence microscopy. Long- and shortpass filters with very sharp transmission slopes are sometimes called edge filters.

Neutral Density Filters

Neutral density filters, also known as ND filters or grey filters, reduce light transmission evenly across a portion of the spectrum. These filters are slightly sensitive to angles, but less so than interference filters. ND filter technology is common in consumer applications, particularly photography.

Spectral Filters

Spectral filters simply transmit select wavelengths. They can be useful in numerous applications requiring emphasis of certain light patterns, including identification of laser patterns and sharpening of infrared images.

The Visible Spectrum

When discussing and selecting optical filters, it is important to consider the visible portion of the electromagnetic spectrum. The human eye responds to wavelengths from around 390 nm (or 430 THz) to 700 nm (or 790 THz); the frequencies within this range are therefore called "visible." (Wavelength literally refers to the length of one electromagnetic "wave", as seen in the full spectrum image below.) Optical filter specifications may indicate a specific range of frequencies or a specific color to be transmitted. A light source's wavelength determines the color perceived by the eye, as shown in the image below.

Image credit: Giangrandi

Infrared and Ultraviolet Light

Optical filters may also be designed to transmit or reflect portions of the infrared (IR) or ultraviolet (UV) spectrum.

Infrared light has longer wavelengths than visible light, and includes most thermal radiation emitted by near-room temperature objects. It includes wavelengths from 1 mm up to 700 nm, and frequencies from 300 GHz to 430 THz. Infrared light has a wide variety of applications in various sectors, including night vision technology, meteorology, thermography, heating, and spectroscopy. The infrared spectrum can be further divided from near-infrared (shorter wavelengths) to far infrared (longer wavelengths).

Ultraviolet light has wavelengths shorter than visible and infrared light. Ultraviolet wavelengths occur from 400 nm to 100 nm, and are further divided into Ultraviolet A (400-315 nm), B (315-280 nm), and C (280-100 nm) designations. Natural sunlight consists of around 10% UV radiation. Ultraviolet light is invisible to the unaided human eye, and high intensities of UV radiation can be harmful to the skin and eyes. Like infrared light, it is useful in a wide range of diverse imaging applications.

The full electromagnetic spectrum, showing the relationship between wavelength and frequency.

Image credit: TechtheFuture

The selection of optical filters involves specifying either a broad spectrum (ie visible, infrared, or ultraviolet) or a specific range (ie 540-541 nm) to be transmitted or rejected.

Filter Surface Quality

The optics industry rates product surface quality based on MIL-0-13830A standards published in 1963. These scratch/dig ratings consist of two numbers separated by a hyphen, as in x - y; x refers to scratches, while y refers to digs, as described below.

A scratch refers to a surface defect whose length is many times its width. The initial number of the scratch-dig rating describes the maximum allowable width, expressed in tenths of microns, of a scratch.

A dig is a defect on an optical surface that is nearly equal in length and width. The second value of the rating refers to the maximum diameter of a dig expressed in hundredths of a millimeter.

For example, a #20-10 indicates that any scratches have a maximum width of .002 mm (or 2 microns), while digs may have a maximum diameter of .10 mm. When considering the two rating values, it becomes clear that smaller values are desirable.

Standards

Optical filters may be manufactured according to different standards or approvals. Some examples include:

ISO 8577 -- Spectral filters 

DIN 9211-2 -- Optics and photonics - Optical coatings - Part 2: Optical properties (ISO 9211-2:2010)

References

University of Edinburgh - Spectral Filtering

Image credit:

Knight Optical (UK) Ltd.