How to Select Optical Bandpass Filters
Image credit: Knight Optical Ltd. | Edmund Optics | Thorlabs Inc.
Optical bandpass filters are optical filters that pass one or more specified wavelength band(s) while blocking others.
Bandpass filters are referred to by the wavelength range, also known as the pass band, which they are designed to transmit. They are common filters suitable for a wide range of optical applications, including environmental testing, colorimetry, flame photometry, fluorescence spectrometry, and laser line separation.
The video below provides an excellent technical overview of filter use in a spectrophotometry application, including a description of several specifications described below.
Video credit: kridnix
The term "bandpass filter" can describe two very different types of filter, depending upon its use in context.
In a broad sense, a bandpass filter may describe the entire family of optical filters used to transmit a select group of wavelengths. These include many different entities, including color filters, hot and cold mirrors, as well as long pass and short pass filters. This term is most often used in broad discussions about optics and filters.
In a narrower sense, a bandpass filter represents a more specific entity: a precision interference filter which passes a well-defined band of light — typically within the visible spectrum — with a bandwidth of one to several hundred nanometers (nm). This selection guide deals with this more specific type of filter.
Bandpass filters are specified and described using numerous parameters, including classification by spectrum and information about their transmitting wavelength. For broader optical filter information, including specifications, filter types, surface quality, and relevant standards, please visit the Optical Filters Selection Guide.
Bandpass filters can be broadly classified by the type of electromagnetic radiation they are designed to filter. These types include ultraviolet, visible, and infrared light.
Filters designed to operate within the ultraviolet spectrum filter light with wavelengths from
4 to 380 nm.
Those designed to operate within the visible spectrum filter wavelengths from
380 to 750 nm.
Filters designed for infrared filtering operate on wavelengths from
750 to 2500 nm.
The whole of the electromagnetic spectrum, including all three types of radiation listed above, is shown below. Note that most optical bandpass filters are constructed to filter a small range of radiation, meaning that the wavelength specifications discussed in sections below would automatically classify them as ultraviolet, visible, or infrared.
The full electromagnetic spectrum. Image credit: Premedia
Pass Band Specs
The range and quality of a filter's pass band is based on the interaction of three important filter specifications: center wavelength (CW), full width at half maximum (FWHM), and peak transmittance (Tpk).
Peak transmittance, also referred to as Tpk, represents the maximum percentage of transmitted light within the passband. When discussing a filter's pass band, one half of the peak transmittance value may be referred to as half maximum (HM).
Full width at half maximum (FWHM) is also referred to as a filter's bandwidth. FWHM refers to the range of wavelengths passed through the filter when the transmission value is at 50% of its specified minimum peak.
Center wavelength represents the midpoint of the bandwidth or FWHM.
The image below graphically describes all three of these values. Note that only wavelengths with positive transmission percentage values, indicated by the red line, are transmitted through the filter; all other wavelengths are blocked.
Bandwidth graph. Image credit: Edmund Optics
A specific filter example helps to illustrate the importance of these three specifications. If a bandpass filter has a center wavelength of 700 nm and a FWHM of 20 nm, the filter will only pass electromagnetic radiation with wavelengths between 690 and 710 nm. Note that this particular filter's center wavelength and FWHM naturally classifies it as compatible with the visible spectrum as described above.
Just as pass band specifications describe a filter's ability to transmit selected wavelengths, a product's optical density represents its effectiveness in blocking unwanted wavelengths.
A bandpass filter's optical density is related to the amount of energy transmitted through it. While a superior filter may feature a high transmission percentage for its pass band, its optical density should naturally be much lower, as described below.
There are two important equations involving optical density.
T = transmission percentage
OD = optical density
By inserting values into these equations, it becomes clear that a higher optical density value represents a lower transmission value and is therefore desirable. The graph below illustrates this by showing the correlation between three OD and percent transmission values.
Image credit: Edmund Optics