What are bandpass filters and why are they important?
Optical bandpass filters are filters that allow a certain spectrum of light to transmit through the substrate, while blocking unwanted frequencies using absorption, reflection or both.
They can be used to transmit signals in a specific range of frequencies, from a very narrow band through to a very wide range.
What is an optical bandpass filter?
Optical bandpass filters transmit (or ‘pass’) a specific range (or ‘band’) of frequencies. This can include visible light, as well as non-visible wavelengths at the infrared and ultraviolet ends of the spectrum.
The result is an output that contains only the desirable frequencies and wavelengths, while the unwanted parts of the spectrum are blocked.
At Envin Scientific the standard range of wavelengths that can be blocked and transmitted by our high-precision optical filters is from 175-200 nm to 8 microns.
How do optical bandpass filters work?
Bandpass filters work by absorbing or reflecting unwanted wavelengths, and only transmitting the desirable parts of the visible light spectrum.
They can be defined using a number of characteristics:
- Blocking Level: How effectively the unwanted wavelengths are eliminated.
- Peak Transmission: How effectively the incident light is transmitted.
- Central Wavelength: The wavelength at the centre of the transmission profile.
- Full Width at Half Maximum: The bandwidth limits between which 50% or more light is transmitted.
For example, very high transmission across the entire FWHM range, with steep drop-off and very low transmission outside of that range, is a sign of a highly effective filter with very little ‘noise’ in the output.
How are optical bandpass filters made?
Optical bandpass filters can be made in several ways:
- Thin film filters and coated filters are made by vacuum deposition of multilayer dielectric coatings on to a substrate.
- Spectral and absorption filters are produced by a combination of lamination, cemented layers and thin film coating.
By combining filters and methods, it is possible to produce single laminated bandpass filters with very specific and complex properties, to achieve all kinds of electro-optical applications.
While there are some limitations on how thin a bandpass filter can be, Envin Scientific’s optical thin-film filters range from as little as 0.1 mm to several centimetres thick and can be from 1 mm to over 100 mm in size.
Custom optical filters can be circular, square/rectangular, or other shapes as required to suit your application.
What are optical bandpass filters used for?
Depending on the range of wavelengths transmitted, optical bandpass filters can be used for an incredibly diverse range of applications, including for example:
- Anti-reflection and anti-glare coatings
- Chemical analysis
- Dielectric mirrors and high reflectors
- Fluorescence filters
- Infrared (IR) applications
- Laser optical filters
- Longwave pass (transmits long wavelengths only)
- Shortwave pass (transmits short wavelengths only)
- Telecoms and fibre optics
- Ultraviolet (UV) applications
We welcome all enquiries relating to bespoke bandpass filters for all kinds of optical applications and can advise on the most suitable substrates, thin films and other methods to produce the optical bandpass filters you need.