Bandpass filters or band-pass filters work by allowing a specific range of wavelengths of light to be transmitted through the filter – they literally ‘pass’ a ‘band’ of wavelengths. The filter allows signals to be transmitted between two frequencies.
Some bandpass filters require external sources of power to work such as filter circuits, these are known as active filters.
Unwanted wavelengths can either be absorbed or reflected, or a combination of both methods can be used to block them.
Bandpass filters can be manufactured to a high degree of accuracy in a variety of different shapes, sizes and thicknesses, using different construction technologies as appropriate.
Types of filter such as interference filters and high pass filter may be applied, and a band of frequencies may be targeted or unwanted frequencies omitted depending on your project.
It is also possible to combine bandpass filters with other optical filters and structures, to apply numerous different effects to the light source as required by the application.
Optical bandpass filters are often categorised first by the range of wavelengths they pass and particularly how specific they are to a particular wavelength.
Narrowband filters typically pass less than 1% of the incident wavelengths, giving an output light that is highly specific to a particular wavelength or frequency.
Wideband or broadband filters typically pass 10% or more of the incident light, giving an output that contains a wider range of frequencies or wavelengths.
Envin Scientific manufacture bandpass filters that operate in the range from 200 nm to 8,000 nm, allowing for a very broad range of different applications to be catered for.
As well as the overall range of wavelengths they pass, bandpass filters can be defined in several other ways by the shape of their transmission profile.
These characteristics include:
In many cases it is desirable to have very high blocking of unwanted wavelengths along with very high transmission of the peak wavelength or wavelengths.
It is also common for bandpass filters to have a very steep gradient from blocked to transmitted wavelengths, with almost zero transmission of wavelengths outside of the FWHM range but near-peak transmission of all wavelengths within that bandwidth.
Bandpass filters are also affected by their cone angle of use. Essentially, the closer to perpendicular the incident beam is, the more accurate the filter’s effect will be.
Historically this meant bandpass filters worked over a very narrow cone angle of just a few degrees at most.
More recently, advanced manufacturing and construction techniques have allowed bandpass filters to be designed with a cone angle of use that is much larger than was previously achievable.
In applications where the incident beam may not always be exactly perpendicular to the filter’s surface, this can ensure physical effects do not lead to a shift in the wavelengths passed through the filter.
To find out more about bandpass filters or to make an enquiry about a particular application, please contact Envin Scientific today and let us know what you need.