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Interference Bandpass Filters


Knight Optical can supply custom interference bandpass filters to meet individual requirements. We also have an extensive stock catalogue of interference bandpass filters so we may already be able to offer filters that meet your specifications, you will find our catalogue of interference filters here.

Interference filters are manufactured by depositing layers of material (usually a metal or dielectric material), with differing refractive indices and thickness, on to a glass substrate. Multiple stacks of these coated substrates may be needed to get the desired wavelength and steep slope. These stacks are then spaced by a dielectric spacer layer (filter cavity), one filter can often include 3-7 cavities.

A bandpass filter is usually defined as having a centre wavelength (CWL), peak transmission and a full width-half maximum (FWHM).

The centre wavelength (CWL) is the midpoint of the bandwidth, the full width-half maximum (FWHM) defines the bandwidth of which the filter will transmit – half of the peak transmission (the maximum transmission achieved by the filter) will define the minimum and maximum limits of the bandwidth

Either side of the transmission band there are blocking bands (or out-of-band blocking), usually specified as an OD value. Knight Optical’s stock filters offer excellent OD blocking of OD4 (<0.01% T) and OD5 (< 0.001% T), but custom interference filters can be manufactured to meet customer specification. In most cases the blockers are metal based from 200nm to Far IR (3000+nm) at OD4, however metal (often silver) blockers degrade over time, Knight Optical can also supply custom filters with dielectric blocking which, though more expensive, gives your filter an extended life span.

Peak transmission can be regulated to perform within a certain range for consistency in setting up your instrument, for example 50%+/-5%, full width-half maximum can be controlled to within a few nanometres, and centre wavelength can be controlled to within 10% of FWHM.

Custom narrowband (FWHM <10nm), standard (FWHM between 10-50nm) and broadband (FWHM > 50nm) interference filters can be carefully manufactured to meet extremely specific requirements, with centre wavelengths in the UV, VIS and IR regions. Ultra narrowband filters are also possible with an FWHM of less than 2nm.

Our custom range of UV Interference bandpass (with a CWL of less than 400nm) filters are typically 3-7 cavity.

Bandpass filters have a wide variety of applications including fluorescence spectroscopy as excitation and emission filters, in Raman spectroscopy as laser cleaning or laser blocking filters. Narrowband and ultra narrowband filters are often used in astrophotography to only filter certain emission lines, such as the hydrogen-alpha emission line at 656.28nm – this would only allow a fraction of red light through whilst blocking other wavelengths).

IR bandpass filters are especially useful in ANPR cameras where their design centre wavelengths coincide with the most commonly used LED wavelengths – 740nm, 850nm, 870nm or 940nm. Knight Optical stock NIR ANPR filters, but we can also supply custom filters to meet individual requirements. They are also used in applications such as CCTV cameras and CCD/CMOS sensors.

Our interference bandpass filters are individually CNC scribed and hermetically sealed. CNC scribing involves removing the outer diameter of coating and so enabling a glass-to-glass epoxy seal, rather than a glass-coating seal, which then means 100% of moisture and humidity is blocked in stable temperature cycles. By further hermetically sealing the mounts increases the long-term stability

by protecting further against humidity. Please see our information sheet for further information.

We can offer custom mounts, and though standard mounting options are generally available as 12.5mm, 12.7mm, 25mm, 25.4mm, 50mm and 50.8mm diameters we can supply any diameter of mounted interference filter.

Unmounted options are also available, including with additional painted edges in aerospace paint for stability and performance.

Knight Optical can offer individual and sets of interference filters and hold a large range of products not listed on our website, all of which come quality assured with 100% testing in our state-of-the-art metrology laboratory.

Contact our multilingual technical sales team and discover how Knight Optical’s high quality Interference Bandpass Filters and service can improve your instrumentation and supply chain experience.

Our 2018/2019 Catalogue

Inside Knight Optical’s catalogue you’ll find details of the industries we serve as well as the many other services we can offer. It also showcases hundreds of our most popular products, supported by useful technical information, to help you choose the right part for your application.
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