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Knight Optical(UK) Ltd - sales@knightoptical.co.uk - +44 (0)1622 859444

Band pass filters

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Optical bandpass filters are used to transmit a selected wavelength while attenuating (blocking) unwanted shorter and longer wavelengths.

Knight Optical supply a massive range of optical bandpass filters with a range of bandwidths from standard Schott or equivalent and Hoya colour glass bandpass filters  though to high quality dichroic  and interference bandpass filters with sharp cut offs and high transmissions with OD7 out of band blocking for high demanding applications.

Interference bandpass filters: Our high quality interference bandpass filters pass light of a narrow bandwidth/wavelength and reflect/block all other light.  In principle interference filters can be designed and manufactured to almost any specification of centre wavelength and band pass. In practice bespoke/custom interference filters can be expensive and so should be avoided unless there are real advantages. Although our prices for custom interference filters are extremely competitive and we are happy to produce, it is best to utilise our extensive stock range of interference filters on offer.  Our standard ranges of stock filter are available from 214nm-2000nm in various bandwidths from narrow 3nm FIN type up to 65-100nm FIW type to meet most requirements.


It is also important to note our interference bandpass filters are of the higher quality type. Where most manufacturers filters are simply sealed into a metal housing with just resin and so susceptible to leakage of moisture, our range of interference filters are individual CNC scribed and hermetically sealed  Info sheet scribed interference filters to form a secure stack in their rings to ensure maximum long-term stability.

Although our standard range of stock interference band pass filters are mounted, un-mounted types with aerospace painted edges can be supplied at special request.  These interference bandpass filters are all  typically  3-7 cavity. In most cases blocking is metal-dielectric from 200nm to Far IR (3000+nm) at OD4.  However, we can offer dielectrically blocked interference filters for even longer extended life spans. Two issues with interference filters need to be considered; firstly any light not passed is reflected and so attention must be paid to where this light will finish up. Secondly the filter performance is sensitive to incidence angle with the characteristic moving to shorter wavelengths as the incidence angle is increased.

Our narrow band interference laserline filters are typically used in imaging applications whilst our fluorescence and dichroic excitation and emission filters are typically used in spectroscopy, microscopy and medical fluorescence applications.

Colourglass bandpass filters: Glass colour bandpass filters are polished sheets of special glass that have been cast with a dye. Colour glass bandpass filters generally are relatively cheap and very stable. The disadvantage with them is that they are only available in a relatively small number of variants, and so can be restrictive in bandpass selection.  These filters have uniform spectral transmission properties over their entire aperture. A wide range of band-pass Schott colour glass filters and Hoya colour glass filters band pass filters are available with circular filters of 25mm diameter and square 50x50mm filters. These filters are primarily used in applications such as agriculture analysis, medical analysers and fluorescence applications.  Colour glass filters also have high transmissions at selected wavelengths therefore ideal where the angle of incidence of the incident light cannot be tightly controlled.

Polyester bandpass filters:  Polyester filters have the dye held in a thin sheet of polyester.  Polyester colour filters have a wide range of characteristics available and the sheets can easily be cut in the laboratory. Whilst polyester filters are cheap and simple to use, the characteristics are not as well controlled as other absorption types and they can be susceptible to bleaching if large amounts of UV is present. Polyester filters are reasonably robust and inexpensive. They are available in sheets up to 1000x1000mm and can be cut to custom dimensions.  At 0.1mm thick these polyester filters are flexible and the range offered covers the visible spectrum. They are used in displays and to protect from unwanted ambient light.

Wratten bandpass filters: The colour effects within Wratten colour bandpass filters are achieved using organic dyes in a gelatine material, which is not possible in colour glass. This filter type can be manufactured to as little as 0.1mm thick and are more accurately controlled and documented. Wratten filters were originally designed for use in photography but are also used in imaging applications from industrial sensor to medical applications. Whilst less robust than polyester filter, their spectral data is more accurately documented.  Wratten gelatin filters are now being replaced by Wratten 2 type filters which are manufactured using a ridged polymer. Wratten 2 filters are more robust and unaffected by water and moisture as gelatine type.

Dichroic bandpass filters:  Dichroic bandpass filters are used to split the light into two or three bands and reflect one or more and transmit the others.  Dichroic bandpass filters are generally used at zero incidence and are generally used to select light of one of the primary colours. For selective stronger blocking a range of Interference filters are available from stock.

Click here to view our Info sheet scribed interference filters


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Our Multi-Lingual Sales Team are here to help with your requirments:

UK +44 (0)1622 859444
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UK +44 (0)1622 851078
+001 401-583-7846

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