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

Long pass filters

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Optical long pass filters are edge filters used to cut out wavelengths that are not required. The filter removes all wavelengths from the desired cut off point and transmit the longer wavelengths. For example, a long pass filter with an 850nm cut off point would transmit virtually no power from the UV, visible and NIR up to 850nm, beyond which the transmission would rise steeply to the filter’s peak value generally better than 80%.

Because longpass filters isolate specific wavelengths, long-pass filters are desirable in systems where they can be used to trim excess wavelengths or stray light to reduce unwanted near band emissions. Industry applications include order sorting in photometry and fluorescence.

Knight Optical supply a massive range of optical long-pass filters at a range of wavelengths from standard Schott and Hoya colour glass longpass colour filters to high quality dielectric coated dichroic longpass filters with sharp cut on and high transmissions with OD7 out of band blocking for high demanding applications.


Colour glass longpass filters: Schott or equivalent colourglass longpass filters are useful for selective wavelength absorption and therefore achieve high out of band blocking. Colour glass type longpass filters have very low transmission in short wavelengths and high transmission in the long wavelengths. Our long-pass range of color glass filters listed here has uniform spectral transmission properties over their entire aperture. A wide range of long-pass Schott colour glass filters and Hoya colour glass longpass filters are available as 50x50mm and 25mm diameter filters. These filters are used in range of spectral selection scientific instrument applications.

Acrylic longpass filters: Acrylic longpass filters are similar to glass filters but the host for the dye is now a sheet of acrylic.  Acrylic longpass filters are cheaper than glass filters, easier to cut to special sizes and shapes but not quite as robust as glass. A rather wider range of spectral characteristics is available than with glass. Two particularly useful types of acrylic longpass colour filters are those which block the visible but pass near IR and the ones that pass the visible but block UV light.

Polyester longpass filters: Polyester longpass filters have the dye held in a thin sheet of polyester.  Polyester longpass 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. Our range of polyester longpass filters are reasonably robust and inexpensive. They are available in sheets at 75mm square 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. Polyester Filters are reasonably robust and inexpensive. They are available in large 1000x1000mm sheets and can be cut to custom dimensions. At 0.1mm thick these polyester longpass filters are flexible and the range offered covers the visible spectrum.

Wratten longpass filters: The colour effects within Wratten longpass colour 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 longpass colour filters were originally designed for use in photography but are also used in imaging applications from industrial manufacture to medical. 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 longpass filters (0° AOI): These dichroic longpass colour filters consist of thin-film dielectric coatings on glass with sharp transitions between the transmitted and reflected bands. Having negligible absorption they are suitable for high powers and does not show the strong fluorescence of certain glass filters. Where strong blocking is needed, however, glass filters or band pass interference filters are recommended.

Dichroic beamsplitters longwave reflecting filters (45° AOI): Dichroic beamsplitters are used to separate two spectral regions which require detection at the same time. Unlike dichroic filters, the beamsplitters are designed for use at 45° and find an application within fluorescence microscopy. 


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