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

Short pass filters

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Shortpass Filters

Short pass filters are an edge filters, which are used to cut out wavelengths that are not required.  The filter removes all wavelengths, such as NIR from visible, from the desired cut off point.  For example, a short pass filter with an 650nm cut off point would transmit from the UV and visible at better than 80% up to 650nm, beyond which the transmission would fall steeply to the filter’s minimum of better than a blocking OD6 in the NIR.

Because shortpass filters isolate specific wavelengths, short-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 range of stock and custom shortpass optical filters for a range of applications and suited to your budget restraints, uses of which span medical, defence, Industrial, commercial, electronics, research and entertainment markets and many sectors and applications.


Our range of shortpass filters are high quality and available from 350nm (UV) to 1.55µm (NIR) to meet the required edge cut-off, steepness of the transition, and the amount transmitted or blocked.  For the dichroic filters the unwanted region is reflected. 

Colour glass Short-pass FiltersGlass colour filters also known as colour glass filters are polished sheets of special glass that have been cast with a dye included in it. Colour glass 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 shortwave selection. 

Colour glass filters are useful for selective wavelength absorption and therefor achieve high out of band blocking.  Colour glass type shortpass filters have very low transmission in long wavelengths and high transmission in the short wavelengths.

The short-pass range of stock color glass filters listed have uniform spectral transmission properties over their entire aperture. A wide range of stock shortpass Schott colour glass filters and Hoya colour glass shortpass filters are available as 50x50mm and 25mm diameter filters.

Polyester Short Pass Filters:  Polyester colour filters have the dye held in a thin sheet of polyester, less than 0.1 mm thick.  Polyester color 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.  We stock a range of shortpass polyester filters in 75x75mm squares.

Wratten Shortpass Filters:  Wratten colour filters are manufactured by using organic dyes in a gelatine material to achieve the desired colour effects, which is not possible in colour glass, especially band-pass types.  Thinner substrates can also be achieved and at 0.1mm thick may be useful where space is limited. The well-known `Wratten` gelatine filters are the wider range and are more accurately controlled and documented. The polyester filters are much more robust and less expensive.  We stock a range of shortpass Wratten colour filters in 75x75mm squares.

Dichroic Short-pass Filters:  Our dichroic filters are comprised of a thin-film dielectric coating on glass, resulting in sharp transitions between the transmitted and reflected bands.  With negligible absorption, this type of filter is ideal for high powers and lacks the strong fluorescence of certain glass filters.  If more selective, stronger blocking is required we recommend one of our stocked glass or band-pass interference filters. The stock dichroic filters listed below are of the short-pass type. The data given is relevant for normal incidence, when used at 45° the shift towards shorter wavelengths is about 35-50nm.   We stock a range of dichroic shortpass filters in a range of standard sizes to meet most applications. 

Hot Mirrors:  At 0° incidence hot mirrors transmit the visible and reflect the near infra-red (heat) spectrum typically found in tungsten light sources.  Hot mirrors therefore remove much of the energy that can cause heating in a lighting system.   Hot mirrors can be used as an alternative to KG1 filters where the light is focussed near to the filter giving a risk of cracking.  Hot mirrors reflect the heat rather than absorb the heat as in KG1 heat filters.  Hot mirrors also have a sharper cut off between IR and Visible because they have a dielectric coating that can be controlled.  Our stock range of hot mirrors are available in a host of sizes to cover most applications. 

KG1 Shortpass Heat Filters:  Schott KG1 filters or equivalent are used in lighting systems to remove the near IR thermal energy from the light. As the heat is absorbed in the filter care must be taken that the filter does not overheat and crack.  Other KG glasses such as Schott KG2, Schott KG3, Schott KG4 and Schott KG5 or equivalent are also available in custom sizes.  Our KG1 toughened shortpass filters are available in standard sizes to meet most applications.

Knight Optical supply high quality stock and custom shortpass filters made to our exact specification.  Every component is individually tested on our Agilent Cary 5000 spectrometers fitted with a UMA device for testing transmission and reflection at any given angle.  Our Agilent 660 FTIR spectrometer is used for coatings between 3.5µm and 45µm.  All tested by our highly skilled technicians in our state of art metrology lab to ensure all components meet our high quality standards. We can provide transmission and reflection curves at given wavelengths from 175nm up to 3200nm in TXT or CSV format, or other formatted options.

Please contact our multilingual technical sales team and discover how Knight Optical’s high quality stock and custom shortpass filters and service can improve your instrumentation and supply chain experience.


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

UK +44 (0)1622 859444
UK +44 (0)1622 850614
UK +44 (0)1622 851078
+1 401 583 7846

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