Infrared Filters

Knight Optical can supply a wide range of custom infrared filters which are designed for the use in wavelengths between 1-100µm.

Custom infrared longpass filters – these filter types are ideal if you want to block the shorter UV-VIS wavelengths whilst transmitting over a broad IR waveband. These often comprise of a thin film coating on germanium or silicon substrates, which already block the UV-VIS regions as they start transmitting at 1.7µm and 1.2µm respectively.

Custom IR shortpass filter – infrared shortpass filters are beneficial if you want to block some of the longer IR wavelengths whilst still transmitting the shorter IR wavebands. Again, an infrared material such as silicon, germanium or sapphire may be used as the substrate on which the coating will be applied.

IR custom interference bandpass filters – bandpass filters are designed to only transmit a certain waveband whilst blocking the rest. A coating, usually consisting of many thin film layers of material of varying thickness and refractive indices, will be applied on top of an infrared material such as germanium, silicon, sapphire and IR quartz. An alternative is combining long wave pass IR filters and short wave pass IR filters can form bandpass filters with tuneable bandwidths depending on the filters used. Knight Optical also offers mounted bandpass filters designed for ANPR/ALPR camera applications, centre wavelengths within the NIR region. These can be custom manufactured to fit individual systems.

Narrowband gas band filters are a type of bandpass filter used in gas detection and analysis applications, it is essential for these IR filters to have a narrow FWHM and usually block to at least OD3. The centre wavelength will correspond to a particular gas (or gases) peak absorption and can therefore detect it from the background signal, such as our atmosphere.

Custom infrared neutral density (ND) filters are usually manufactured by vacuum depositing metallic based thin films onto an infrared substrate, such as germanium. ND filters are designed to provide a linear transmission over a certain wavelength range.

Infrared ND filters usually increase in 0.1 increments up to OD1, and then in increments of 1 from OD1 to OD10. An OD is defined as ten times the log (base 10) of the transmission, for example, OD1 transmits 10%, and OD2 transmits 1%. Knight Optical offers sets of custom IR ND filters, individual sets with OD0.1 to OD1 or OD to OD10, as well as a combination of the two. The wavebands can be 1-6µm, generally on a silicon substrate, or in the thermal waveband of 8-12µm, generally on a germanium substrate. However, Knight Optical can offer alternative densities and substrates to meet individual needs.

Custom infrared semiconductor filters have long wave pass filter characteristics. They are manufactured by applying an AR coating to a semiconducting substrate. These filters have an abrupt cut-on, known as the intrinsic edge. In an intrinsic semiconductor, they have an empty conduction band and a full valence band, with a small (direct or indirect) gap separating the two. When photos have enough energy to cross this gap, they are absorbed by the conduction band, however when the wavelength corresponds to the energy gap, the photons do not have enough energy to cross from the valence to the conduction band and are therefore transmitted. This is when we see the sudden change from absorption to transmission. The following materials are typical substrates used in semiconducting filters: GaAs with a cut on at 0.9µm, Silicon at 1.04µm, Germanium at 1.7µm and Indium Arsenide at 3.6µm. The AR coating is applied to minimise reflection at the surface due to the high refractive indices of these materials.

Knight Optical supply high quality IR Optical Filter components made to your specific bespoke custom specification.  Every component is individually tested by our highly skilled technicians in our state of art metrology lab to ensure all components meet our high quality standards. We also hold a large range of stock not listed on our website, so please enquire.

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