Custom Interference Bandpass Filters
Interference bandpass filters are excellent for applications requiring specific spectral tolerances. The manufacturing process allows a high level of control on the transmission parameters.
Though we have an extensive stock range of interference bandpass filters, Knight Optical can also provide custom filters manufactured to your specification. We can tailor the spectral properties to meet your application demands including:
- Peak transmission – either as a minimum value, or regulated to perform within a certain range.
- Centre wavelength (CWL) within the UV, visible or infrared waveband.
- Bandwidth or FWHM – we can supply narrowband (< 10nm), standard (10-50nm) and broadband filters (> 50nm).
- Out-of-band blocking – Typically OD4 (T < 0.01%) but
Knight Optical can also provide differing dimensions to fit into your application, including mounted and unmounted options. We typically supply 12.5mm, 12.7mm, 25mm, 25.4mm, 50mm, and 50.8mm diameter mounts, which can be etched with your unique batch number if required, however custom mount sizes can also be supplied.
Our filters are:
- CNC scribed which involves removing the outer diameter of coating and so enabling a glass-to-glass epoxy seal, rather than a glass-to-coating seal, which provides an excellent barrier against moisture.
- If mounted, they are hermetically sealed to further protect against moisture and there increase the long-term stability.
You can find our typical manufacturing capabilities below, however, please enquire for further information. Please note, the spectral properties are dependent on factors such as the design wavelength and bandwidth size.
Centre wavelength (CWL)
5mm to 100mm+
200nm to 3000nm+
3nm to 100nm+
-50°C to 80°C
Knight Optical can offer individual and sets of interference filters, and stock storage boxes to hold 12.5mm, 25mm and 50mm filters.
Contact our technical sales team for further information on our interference bandpass filters and how we can help you with your enquiry.
- 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.
- The incident light will behave differently at each layer, due to interference, and ultimately only the desired wavelengths will exit the filter.
- Often multiple stacks of these coated substrates, separated by a dielectric spacer layer (filter cavity), are needed to get the desired wavelength and steep slope. One filter can often include 3-7 cavities.
- Centre wavelength (CWL) is the midpoint of the bandwidth
- Full width-half maximum (FWHM) defines the bandwidth of which the filter will transmit – half of the peak transmission 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.
- Peak transmission is the maximum transmission achieved.
Knight Optical verifies all optical components within our metrology laboratory:
- Using our Agilent Cary 5000 spectrophotometer we verify the peak transmission, centre wavelength and FWHM, as well as the out-of-band blocking (OD).
- For infrared bandpass filters, we use our Agilent Cary 660 FTIR to check the transmission.
- We are able to provide the transmission data as raw ASCII (.csv) files or present it as a graph.
- Dimensions are verified with handheld measuring devices, typically our micrometres.
- All filters are visually inspected by our QA department prior to packaging.
- ANPR cameras where the centre wavelengths coincide with commonly used NIR LEDs – 740nm, 850nm, 870nm and 940nm. Knight Optical stock NIR ANPR filters with 25.5mm/30.5mm mounted options.
- NIR and IR bandpass filters are often used as gas band filters where the centre wavelength coincides with the absorption of a particular a gas molecule.