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Filters
Optical filters select radiation according to defined criteria
Filter coatings play a crucial role in numerous optical applications, where optical filters are utilised to selectively isolate incident radiation based on specific criteria. Key factors influencing filter performance include wavelength and angle of incidence. Typically, optical filters are designed to be transparent to certain segments of the optical spectrum while blocking others, thereby enabling light to be either transmitted, absorbed, or reflected depending on its wavelength.
Coatings employing interference layer systems are pivotal in achieving the desired optical properties for these filters. Thin films enable surfaces to exhibit defined transmission, absorption, or reflection behaviour. They are predominantly applied on glass substrates, as well as silicates, and selected plastics such as polycarbonates. We specialise in tailoring the spectral characteristics of thin film filters – including wavelength, edge steepness, transmission or blocking width, and blocking efficiency – to meet customer-specific requirements, accounting for factors such as geometry, dimensions, and specifications. Depending on the application, we utilise various coating technologies and materials to develop the optimal coating design.
Optical filters are used across a diverse array of applications, spanning sensing, medical diagnostics, fluorescence microscopy, spectroscopy, machine vision inspection, and photography.
Types of optical filters
Optical filters encompass two primary categories: short-pass and long-pass filters, commonly referred to as edge filters, as well as bandpass filters (broadband and narrowband filters).
Short-pass and long-pass filters are often termed dichroic beamsplitters, as they transmit specific spectral ranges while blocking or reflecting others. Short-pass filters exhibit high transmission for short wavelengths, with longer wavelengths being effectively blocked by high reflection. Conversely, long-pass filters transmit longer wavelengths while reflecting shorter wavelengths.
Bandpass filters, on the other hand, selectively transmit light within a restricted spectral range while blocking wavelengths outside of this range. These filters can vary in spectral width, leading to distinctions between broadband and narrowband filters.
Notch filters, also called band-reject or band-stop filters, reflect or block specific predefined spectral ranges while transmitting wavelengths to the left and right of this band.
Additionally, neutral filters are available, which attenuate radiation without altering the spectral characteristics of transmitted light, maintaining a constant light transmission, particularly within the visible range. These filters typically appear colourless, black, or grey to the human eye. Most absorbing neutral density filters or neutral density splitters employ metallic coatings.
Lastly, IR filters and UV filters serve to either block or transmit infrared or ultraviolet radiation, depending on the specific application requirements.
Overview optical filter
Notch-filter (Band-reject filter, band-stop filter)
Infrared filter (including heat protection filter)
FAQ: Frequently asked questions about optical filters
Optical filters are coatings that select light radiation according to certain criteria such as wavelength and angle of incidence. They allow certain spectral ranges to pass through and block others.
They use interference layer systems made of thin films to generate defined transmission, absorption or reflection behaviour – usually on glass, silicates or plastics such as polycarbonate.
The main types include short-pass filters, long-pass filters, band-pass filters (wideband and narrowband), band-blocking filters (notch filters), neutral filters, UV filters, and IR filters.
A short-pass filter allows short wavelengths to pass through and reflects longer wavelengths. It offers high transmission in the lower spectral range.
A long-pass filter transmits long wavelengths and reflects shorter ones. It is suitable for applications that use light in the upper spectral range.
A bandpass filter only allows light to pass through in a defined spectral band and blocks all other wavelengths. There are broadband and narrowband variants.
This filter specifically blocks or reflects a certain spectral range and allows adjacent wavelengths to pass through.
Neutral filters reduce the intensity of light evenly across the entire visible spectrum without changing its color. They appear colorless, black or gray.
UV filters block or let UV radiation through, IR filters regulate infrared radiation. Depending on the application, both are used in a targeted manner, e.g. for thermal insulation or image processing.
Typical applications include sensor technology, medical technology, fluorescence microscopy, spectroscopy, machine vision inspection, and photography.
