What is an optical filter?


An optical filter is a component in an optical setup that is (partially) permeable to certain wavelengths or polarization directions, while at the same time reflecting or absorbing other wavelengths (partially). Optical filters are divided into two classes: filters that target the wavelength spectrum of the light (bandpass filter, long pass filter, short-pass filter, notch filter and neutral density filter), as well as filters that change the polarization properties of the light (polarization filter).

Optical filters are made in different ways. Interference filters, also called dielectric filters, consist of numerous superimposed layers of transparent material with different refraction indexes applied to a carrier substrate. This allows some wavelengths of light to pass through the layers of the filter while others are reflected. In addition, some layers are only permeable to light for parts of individual wavelengths.

Coloured glass filters are made of coloured glass that absorbs certain wavelengths. Certain crystals or macromolecular foils that are plastic and have been stretched in one direction are used for polarization filters.

How do optical filters work?

Optical filters allow certain wavelengths through while reflecting or absorbing other wavelengths. Some filters only allow part of a single wavelength to pass through. Some optical filters also influence the polarization properties of light.

What optical filters are there?

There are bandpass filters, long pass filters, short-pass filters, notch filters, neutral density filters and polarization filters.

What are optical filters used for?

Optical filters are used in a wide range of applications. They can be found, for example, in optical setups in which certain wavelengths must be isolated, in optical measurement technology, and in imaging processes.