An interference filter, dichroic filter, or thin-film filter is an optical filter that reflects some wavelengths (colors) of light and transmits others, with almost no absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection. They are used in scientific applications, as well as in architectural and theatrical lighting.
An interference filter consists of multiple thin layers of dielectric material having different refractive indices. There may also be metallic layers. Interference filters are wavelength-selective by virtue of the interference effects that take place between the incident and reflected waves at the thin-film boundaries. The principle of operation is similar to a Fabry-Perot etalon.