(See Fabry–Pérot interferometer for a mathematical description of the effect.) Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and so do not become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband). By controlling the thickness and number of the layers, the frequency of the passband of the filter can be tuned and made as wide or narrow as desired. The layers are usually added by vacuum deposition. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. In a dichroic mirror or filter, instead of using an oil film to produce the interference, alternating layers of optical coatings with different refractive indices are built up upon a glass substrate. Because the light reflecting from the bottom travels a slightly longer path, some light wavelengths are reinforced by this delay, while others tend to be canceled, producing the colors seen. When light strikes an oil film at an angle, some of the light is reflected from the top surface of the oil, and some is reflected from the bottom surface where it is in contact with the water. Dielectric mirrors operate on the same principle, but focus exclusively on reflection.ĭichroic filters use the principle of thin-film interference, and produce colors in the same way as oil films on water. The principle of operation is similar to a Fabry-Perot etalon.ĭichroic mirrors and dichroic reflectors are the same type of device, but are characterized by the colors of light that they reflect, rather than the colors they pass. 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. They are used in scientific applications, as well as in architectural and theatrical lighting.Īn interference filter consists of multiple thin layers of dielectric material having different refractive indices. An interference filter may be high-pass, low-pass, bandpass, or band-rejection. Band-pass interference filter for laser experiments Dichroic filtersĪn 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.
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