Interference filter

An interference filter consists of multiple thin layers of dielectric material having different refractive indices.

Dichroic filters use the principle of thin-film interference, and produce colors in the same way as oil films on water.

The color transmitted by the filter exhibits a blue shift with increasing angle of incidence, see Dielectric mirror.

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.

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.

Recessed or enclosed luminaires that are unsuitable for use with dichroic reflector lights can be identified by the IEC 60598 No Cool Beam symbol.

They separate the various harmonic components of frequency doubled laser systems by selective spectral reflection and transmission.

[2] With a technique licensed from Infitec, Dolby Labs uses dichroic filters for screening 3D movies.

[3] Long-pass dichroic filters applied to ordinary lighting can prevent it from attracting insects.

In some cases, such filters can prevent attraction of other wildlife, reducing adverse environmental impact.