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.

Dichroic 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. Dielectric mirrors operate on the same principle, but focus exclusively on reflection.

Dichroic filters use the principle of thin-film interference, and produce colors in the same way as oil films on water. 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. 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. 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. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually added by vacuum deposition. 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. 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). (See Fabry–Pérot interferometer for a mathematical description of the effect.)

Where white light is being deliberately separated into various color bands (for example, within a color video projector or color television camera), the similar dichroic prism is used instead. For cameras, however, it is now more common to have an absorption filter array to filter individual pixels on a single CCD array.

Dichroic filters can filter light from a white light source to produce light that is perceived by humans to be highly saturated in color. Such filters are popular in architectural and theatrical applications.

Dichroic reflectors known as cold mirrors are commonly used behind a light source to reflect visible light forward while allowing the invisible infrared light to pass out of the rear of the fixture. Such an arrangement allows intense illumination with less heating of the illuminated object. Many quartz-halogen lamps have an integrated dichroic reflector for this purpose, being originally designed for use in slide projectors to avoid melting the slides, but now widely used for interior home and commercial lighting. This improves whiteness by removing excess red; however, it poses a serious fire hazard if used in recessed or enclosed luminaires by allowing infrared radiation into those luminaires. For these applications non-cool-beam (ALU or Silverback) lamps must be used. Recessed or enclosed luminaires that are unsuitable for use with dichroic reflector lights can be identified by the IEC 60598 No Cool Beam symbol.