The purpose of optical coatings is to change the reflectivity of optical surfaces. According to the materials and physical phenomena which are used one can in principle distinguish metallic and dielectric coatings. Metallic coatings are used for reflectors and neutral density filters. The reflectivity which can be achieved is given by the properties of the metal. Dielectric coatings use, however, optical interference to change the reflectivity of the coated surfaces. Another major difference is that the materials used for this kind of coatings show very low absorption. Using optical interference coatings the reflectivity of optical surfaces can be varied from nearly zero (anti reflection coatings) to nearly 100% (low loss mirrors with R > 99.999%). However, these reflectivity values are achieved only for a certain wavelength or wavelength range.

     A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified reflectivity at different wavelengths of light. Dielectric mirrors are also used to produce ultra-high reflectivity mirrors: values of 99.999% or better over a narrow range of wavelengths can be produced using special techniques. Alternatively, they can be made to reflect a broad spectrum of light, such as the entire visible range or the spectrum of the Ti-sapphire laser.

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