Thin film coatings utilize the interference effects of electromagnetic energy. Waves that are in phase undergo constructive interference, and waves that are 180° out-of-phase undergo destructive interference. When light is incident on a thin film, some of the light will be reflected from each of the interfaces, while the remainder is transmitted. By careful selection of materials and thicknesses and the overall film structure, optical thin film coatings control reflected and transmitted phase at each layer interface over a given range of wavelengths and incident angles. Most coatings employ many layers consisting of two or more different materials of varying thicknesses to produce the desired effects.