Mirrors are available in a variety of sizes and shapes. In addition to shape and dimensions, the other primary specifications include the reflective coating, the substrate material, the flatness or surface accuracy, and cosmetic surface quality. From the perspective of manufacturing, coatings can be classified as either metallic, dielectric or hybrid and as single-layer or multi-layer. Metallic coatings are usually deposited by evaporating a metal, such as aluminum or gold. Dielectric coatings are made of dielectric materials (electrically non-conductive) such as magnesium fluoride (MgF2). Hybrid coatings consist of dielectric layers deposited upon a metallic base layer. Purely dielectric coatings may be single layer. Hybrid coatings are, of course, always multi-layered.
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