Flat mirrors are smooth, highly polished, flat surfaces, for reflecting light. The actual reflecting surface is usually a thin coating of silver, aluminum, or other material. Most flat mirrors are coated, and then overcoated a second time to add a level of scratch resistance. Flat mirrors are found in a wide range of optical systems including laser light and diode applications, holography, imaging systems, and autocollimating applications.
Flat mirrors may be fashioned from a number of materials. The material used influences the reflectivity characteristics of the mirror. Commonly used mirror materials include BK7 glass, also referred to as boro-crown glass, or borosilicate glass; copper, which is often used in high power applications because of its high thermal conductivity; fused silica and UV grade fused silica, which have very low coefficients of thermal expansion and are ideal for use with moderately powered lasers or changing environmental conditions; nickel, which is considerably more durable than glass substrates, both to thermal and physical damage; optical crown glass, which is used in non-imaging applications, including light gathering or conventional beam manipulation tasks. Crown glass is generally used when thermal stability is not a critical factor. Flat mirrors may often be crafted from proprietary materials each of which brings their own advantages to the mirror.