As important as the transmitting optical components ,high-reflectance mirror coatings can reflect a major portion of the incident

light. Usually we want the specular reflectance should be as high as conveniently possible,although in some specialised

applications We need not only the reflectance be high,but also the absorption should be extremely low.

Reflectance mirrors may be simple metallic layers . in some case ,multilayer all-dielectric mirrors can give the higher reflectance.