Mirrors also can be divided into two kinds: Dielectric HR Coating and metal HR Coating. A character of the HR coating-mirrors is that the reflectivity is very high in a special wave band. The dielectric HR coating is made with lots of layers whose material is insulating medium .The polarization of this kind of mirror can be very small and the reflectivity can be very high. The metal HR coating is made with metal such gold or silver or aluminum. The surface of this kind of mirror coated with metal takes on a special color.