Optical mirrors have a smooth, highly-polished, plane or curved surface for reflecting light. Usually, the reflecting surface is a thin coating of silver, or aluminum on glass. Product specifications for optical mirrors include diameter, radius of curvature, thickness focal length, and surface quality. The diameter or height of an optical mirror is measured straight on. If the optical mirror’s curvature was extrapolated into a sphere, then the radius of that sphere is the radius of curvature for the mirror. There are two thickness measurements for optical mirrors: center thickness and edge thickness. Units of measure include inches, feet, and yards; nanometers, centimeters, and millimeters, and miles and kilometers. With optical mirrors, focal length is the distance from the mirror at which light converges. Surface quality describes digs and scratches. A dig is a defect on a polished optical surface that is nearly equal in terms of length and width. A scratch is a defect whose length is many times its width.
Optical mirrors are made from many different materials, each of which influences the mirror’s reflectivity characteristics. Choices for materials include borosilicate glass, copper, fused silica, nickel, and optic crown glass. Borosilicate glass is also known as BK7 and boro-crown glass. Copper is used in high-power applications because of its high thermal conductivity. Fused silica has a very low coefficient of thermal expansion and is suitable for use with moderately-powered lasers or changing environmental conditions. Ultraviolet (UV) grade optical mirrors are also commonly available. Nickel is used in applications which require resistance to both thermal and physical damage. Proprietary materials for optical mirrors include Pyrex (Corning Inc.) and Zerodur (Schott Glaswerke).
Optical mirrors are sometimes coated to enhance their reflectivity. Choices include bare, enhanced, and protected aluminum; silver, bare gold and protected gold; and coatings made from rhodium and dielectric materials. Enhanced aluminum coatings are used to improve reflectance in the visible and ultraviolet regions. Protected aluminum coatings provide abrasion resistance while protecting the surface of the aluminum, an excellent reflector in the upper UV, visible and near-infrared (IR) regions. Optical mirrors with bare gold and protected gold coatings are used in the near-IR to far-IR regions. Silver coatings provide better reflectance than aluminum; however, silver’s tendency to oxidize and tarnish requires thorough sealing from the atmosphere. Rhodium coatings have a reflectivity of approximately 80% of the visible spectrum.