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 auto collimating 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.
Mirror coatings are used to enhance the reflectivity of flat mirrors. The most common types of coatings include a variety of aluminum films, precious metals, and specialized coatings such as rhodium or dielectrics. Bare aluminum is an excellent reflector in the upper UV, visible, and near infrared. The surface is soft and must be handled with care. Enhanced aluminum is a multilayer film of dielectrics on top of aluminum is used to enhance the reflectance in the visible or ultraviolet region. The multilayer film also provides the handling characteristics of the protected aluminum coating. Protected aluminum coating is used as an overcoat to protect the delicate aluminum. This treatment provides an abrasion resistant surface while maintaining the performance of aluminum.
Precious metal coatings for flat mirrors include a few types of gold coatings and silver oxides. Bare gold coatings provide consistently high reflectance in the near-IR to far-IR, and are the most widely used materials in these regions. Gold however is soft and easily scratched. As a result, protected gold coatings are available, which have an overcoat to provide abrasion resistance without limiting the performance of the gold coating. Silver coatings are superior in reflectance to aluminum; however its strong tendency to oxidize and tarnish means that it must be thoroughly sealed from the atmosphere in order to avoid degradation.
Rhodium coating has a reflectivity of approximately of 80% throughout the visible spectrum. Rhodium coated nickel is considerably more durable then glass substrates both to thermal and physical damage. Dielectric coatings for flat mirrors are multi-layer coatings, which offer excellent performance over a specific wavelength range and are relatively insensitive to small angle changes. Dielectric coatings tend to be more durable than metal coatings.