Metallic mirror coatings are optimized for different regions of the spectrum. Standard metallic mirror coatings include Protected Aluminum, Enhanced Aluminum, UV Enhanced Aluminum, DUV Enhanced Aluminum, Bare Gold, Protected Gold, and Protected Silver. Protected Aluminum and Enhanced Aluminum are typically used for visible applications. UV and DUV Enhanced Aluminum can be used for UV and visible applications. Bare or Protected Gold offers high reflectance for near¬Infrared (NIR) and Infrared wavelengths. Protected Silver provides the highest reflectance between 500 – 800nm and also performs well in near-Infrared and Infrared applications.
Introduction to First Surface and Second Surface Mirrors
All of our mirrors are first surface mirrors, featuring a high reflectance coating deposited on the front surface of a variety of different types of glass, metal, or semiconductor substrates. First surface mirror or front surface mirror (also commonly abbreviated FS mirror or FSM) is recommended for use in precision optics applications. Second surface mirror or rear surface mirror can be manufactured using similar coating technology, but the incident light first passes through a transparent substrate material before it is reflected by the coating. This geometry helps protect the coating layer from scratches and oxidization but leads to several other important issues that make this type of mirror unsuitable for most precision optics applications.
Light incident on a second surface mirror is subject to chromatic dispersion from the substrate material.
Reflection at the substrate leads to ghost images, indicated by the dashed orange line. A second unwanted reflection occurs as the light exits the substrate decreasing the net reflectance of the mirror. Additional stray ghost reflections may also be seen as the light bounces between the coated and non-coated surfaces of the substrate.
Metal coatings are typically very delicate without a protective coating and require extra care during handling and cleaning. The surface of an unprotected metal coating should never be touched or cleaned with anything but clean, dry air. A dielectric overcoat on a metallic mirror allows for improved handling of the component, increases the durability of the metal coating and provides protection from oxidation with little impact to the performance of the metal coating. The dielectric layer(s) can also be designed to enhance the reflectance of the metal coating in specific spectral regions.
Standard protected aluminum is our most popular mirror coating for applications in the visible and near infrared. A λ/2 coating of Silicon Monoxide (SiO) is typically used as an overcoat to protect the delicate aluminum. This treatment provides an abrasion-resistant surface while maintaining the performance of aluminum mirror.
In an Enhanced Aluminum coating, a multi-layer film of dielectrics on top of aluminum is used to increase the reflectance in the visible or ultraviolet regions. This coating is ideal for applications requiring increased reflectance from 400 – 650nm while the UV and DUV Enhanced Aluminum coatings yield increased reflectance from 120 – 400nm range. The multi-layer film also provides the improved handling characteristics of the protected aluminum coating.
Silver offers high reflectance in the visible and infrared spectral regions, making it an excellent choice for broadband applications that span multiple spectral regions. A protective coating reduces silver's tendency to tarnish but the coating still performs best in low humidity environments.
Bare and Protected Gold
Gold coatings are effective for applications requiring high reflectance in the NIR and IR regions. Since a durable coating is necessary in many applications, we offer gold with a protective overcoat. The performance of gold (96% reflectivity from 750 – 1500nm) is maintained, but the optic has a more durable finish.