Manufacturers of optical coatings and components

All dielectric maximum reflectance mirrors for 45 degrees angle of incidence and random polarization (purple: visible spectrum, red: 532 nm)

Dielectric mirrors are manufactured from insulating (non-metallic) materials, and rely solely on interference phenomenon to create high reflectance.  As such, they operate over a much narrower wavelength region than metal mirrors.  Their primary advantage is that they are extremely durable and can operate in high temperature environments.   Dielectric coatings are impervious to organic solvents and are not readily attacked by acids or bases.  Another advantage to dielectric mirrors over metal mirrors is that they exhibit exceptionally high reflectance for both "s" and "p" polarization's, which is important for many mirror applications.