An optical supermirror is a Dielectric mirror that is optimized for an extremely high reflectivity which in extreme cases can be larger than 99.9999%. This means that the Reflection losses are below 1â€‰ppm. Two such mirrors form a Fabryâ€“Perot Interferometer with a Finesse larger than 3 millions and a strong field enhancement within the cavity. The Q Factor of a supermirror cavity can be above 1011. Supermirrors can be used in certain Quantum Optics experiments and for some measurements with extremely high precision. The term supermirror is more common for X-ray and neutron reflectors. In that field, it was originally very difficult to achieve high Reflectance values. Multilayer mirrors have then been developed, which offer much better performance.