ABSTRACT
There is no doubt that the broadest and simplest high-reflectance coatings are the front-surface metallic mirrors. A thin film of aluminum, for example, gives high reflectance from the extreme ultraviolet to the far infrared and beyond. Why, then, should we need anything else? Unfortunately, even metals such as silver, with highest possible performance, exhibit losses of at least several percent, and there are many applications where such levels of loss are completely unacceptable. There are also many applications where what is not reflected must be transmitted rather than absorbed. Interference structures using dielectric materials are the answer, but although interference can achieve incredible levels of performance over limited wavelength regions, the wide performance characteristic of metal layers is beyond it. We need both.
