The first films produced by vacuum deposition - as we know it - were aluminum films for mirrors made by John Strong in the 1930s; he coated mirrors for astronomical use. This chapter discusses the reflective qualities of mirrors, not the optical qualities. The films can be applied to virtually any material as long as the shape of the blank does not bend too far because the coatings lose their reflection at severe angles. Also, the surface needs to be polished or smooth drawn.
The metals used for most reflectors are gold, silver, or aluminum. There are advantages for each (Fig. 6.1). If a mirror is to be used only from 700 nm to the far-infrared spectrum, then gold is a good choice. For the visible and most of the infrared spectrum, the highest reflection is achieved with silver. Aluminum is a good choice if durability is a concern; it is also the only choice for the ultraviolet spectrum (Fig. 6.2).
Protection of a mirror surface is a necessity for most applications. Adding one layer of any index of refraction to a metal surface reduces the reflectance for the whole spectrum except at one wavelength. If the overcoat is thin (in comparison to the wavelength), the loss may be neglected.