Metallic reflecting surfaces from which light is to be reflected directly may be prepared by chemical or physical methods. The deposits may be opaque or translucent. There are two physical methods by which metal reflecting films are deposited on glass. In both methods the metal is volatilized and condenses on the polished glass surface. The volatilization may be induced by bombarding the metal with gaseous ions in a moderate vacuum in this process, called "sputtering," the metal source remains comparatively cool. While the reflectivity of a glass surface can be reduced by a film of lower refractive index, it can be increased by coating it with a transparent substance of higher refractive index. The glass surfaces are first exposed to a glow discharge in a vacuum chamber at a pressure of some 10–2 mm Hg and rhodium is evaporated from a hot filament and condenses on the glass surfaces facing the filament.