ABSTRACT
The word "vacuum" is used to describe a very wide range of conditions. At one extreme, it refers to nearly complete emptiness, a space without matter, or more specifically, space in which air and other gases are absent. The basic property is the density of the gas. The degree of vacuum can easily be described in terms of gas particle density instead of pressure. A television picture tube is a vacuum tube. In addition to the increased travel distance of a coherent stream of electrons, vacuum environment also prevents rapid oxidation of the electron emitters. The same two basic requirements are used for thin-film coating applications. Vacuum can be produced by a few basic methods. It can be produced by mechanical displacement of gases from an enclosed space by means of pistons, rotating vanes, lobes, and so on.
