It was Albert Einstein who in 1917 defined the theory of the Stimulated Emission of Radiation, developing and expanding on the work of Niels Bohr, who in 1913 had formulated the Spontaneous Emission theory. Einstein described the electrons of molecules being excited by a source of energy, usually heat, and directed in a specific way. The excited electron releases a spontaneously emitted photon which interacts with a molecule of the active medium, causing those electrons to move to a less stable, higher energy state and producing further photons. This process exponentially increases the number of identical photons which are focused by mirrors at either end of the laser tube and emitted into the delivery system. The acronym “LASER” represents “light amplification by stimulated emission of radiation”.