A helium-neon laser produces a continuous, intense beam using gas lasers. It consists of a quartz discharge tube with helium and neon in a 1:4 ratio at about 1 mm Hg pressure, fitted with perfectly reflecting and partially reflecting mirrors. A radio frequency generator excites the helium atoms to higher energy levels; these excited atoms transfer energy to neon atoms by collision, achieving population inversion. When an excited neon atom drops from its metastable state (20.66 eV) to lower state (18.70 eV), it emits photons that stimulate further emissions through reflection between the mirrors until coherent light escapes via the partially reflective mirror.