The laser transition occurs between two defined levels or level groups - the upper (E2) and the lower (E1). Important in terms of laser operation is that an inverted condition is achieved between the two energy levels: the higher energy level must be more densely populated than the lower. This condition is never achieved in systems in thermodynamic equilibrium. Thermal equilibrium is thus characterised by the fact that the lower energy level is always more densely populated than the higher. Lasers must therefore operate in opposite conditions to those which prevail in thermal equilibrium.