Phosphorescence is a form of luminescence in which a substance emits visible light some time after it has been stimulated or excited by electromagnetic radiation of a different wavelength or by certain other means not involving a rise in temperature. The light is given off within a period arbitrarily set at greater than 10 nanoseconds (10-8 s) – after the stimulation ends; in this respect, the phenomenon differs from fluorescence. The slower time scales of the re-emission of the radiation are associated with quantum mechanically forbidden energy state transitions. Phosphorescence is the mechanism used for "glow in the dark" materials which need to be charged up by exposure to light.