Luminescence is a spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment.  A luminescent object emits cold light in contrast to incandescence, where an object only emits light after heating. Generally, the emission of light is due to the movement of electrons between different energy levels within an atom after excitation by external factors. However, the exact mechanism of light emission in vibrationally excited species is unknown. The dials, hands, scales, and signs of aviation and navigational instruments and markings are often coated with luminescent materials in a process known as luminising. Source: Wikipedia (en)