Radioluminescence occurs when an incoming particle of ionizing radiation  collides with an atom or molecule, exciting an orbital electron to a higher energy level.  The particle usually comes from the radioactive decay of an atom of a radioisotope, an isotope of an element which is radioactive.  The electron then returns to its ground energy level by emitting the extra energy as a photon of light.  A chemical that releases light of a particular color when struck by ionizing radiation is called a phosphor.  Radioluminescent light sources usually consist of a radioactive substance mixed with, or in proximity to, a phosphor.