Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as beta particles. Radioluminescence is used as a low level light source for night illumination of instruments or signage or other applications where light must be produced for long periods without external energy sources. Radioluminescent paint used to be used for clock hands and instrument dials, enabling them to be read in the dark. Radioluminescence is also sometimes seen around high-power radiation sources, such as nuclear reactors and radioisotopes.