Radioluminescent paint is a self-luminous paint that consists of a small amount of a radioactive isotope (radionuclide) mixed with a radioluminescent phosphor chemical. The radioisotope continually decays, emitting radiation particles which strike molecules of the phosphor, exciting them to emit visible light. The isotopes selected are typically strong emitters of beta radiation, preferred since this radiation will not penetrate an enclosure. Radioluminescent paints will glow without exposure to light until the radioactive isotope has decayed (or the phosphor degrades), which may be many years.
Because of safety concerns and tighter regulation, consumer products such as clocks and watches now increasingly use phosphorescent rather than radioluminescent substances. Previously radioluminicesent paints were used extensively on watch and clock dials and known colloquially to watchmakers as "clunk". Radioluminescent paint may still be preferred in specialist applications, such as diving watches.