In physics, induced gamma emission (IGE) refers to the process of fluorescent emission of gamma rays from excited nuclei, usually involving a specific nuclear isomer. It is analogous to conventional fluorescence, which is defined as the emission of a photon (unit of light) by an excited electron in an atom or molecule. In the case of IGE, nuclear isomers can store significant amounts of excitation energy for times long enough for them to serve as nuclear fluorescent materials. There are over 800 known nuclear isomers[1] but almost all are too intrinsically radioactive to be considered for applications. As of 2006[update] there were two proposed[citation needed] nuclear isomers that appeared to be physically capable of IGE fluorescence in safe arrangements: tantalum-180m and hafnium-178m2.