Chemiluminescence (CL) is the emission of light (luminescence) caused by a chemical reaction. In a chemiluminescent reaction, reactants collide to form an excited electronic state, which is the direct product of the reaction. This excited state then decays into an electronic ground state with lower energy level, causing light emission. A well-known example of chemiluminescence is the luminol test used in forensics, where trace amounts of blood at crime sense can be indicated by luminescence upon contact with iron in hemoglobin.