Luminescent immunoassays are variations of the standard ELISA, just like fluorescent immunoassays. An enzyme converts a substrate to a reaction product that emits photons of light instead of developing a visible color. Luminescence is described as the emission of light from a substance as it returns from an electronically excited state to ground state. The various forms of luminescence (bioluminescence, chemiluminescence, photoluminescence) differ in the way the excited state is reached. For example, photoluminescence is simply fluorescence; the excitation is initiated by light at a particular wavelength. Bioluminescence is characterized by the use of a bioluminescent compound, such as luciferin and firefly luciferase. Chemiluminescence is light produced by a chemical reaction. The chemiluminescent substance is excited by the oxidation and catalysis forming intermediates. When the excited intermediates return back to their stable ground state, a photon is released, which is detected by the luminescent signal instrument.