The biological emission of photons (biophotons) is a term used to describe the permanent ultraweak (1-100 photons/sec/cm2) emission of coherent (phase-locked and/or frequency-locked) photons from living systems. (F.A.Popp 1976) Popp considered it to be a quantum biological phenomenon with bio-informational character distinct from the non-coherent emission of photons as by-products of metabolism, like thermal radiation and bioluminescence/chemiluminescence caused by radical reactions, oxidation etc.

Biophoton/ultraweak photon emission originates from relaxation of electronically excited states of the constituents of living cells, which are generally associated with the presence of an oxidative metabolism that accompanies the production of reactive oxygen species (ROS) which participate in the regulation of a wide spectrum of biochemical and physiological functions.