Ultra‐Weak Photon Emission as a By‐Product of Mitochondrial Oxidative Metabolism: Mechanisms, Limits, and Implications

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ABSTRACT Ultra‐weak photon emission (UPE) is spontaneous, low‐intensity light produced during aerobic metabolism. UPE has multiple cellular sources; we focus on the mitochondrial contribution, which arises predominantly from reactive oxygen species (ROS) chemistry and electronically excited products, linking UPE to redox state and mitochondrial function. We treat UPE as a by‐product of oxidative metabolism, not as a dedicated signalling channel. We outline mitochondrial oxidative pathways generating triplet excited carbonyls and singlet oxygen, and distinguish baseline UPE from stimulated emission under oxidative stress, lipid peroxidation and cell death. An energy budget for the mitochondrial electron transport chain shows that no single transition can emit a visible or near‐infrared photon: the largest step corresponds to 2180 nm, and the residual after proton‐pumping to 3000 nm, where the sample's thermal emission dominates. Measured UPE, at rest and under stress, therefore reflects ROS‐driven chemiexcitation, and the two regimes differ chiefly in flux. We review detectors, from photomultiplier tubes and low‐light cameras to single‐photon avalanche diodes and emerging cryogenic and nanophotonic sensors, and organise applications across medicine, neuroscience, agriculture, food science, dermatology and fundamental biology within a two‐regime framework (spontaneous vs. stimulated). We conclude with experimental priorities for establishing UPE as a quantitative reporter of oxidative metabolism.