Incident and emitted photons

Incident and emitted photons refer to the particles of light that interact with biological matter, serving as the fundamental basis for optical imaging and light-based therapies. Incident photons are the incoming light energy used to excite molecules, while emitted photons are those released during processes like fluorescence, phosphorescence, or bioluminescence (NCBI, 2023). In a clinical context, photons are not biological targets such as receptors or enzymes; instead, they act as the energy source or physical modality for photodynamic therapy (PDT) and photobiomodulation (PubMed, 2021). During PDT, incident photons of a specific wavelength are absorbed by a photosensitizer drug, leading to the production of reactive oxygen species that induce localized cell death in tumors or abnormal vasculature (StatPearls, 2023). Because they are physical phenomena rather than biochemical entities, they do not possess a molecular classification or traditional pharmacological binding site. Their role is primarily as a trigger for exogenous drugs or endogenous chromophores like hemoglobin or melanin. Consequently, while critical to medical technology, they are categorized as a physical stimulus rather than a molecular therapeutic target.

Other names

LightElectromagnetic radiationOptical radiationPhotons

Incident photons of specific wavelengths excite photosensitizing agents to a higher energy state, which then transfer energy to molecular oxygen to generate cytotoxic reactive oxygen species (ROS) or emit light for diagnostic imaging (StatPearls, 2023).