Photochemical interaction of light and tissue is of great interest for the study of tissue damage induced by solar radiation, in particular in skin-aging process, as well as for the designing of controllable technologies for tissue repairing and rejuvenation. Such interaction depends on the type of endogenous or exogenous chromophore (photosensitizer) involved in photochemical reaction, oxygen tension, and light wavelength, intensity, and exposure. To characterize a photochemical reaction a quantum yield is introduced. For a radiation-induced process, quantum yield is the number of times that a defined event (usually a chemical reaction step) occurs per photon absorbed by the system. It is a measure of the efficiency with which absorbed light produces some effect. Since not all photons are absorbed productively, the typical quantum yield is less than 1. Quantum yields greater than 1 are possible for photo-induced or radiation-induced chain reactions, in which a single photon may trigger a long chain of transformations.
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