ABSTRACT
Photodynamic action, which is the expression of the photodynamic effect, refers to the damage or destruction of living tissue by visible light in the presence of a photosensitiser and oxygen. In principal, light in the very near infrared region could be employed, but sensitisers absorbing there are rather uncommon, and in most cases irradiation is with visible light. The light source may be incoherent (eg a tungsten lamp) or coherent (eg a laser): the sensitiser must absorb the light and the excited sensitiser then activates oxygen in some way (section 4.4). Molecular oxygen is essential for the process to occur, and in most examples is derived from the atmos phere.
