Optical radiation ranges from short wave ultraviolet via visible light to the far
infra-red. Both natural and artificial light sources may cause desired and undesired
effects, both on the skin and on the organism as a whole. The nature and extent of
such effects depend primarily on the energy of the photons, the absorption capacity
of the chromophores, the mean depth of penetration, and on the density and dose of
the radiation. The present paper discusses the mechanisms of action and desired and
undesired effects in the organism, including therapeutic applications, separately
for ultraviolet radiation, visible light and infrared radiation.