ABSTRACT
Application of photobiology to dermatology relies on knowledge from a wide range of areas, including climatology, photochemistry, cellular and molecular biology, and pathology. These three components of the solar spectrum—ultraviolet (UV), visible, and infrared—are referred to collectively as optical radiation. But it is the UV rays, comprising about 5% of terrestrial sunlight, which hold the greatest interest in photodermatology. The two most common dosimetric terms found in photodermatology are irradiance and dose. The problem of dosimetry in photodermatology lies in the fact that the ability of UV radiation to elicit erythema in human skin depends strongly on wavelength, encompassing a range of four orders of magnitude between 250 and 400 nm. Experiments in which the photoresponse of a material is investigated as a function of irradiance are commonly called reciprocity law experiments. UV and visible photons enter the skin where they may be absorbed by molecules in the epidermis and dermis or scattered by structures, such as collagen fiber bundles.
