7 Visible Light and UV Radiation

The sun is the most important source of visible and ultraviolet (UV) radiation. Even though the distance from the sun to the Earth is large – about 150×10 km – the fluence rate of solar radiation on Earth, the solar constant, is about 1 360 W/m 2 . Approximately 40% of this radiation is reflected back into space. The remaining 60% is the driving force of all life on Earth. A number of pigments have been developed by life to harvest solar energy: chlorophyll a (350-700 nm), phycoerythrin (45-580 nm), phycocyanin (460-650 nm), bacteriochlorophyll a (750-850 nm) and bacterio-chlorophyll b (950-1 050 nm) are some of the most important ones. Furthermore, animals have developed visual pigments, the rhodopsins, to see light. By its action on DNA, UV radiation from the sun has induced mutations to speed up generation and development of new species. UV radiation acts positively and negatively on the human immune system, e.g. induces cancer and takes part in the production of vitamin D. This chapter reviews some of the basic facts about visible and UV solar radiation: spectra and their variations with the phases of the solar cycle, ozone level, time, latitude, altitude, albedo (reflection), and sky cover. Furthermore, scattering and absorption of optical radiation in the atmosphere and in human skin are discussed. Finally, there is a review of the action spectra for erythema, skin cancer of different types, effects on the human immune system and photoreactivation (light-induced repair of DNA damage). The biology of these phenomena will be dealt with in Chapter 31 'Effects of UV Radiation and Visible Light'.