Light is defined as that part of the electromagnetic spectrum (similar to 380-780 nm) that gives rise to a visual sensation. Lighting in buildings, whether through use of daylight or by artificial means, is designed primarily for the visual needs of the occupants and their expected tasks within a given space. However, solar radiation, and, depending on spectral output of the source, artificial radiation, has other effects on human physiology and behaviour. Blue light affects the circadian rhythm, mood and behaviour; at shorter wavelengths in the ultraviolet (UV) the detriments of photoaging and sunburn are balanced by the benefits of Vitamin D synthesis. (c) 2006 Elsevier B.V. All rights reserved.