‘‘Its not the substance, it’s the dose which makes something poisonous!’’ When
Paracelsius, a German physician of the 14th century made this statement he
probably did not think about light as one of the most obvious environmental
factors. But his statement applies as well to light. While we need light, for
example for vitamin D production, too much light might cause skin cancer. The
dose makes the difference. These diverse findings of light effects have attracted
the attention of scientists for centuries. The photosciences represent a dynamic
multidisciplinary field that includes such diverse subjects as behavioral re￾sponses of single cells, cures for certain types of cancer and the protective
potential of tanning lotions. It also includes photobiology and photochemistry,
photomedicine as well as the technology for light production, filtering and
measurement. Light is a common theme in all these areas. In recent decades a
more molecular centered approach has changed both the depth and the quality
of the theoretical as well as the experimental foundation of photosciences.