‘‘It’s not the substance, it’s the dose which makes something poisonous!’’
When Paracelsius, a German physician of the fourteenth 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 photo￾sciences represent a dynamic multidisciplinary field which includes such
diverse subjects as behavioural responses of single cells, cures for certain
types of cancer and the protective potential of tanning lotions. It 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 centred approach changed
both the depth and the quality of the theoretical as well as the experimental
foundation of photosciences.