Most of the biological systems and most of materials emit, on being illuminated optical photons characterized by an ultraweak intensity and relatively long duration. This photo induced ultraweak luminescence is often called delayed luminescence (DL).
The "Delayed light" was observed for the first time in 1951 by W.A. Arnold, one of the giants of reasearch on photosynthesis, and B. Strehler. They observed a totally unexpected light emission from green plants seconds after illumination (Arnold W. 1986).
Nowadays we know that this "Delayed light" (called also Delayed Fluorescence or Delayed Luminescence) is connected to the electron transfer chain of the photosystem II (PSII) of plants and that probably it is characteristic not only of photosynthetic systems.