Luminescence - photoluminescence - phosphorescence

Luminescence refers to light emission caused by electron transition from excited state to ground state; not by a rise in temperature (light emission from pure heat source is incandescence).

By different excitation(pumping) mechanism, luminescence has various types, for example:

• Photoluminescence: material is pumped by absorption of photons (e.g., fluorescent lamp).


• Cathodoluminescence: pumped by electron beam (e.g., CRT monitor).


• Electroluminescence: pumped by electric current or field (e.g., LED).

Photoluminescence:
From the transition speed, photoluminescence is divided into two types:

• Fluorescence: an almost instantaneous effect; the emission ends within ~10^-8 second after excitation.


• Phosphorescence: this term describes the persistent luminescence (afterglow) of phosphors (the "glow-in-the-dark" phenomenon). The upper state is either metastable with long lifetime or is transition forbidden to the ground state.

Phosphorescence:
Materials exihibiting phosphorescence are known as phosphors. Phosphors are usually some microcystalline materials. In these crystals, there are impurity ions called activators, which replaces some host ions in the crystal lattice. These activators form luminescing centers and are where the excitation-emission process occurs.

References
[1] John Wilson and John Hawkes, Optoelectronics, an introduction, 3rd ed., Prentice Hall Eruope, 1998. Chapter 4.
[2] Luminescence article at www.britannica.com
[3] Other sites that explain the phosphorescence physics:
http://math.ucr.edu/home/baez/spin/node17.html
http://en.wikipedia.org/wiki/Phosphorescence