Luminescence:

When the atoms in the solid state are excited by the some means (optical, electrical exciton etc.,) and if the atom return from excited state to the ground state by emitting light is called Luminescence. Based on their exciton, we can clarify the luminescence into many types.

Photo luminescence:

    Here the excitation of atoms occurs due to photons (or) light.

Electro luminescence:

    Here the excitation of atoms occur due to electric field .

Cathode luminescence:

    Here the excitation of atoms occurs due to electron bombardment.

Thermo luminescence:

    Here the excitation of atoms due to heating.

Chemi luminescence:

    Here the excitation of atoms occurs due to chemical reaction.
   
Injection luminescence:

    Here due to the application of forward biasing, the majority charge carrier moves from n to p and p to n region and becomes minority charge carriers. Then these excess minority charge carrier diffuses and combines with the majority charge carriers in n and p regions to emit light.

Characteristic and non-characteristic luminescence:

    We know the phosphorescence occurs due to the time that an atom spends in a trap at a given temperature and the materials exhibiting phosphorescence are called phosphors.

    Normally these materials are produced by adding impurities which leads to traps. Therefore, the impurity ions which give rise to the production of traps are called activators.

    Normally the activators (impure ion) will replace he host ions and the charges will also be in the balanced condition. Sometimes while replacing host ions by activators ions, the charges may not be balanced properly. In that case many impurity ions with different ionic charges are produced and they are termed as co-activators.

    Therefore based on the presence and participation of activators and co-activators. We have two types of Luminescence.

Characteristic Luminescence:

     If the excitation energy is very quickly (say less than  secs) transferred to the activators ions and thereby luminescence occurs due to the life time of the activators in the excited level, it is called characteristics luminescence.

    Therefore we can say that in the characteristic luminescent materials activators alone participate in excitation process.


Non-characteristic luminescence:

    If the excitation energy is transferred to both activator ions and co-activators ions and thereby luminescence occurs, it is called non-characteristic luminescence.
    Therefore here both activator and co-activator participate in the exciton process.