Instead of considering only one atom interacting with a radiation field, consider a large number of atoms. By including the interaction of the atoms among themselves, in addition to the atom--field interactions, a number of interesting results can be observed.
There are two effects that can be included: collisions and spontaneous emission. Collisions will be examined first because they are simpler to visualize. Two types of collisions have been included in this model: elastic and inelastic. Each has a unique effect on the system.