A fundamental investigation of the problem of an incoherently driven two-level atom is presented. In the case of a black body (thermal) broad band field the Einstein radiation rate equation is derived once a statistical decorrelation between matter and field variables is made. This is also shown to be true for N #62; 1 independent (non-cooperative) atoms. The normalised second order correlation function g(2)(0) = 2 for N ⪢ 1, characterising the chaotic nature of the scattered field. We also investigate the hierarchy of approximations to the rate equation. To first order in the coupling constant e2 (e is the electron charge) a generalised (correlated) rate equation, for a single atom, is derived. The statistical correlation of field and matter has its effect on the atomic saturation as well as on the scattered radiation where g(2)(∞) #62; 1 (partial coherence) compared with full coherent radiation, g(2)(∞) = 1, in the decorrelated case.
