In this paper we discuss intensity mechanisms of interest for rare-earth ions in optical materials. More specifically, we investigate the character of two models-first, a semi-classical dynamic intensity model, and second, a more traditional quantum mechanical vibronic intensity model. We focus on the case of f-f transitions and show that the predicted oscillator strengths then are the same under very reasonable approximations. We emphasize that this connection between the models cannot be made in the case of f-d transitions. The dynamic intensity model has an interesting classical interpretation. We show that it is very convenient to apply this approach together with classical dynamical techniques such as molecular dynamics simulation or the classical Monte Carlo method. The classical approach simplifies both the interpretation and calculation of vibronic oscillator strengths in complex systems. The concept of an effective temperature works as a bridge between the two models; so realistic predictions can be obtained even at low temperatures.