This physical model is a generalized model which can simulate the effects of intensity spectrum of light source and numerical aperture of objective lens. Although it is capable of simulating the interference signal with distinctive feature (as shown in Fig. 2), it does not help in explaining why and how the distinctive feature is generated. This is because the generalized model is hard to be manipulated algebraically. For example, it is difficult to identify and extract the term(s) representing the OPD range where the interference occurs and the fringe contrast function out of the generalized physical model.