By using a simple model based on a linear susceptibility approach and taking into account the dynamic nature of the molecule's electronic structure, the electric field diffracted by the near-field probe is evaluated in the presence or absence of a thin silver layer deposited on the surface of a bulk fluorescent material. This modeling points out that the far-field signal emitted by the molecules situated far from the tip is important in the absence of metal coating and can reduce the near-field detection sensitivity. It shows that surface metal coating enhances the near-field contribution and decreases the far-field one. Cathodoluminescence images obtained on a fluorite (CaF2:Eu) sample confirm the results predicted by modeling.