Nanoscopic characterization of light-emitting materials is essential to realize nano-optical devices, which requires nanoscopic spatial resolution far beyond the diffraction limit of light. Cathodoluminescence (CL) is a powerful means to achieve such nano-optical characterization by combining with electron microscopy. However, discrimination between coherent and incoherent CL emissions, when a phosphor material is combined with a resonator, is not trivial. To solve this general problem in such coupled emitter–resonator systems, we take advantage of optical saturation in incoherent CL in the phosphor and propose a method to extract the incoherent component to distinguish the coherent components purely from the resonator. We demonstrate this CL saturation imaging approach using an integrated system of Zn₂SiO₄ phosphors and a plasmonic resonator array and visualize the resonator-modified luminescence at the nanoscale, which evidence the near-field coupling between the phosphors and the plasmonic resonators.