Ultra-sensitive, super-resolving and non invasive tools are required for studying the cell machinery at the molecular level. Thanks to the use of ultra-sensitive cameras and to the development of the 4PI-microscope, fluorescence microscopy technique is the best tool for this kind of study. Nevertheless, the axial localization of single molecules and the lateral resolution of the confocal microscopes must be improved. In our optical set-up, a 4Pi-microscope coupled to a Michelson interferometer allows to determine the axial position of a luminophore by Partial Coherence Interferometry (PCI). We have also proposed a configuration of 4Pi-microscopes for improving their lateral resolution.
