Spatial resolution of an optical microscope with oblique illumination

This paper presents the theory for numerical evaluation of the spatial resolution along an optical axis of an optical microscope in case of oblique illumination. It considers the optical setup with a coherent light source, a microscope condenser, a grating located in a plane with an optical axis and with grating slits perpendicular to this axis and a microscope objective. It is proposed the analytical expression for calculation of the minimal resolvable period of this grating or the corresponded spatial cutoff frequency that characterizes the spatial resolution along the optical axis. It is demonstrated that this spatial cutoff frequency is not proportional to the angle of beam inclination. The proposed theory clearly explains why an optical microscope has the limited spatial resolution along an optical axis and how illumination can maximize this resolution.