Analysis and improvement of the lateral resolution of CSI instruments based on the universal Fourier optics (UFO) model

This contribution discusses lateral resolution capabilities of CSI instruments based on the three-dimensional optical transfer function. The 3D transfer function combines the lateral spatial frequency transfer of a measured surface topography with axial spatial frequency contributions corresponding to the fringe frequencies of CSI signals. A broad bandwidth of fringe frequencies contributing to CSI signals can be achieved by both, low temporal coherence of the light source employed, or high numerical apertures, which reduce the longitudinal spatial coherence while enhancing the lateral resolution capabilities of an instrument. We first analyze theoretical aspects of lateral resolution in CSI using the Universal Fourier Optics (UFO) model and then validate our theoretical findings by experimental results using custom-built CSI systems of high NA. In addition, we introduce a CSI microscope equipped with immersion objectives to further improve the lateral resolution. Results based on separated phase irregularities demonstrate that scattering centers can be resolved even if their distance is less than the minimum distance predicted by the Abbe limit.