Three-dimensional coherent transfer function in reflection-mode confocal microscopy with two-zone phase filters

The three-dimensional (3D) coherent transfer function (CTF) in a reflection-mode confocal scanning microscope using two-zone phase filters has been derived analytically. The numerical results for various phase filters are presented and the characteristics of different cases are compared. Finally, the image sharpness expressed by in-focus CTF and the effect of Gaussian beam are discussed. More information in the high spatial frequencies can be transferred in both cases, which is normally at price of the degrading the axial resolution and the image sharpness, as expected. However, the axial resolution remains unchanged for two equal phase filters with the phase difference of π. In addition, the dark-field imaging may be produced in a system with one phase filter. It is also found that the contrast-reversal phenomenon would occur in focal plane and the maximum value of axial response is not at the origin in some cases.