Optical transfer function of inverse-phase composite zone plate devised for deep-focusing X-ray microscopes

The optical transfer function (OTF) of an inverse-phase composite zone plate under incoherent illumination was numerically evaluated by convoluting the point spread function (PSF) and transmission distribution of a model sample with gradually changing spatial frequency. The PSF used in the simulation was obtained from our previous study on deep-focusing X-ray microscopes. As expected, the contrast in the sample image was lesser than that produced by a conventional zone plate. Phase-reversed contrast-spurious resolution-appeared at a high spatial frequency. Although the calculated OTF is slightly aberrated, its deep-focusing capability is advantageous for X-ray microimaging of thick samples.