Fresnel Zoneplate

Introduction

A fresnel zoneplate is diffractive element that can be thought of as a circular grating. The period of the zoneplate decreases the further away from the center one gets.  Similar to how a grating diffracts light to multiple orders, a Fresnel zoneplate creates multiple focuses.  If one of these focuses can be selected using an aperture, then the zoneplate acts similarly to an optical lens.