Fresnel-Zone-Plate Spectrometer with Central Stop*

The marked chromatic aberration of a Fresnel-zone-plate lens can be used as a variable narrow-band optical filter, and thus a holographic spectrometer. The filtering properties of the zone plate are analyzed, with particular emphasis on the effect of a central stop over the low-order rings. It is shown that the central stop can markedly improve the rejection of frequencies outside the pass band without significant degradation of the spectrometer resolution.