FRACTAL ZONE PLATES FOR X-RAY MICROSCOPY

KEY WORDS: X-ray microscopy, chromatic aberration, depth of field, zone plates Fresnel zone plates are diffractive elements that are essential to form images in X-ray micros-copy[1,2]. One of the main shortcomings of Fresnel zone plates their high chromatic aberra-tion. Recently presented, Fractal Zone Plates (FZPs) [3] are diffractive lenses characterized by a fractal focal structure with an extended focal depth, as compared with Fresnel lenses. This behaviour predicts an improved performance of FZPs as image forming devices with an ex-tended depth of field and a reduced chromatic aberration. Here we examine the incoherent imaging characteristics of these elements using polychromatic light in the visible range. The performance of FZPs is cotrasted with conventional Fresnel zone plates of the same focal dis-tances. We have demonstrated experimentally that the FZP provides an extended depth of field and a reduction of chromatic aberration. This finding is confirmed objectively using the modulation transfer function as a merit function. We show that the polychromatic modulation transfer function (MTF) of a FZP affected by defocus is about two times better than one corresponding to a Fresnel zone plate.