Imaging Properties of Modified Fresnel Zone Plates

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Abstract
The imaging properties of two types of modified Fresnel zone plate are calculated and compared with those of the equivalent normal zone plate, with particular reference to their use in soft X-ray microscopy. The modifications considered are (1) central obstruction of the zone plates, which improves the resolving power but also leads to loss in efficiency and to a higher fraction of the energy in the outer parts of the diffraction pattern; and (2) a central zone plate with first-order focal length ƒ surrounded by outer zones with third-order focal length ƒ. This gives improved resolution and higher focused energy, but a smaller depth of focus and increased background.

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