Abstract
The effects of contrast in the Fresnel zone plate on the amplitude in the focal point due to a normally incident plane wave are obtained by direct evaluation of the Fresnel-Kirchhoff diffraction integral. The change in focal-point amplitude due to smoothing the ordinary, discontinuous, “square-wave” transmission function into a continuous, “sine-wave” function is also calculated. Finally, an expression is derived for the amplitude in the point image due to a plane wave impinging on the zone plate at a small angle of incidence.

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