Mainly on the Fresnel Mode in Lorentz Microscopy

We consider the problem of the determination of magnetic structures by transmission electron microscopy. A stationary phase approximation to the Kirchoff diffraction integral yields a criterion for the validity of geometrical optics in calculating image intensity distributions at any diffraction plane. Our criterion is particularly useful for the Fresnel diffraction case since the usual criterion Δφ ≫ h c/e [1] is strictly valid only for the Fraunhofer diffraction condition.