Laplacian and caustic imaging theories of MEM work-function contrast

We apply geometrical, Laplacian, and caustic imaging theories to simulate the mirror electron microscope (MEM) contrast arising from a surface phase boundary associated with a discontinuity in work function. The key approximations inherent in the theories are highlighted and investigated within strong and weak scattering regimes from the work-function test object. For strongly varying potentials, the approximation that the electron classical turning distance is unchanged fails, invalidating the quantitative accuracy of the geometrical and Laplacian approaches. For sufficiently small defocus and surface height or potential variations, the Laplacian approach facilitates an intuitive interpretation of MEM contrast.