Abstract: An element for the retarding field analysis of the energy distribution of electrons is constructed from three or more similar planar grids superimposed in parallel planes. The element is arranged for rigid location with respect to the beam axis of a scanning electron microscope so that the beam passes through aligned apertures in the grids and so that secondary electrons produced from a specimen are collected by the element. In use in the electron microscope the potential of the retarding grid is automatically maintained at a level such that the ratio of electron collection at high and low energies is held constant. The potential so determined can be related to the potential of the surface at the point of origin of the electrons.