A system for the linearization of voltage contrast in the scanning electron microscope for potentials between +or-10V is described. The linearization is achieved by using two concentric hemispherical grids which form a retarding field analyser with the specimen at the centre of the grids. Collection is by either a solid hemispherical cup, or alternatively the conventional scintillator cage. Results are presented for both cases. A discussion of the minimum detectable potential in terms of the beam current and equivalent noise current is also included.