In a scanning anode field emission microscope the field emission current is extracted by a small spherical anode brought, at micrometric proximity, in front of a planar cathode surface. Therefore, the field over the emission area is not uniform and direct quantitative interpretation from the total field emission current versus applied voltage is misleading. The potential distribution of the system composed of a sphere in front of a plane have been calculated to determine the field distribution over the surface in order to define the active field emitting zone and to extract the current density versus the local applied field from the measured total field emission current versus applied voltage characteristics. (c) 2005 American Vacuum Society.