This article describes the characterization of field emission from a planar cathode to a spherical anode with the approach curve method (ACM). In such a diode configuration the electric field strength at the cathode surface is nonuniform. This nonuniformity gives an extra degree of freedom and it allows the interpretation of the current-voltage and voltage-distance (Vxd) curves in terms of nonuniformity. The authors apply the ACM to Cu emitters to explain the nonlinearity of the Vxd curve in ACM measurements. This analysis provides a good insight into field emission phenomena, supporting a method for nonuniformity characterization based on field emission behavior. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3327928]