Energy spread is critical to the performance of probe forming systems working at high resolution. The energy spread in the probe is the convolution of the intrinsic spread upon emission and the broadening due to Coulomb interactions in the beam. In order to optimize the design, the two need to be distinguished. The Boersch effect in a Schottky electron gun is difficult to calculate because of the low initial velocity at emission and the strong potential gradient. This article presents a method to extract it from experimental data. Extracting the Boersch effect by deconvoluting the experimental spectrum with the intrinsic distribution is difficult because of noise in the data. Instead, the Boersch effect was determined by fitting convolutions to the measured spectra. A bell shaped function with two free parameters was selected to describe the Boersch contribution. By fitting the convolution of the intrinsic energy distribution and this bell with its free parameters as fitting parameters, the Boersch contribution of experimental spectra could be adequately determined. (c) 2007 American Vacuum Society.