A quantitative thermionic projection microscope, employing computer image processing, is described. In this system, thermionic emission from a single‐crystal hemispherical cathode is projected onto a phosphor screen and the resulting image is digitized by means of a video camera. Single‐plane effective work functions for the highest emitting planes of the cathode may be measured to within a tenth of an electron volt. ©1994 American Institute of Physics.