An interferometric technique designed to measure optical homogeneity in an electron‐beam‐pumped plasma is described and characterized. The system provides a two‐dimensional transient phase shift contour mapping of the excited region, at any time during or after the excitation pulse. The basic intrument is a 7‐cm‐field, 5‐m Mach‐Zehnder type interferometer coupled to a specially configured 2‐m e‐beam‐pumped sample chamber. By virture of ’’e‐beam partitioning,’’ absolute fringe shifts can be measured in a single shot. This relaxes the usually severe requirements on vibration isolation associated with systems of this size, and simplifies the overall mainframe construction. A short‐pulse XeF discharge laser, developed specifically for this application, is also described, and sample interferograms are presented for e‐beam‐pumped noble gas and noble gas–NF3mixtures. A ray calculus model is applied to the analytical interpretation of the interferograms produced by this technique. This model has general applicability to ray propagation in graded index media and is discussed in some detail.