We report the measurement of the x‐ray power from four transition radiators. The average power from a 44 &mgr;A, 104 MeV electron beam penetrating a stack of fifteen 1.5‐&mgr;m‐thick aluminum foils was 0.7 mW at a peak photon energy of 1.56 keV with an approximate bandwidth of 40%. The peak power from the same stack in a 17 ns pulse was 58 W. Foil stacks constructed of Mylar, parylene, or boron were either distorted or destroyed by exposure to the high average current electron beam.