Abstract: Apparatus and method for testing the susceptibility of an integrated circuit (10) to single event upsets caused by high energy heavy ions, such as are found in cosmic rays. The integrated circuit is mounted in a three axes manipulator (20) by which it is positioned at a desired target point. A light pulse generated by source (32) is filtered and collimated by spatial filter (34), and is focused as a spot on the integrated circuit by optics (60). Preferably, the diameter of the focused spot is 5 microns or less and its wavelength is in the range of 850-1100 nanometers. The susceptibility of any point on the integrated circuit is determined by operating the circuit and monitoring it for errors after it is subjected to the focused light pulse. Timing of the light pulse may also be controlled with respect to integrated circuit operation, to determine susceptibility as a function of time.