This mercury‐capsule light‐pulse generator was developed to test and evaluate the high‐speed features of multiplier phototubes and low‐level image tubes. Light pulses and electrical trigger pulses are generated simultaneously in an arc discharge at a usual repetition rate of 60 per second. The electrical pulse is used as a time reference for the light pulse. The time required for both the light and the electrical pulse to rise from 10 to 90% of peak amplitude is less than 5×10−10sec. The light pulse rises to a maximum and falls to 50% peak amplitude in less than 1.5×10−9sec. The electrical pulse power available is large. Time resolution of 10−10sec is typical of measurements made with conventional fast oscilloscopes, while elaboration of technique permits relative time measurements that are better in some cases by at least three orders of magnitude. The light is emitted from a region a few mils in diameter, and thus may often be considered to come from a point source. An S4 photosurface subtending 0.1 sterad at the light source emits photoelectrically about 107electrons per sterad per pulse. For convenience, three decoupled electrical output channels are provided, together with a Polaroid attenuator for the light pulse.