Abstract: A first thyristor gate whose conductive state is determined by the polarity of a signal applied to its control electrode is connected in series with a flash tube for selectively coupling operating voltage from a flash capacitor to the tube. The first gate is opened by a signal of a first polarity derived from the operating voltage source and generated in synchronism with an ignition pulse, which is applied to the trigger electrode of the flash tube to energize the tube. An integrating circuit energized by a transformer coupled to the flash tube output contains a photosensitive element responsive to a portion of the emitted light for developing a control voltage that increases in proportion to the total light integrated. When the output voltage from the integrating circuit reaches a predetermined value, a second thyristor gate whose control electrode is coupled to the output of the integrating circuit is opened to apply a signal of the opposite polarity to the control electrode of the first thyristor gate.