Abstract: The invention provides a brightness control circuit for a periodically flashing xenon lamp. The intensity of the flashing lamp may be selected in response to the selection of a reference voltage. The control circuit has a power source in series with a thristor for selectively controlling an application of a voltage from a power source to a xenon lamp. A capacitor is coupled substantially in parallel with the xenon lamp in order to apply a voltage across the lamp. A comparator is responsive jointly to a charge level on the capacitor and a reference voltage. The comparator switches on/off the thyristor so that the switching voltage may be selected by selecting the reference voltage, thereby selecting the intensity of the reference voltage. The flashing lamp in this particular application is part of an airport runway approach system.