Abstract: A system for controlling power applied to a gas discharge lamp by an AC source, wherein the power dissipated in the lamp during a half cycle is sensed by a measuring circuit. The measuring circuit provides output signals proportional to the current through and voltage across the lamp during its conduction. The measuring circuit output signals are multiplied and the difference between the resultant multiplier signal and a reference signal is time integrated and fed to a pulse width modulator. The pulse width modulator provides a pulse to a pulse transformer to trigger an SCR in series with the lamp at some point in the next half cycle, the point at which the SCR is triggered being determined by the power dissipated in the lamp in the previous half cycle. In another embodiment the SCR is replaced by a transistor and the pulse width modulator shuts off the transistor after the lamp has dissipated the desired amount of power in a given half cycle.