A ballast circuit is suitable for driving a high intensity discharge lamp in at least two power modes including a nominal power mode for driving the high intensity discharge lamp at a nominal power and a reduced power dimming mode for driving the lamp at less than the nominal power. The lamp ballast circuit generates a first alternating current waveform during the nominal power mode and generates a background alternating current waveform in the reduced power mode which is perturbed at intervals by a high intensity pulse which helps to reduce the change in correlated color temperature which would otherwise occur in the dimming mode.