Gas discharge lamp ballast circuit with automatically calibrated light

Abstract

Disclosed is a ballast circuit for a high pressure gas discharge lamp, including a comparator circuit for producing a feedback error signal representing the difference between a feedback signal and a reference signal. The feedback signal is selectively one of a non-light feedback signal and a light feedback signal, as determined by a switching circuit. A power control circuit adjusts the level of power supplied to the lamp in response to the feedback error signal. A first, non-light feedback circuit supplies a non-light feedback signal to the comparator circuit, based on non-light information of the lamp that is fed back to the non-light feedback circuit. A light feedback circuit operative during a lamp warm-up period supplies a light feedback signal to the comparator circuit based on light intensity information that is fed back to the light feedback circuit. The light feedback circuit has an adjustable gain as determined by the difference between measured light intensity and magnitude of the light feedback signal produced. A calibration circuit, operative during steady state lamp operation when the comparator circuit is responsive to the non-light feedback signal, automatically adjusts the gain of the light feedback circuit until a state is reached in which switching to light feedback control would result in substantially no change in light intensity. The calibration circuit includes a calibration memory for storing the gain for use in a subsequent period of lamp warm-up.