Patent ID: 8217593

Claim:
A method of driving a gas-discharge lamp with an alternating current, the method comprising monitoring an arc-length control value, indicating the current length of a discharge arc of the gas-discharge lamp, driving the lamp in a first operation mode (OM p ) with a first current wave-shape when the arc-length control value indicates that the arc-length is higher than a switching threshold, which first current wave-shape is selected to result in a tip growing on an electrode of the lamp, driving the lamp in a second operation mode (OM n ) with a second current wave-shape when the arc-length control value indicates that the arc-length is lower than a switching threshold, which second current wave-shape is selected such that the tip on the electrode is at least partly melted back, switching an operating frequency of the lamp from a first frequency value (f 1 ) to a second frequency value (f 2 ), which second frequency value (f 2 ) is higher than the first frequency value (f 1 ), if a first time criterion pertaining to a duration of driving of the lamp in the second operation mode is fulfilled, and switching the operating frequency from the second frequency value (f 2 ) to the first frequency value (f 1 ), if a second time criterion pertaining to a duration of driving of the lamp in the first operation mode is fulfilled.