Patent ID: 8487540

Claim:
A method of driving a gas discharge lamp to produce a variable light level in a range between a nominal light output level (L M ) and a minimum light output level, comprising the steps of: generating an alternating lamp current (I) with a constant current amplitude; when producing the nominal light output level (L M ), constantly supplying the lamp with the alternating lamp current (I) at a nominal current amplitude (I M ); when producing light having a light output level in a first range below said nominal light output level (L M ), supplying the lamp with spaced apart current bursts having a burst duration Tc and a burst repetition period T where, in each current burst, the lamp is constantly supplied with the alternating lamp current (I) at the nominal current amplitude (I M ) and where, in the intervals between successive current bursts, substantially no current is supplied to the lamp, the light output level in the first range being varied by varying the burst duty cycle (Δ), defined as Δ=Tc/T, within a range between 100% and a minimum burst duty cycle value (Δ T ); when producing light having a light output level in a second range below said first range, supplying the lamp with spaced apart current bursts where, in each current burst, the lamp is constantly supplied with the alternating lamp current (I) at a reduced current amplitude (I R ) lower than the nominal current amplitude (I M ) and where, in the intervals between successive current bursts, substantially no current is supplied to the lamp, the light output level being varied by varying the reduced current amplitude (I R ) within a range between zero and the nominal current amplitude (I M ) while keeping the burst duty cycle (Δ) constant at said minimum burst duty cycle value (Δ T ).