Patent ID: 8461772

Claim:
A high pressure discharge lamp ballast for driving a high pressure discharge lamp with a synthesized-waveform alternating current comprising combination of a first set of current waveforms and a second set of current waveforms, the high pressure discharge lamp including a pair of electrodes disposed to face each other, the ballast being used in a digital lighting processor (DLP) system employing a color wheel, the ballast comprising: control means for controlling a rate of a time period of the first set or second set in the synthesized-waveform alternating current per unit time; detection means for detecting a synchronization signal for a rotation of the color wheel; output means for applying the synthesized-waveform alternating current in accordance with the synchronization signal and the rates to the high pressure discharge lamp; and detection means for detecting a lamp voltage of the high pressure discharge lamp, wherein the first and second sets are each in a waveform inverted so as to correspond to at least one of a rotational speed of the color wheel and divided positions of segments of the color wheel, a period of each of the first and second sets has a length equivalent to one rotation of the color wheel, and the number of inversions of the second set is larger than the number of inversions of the first set, so that an average frequency of the second set is higher than an average frequency of the first set, and the control means is configured to set the rate of a time period or the number of cycles of the second set per unit time to R L % when the lamp voltage exceeds a first predetermined voltage value V, and to set the rate of a time period or the number of cycles of the second set per unit time to R H % when the lamp voltage falls below a second predetermined voltage value V′, where 0≦R L <R H ≦100 and V>V′.