Patent ID: 8482217

Claim:
A high pressure discharge lamp ballast for driving a high pressure discharge lamp with a synthesized-waveform alternating current made of frequency components f 1 and f 2 (f 1 <f 2 ), the high pressure discharge lamp including a pair of electrodes disposed to face each other, the ballast comprising: control means for controlling a rate of a time period of the frequency component f 1 or f 2 to time periods of the frequency components f 1 and f 2 per unit time; output means for applying a synthesized-waveform current in accordance with the rate to the high pressure discharge lamp; and detection means for detecting a lamp voltage of the high pressure discharge lamp, wherein the control means is configured to shift the rate of the f 2 to R L % when the lamp voltage exceeds a first predetermined voltage value V, and to shift the rate of the f 2 to R H % (0≦R L <R H ≦100) when the lamp voltage falls below a second predetermined voltage value V′, and the control means is further configured to change the rate stepwise when the rate is shifted from R L % to R H %, or when the rate is shifted from R H % to R L %.