Patent ID: 8203280

Claim:
A light source device comprising: a discharge lamp that emits light by discharge between a first electrode and a second electrode; and a driver that supplies alternating current to the first and the second electrodes so as to maintain the discharge, and changes a duty ratio of the alternating current in accordance with a predetermined pattern, the predetermined pattern including a plurality of section periods for which the duty ratio maintains mutually different values for a predetermined period, wherein a difference of the duty ratio between a first section period among the plurality of section periods and a second section period among the plurality of section periods following the first section period is greater than a predetermined amount; the duty ratio in the first section period and the duty ratio in the second section period are changed so as to extend across a reference duty ratio determined in accordance with a median value of a changing range of the duty ratio; a length of the first section period and a length of the second section period are mutually different; the predetermined pattern is a pattern that cyclically changes; in a specified period within one cycle of the pattern, a section period for which the duty ratio is higher than the reference duty ratio is longer than a section period for which the duty ratio is lower than the reference duty ratio; and in a remaining period within the one cycle of the pattern, a section period for which the duty ratio is higher than the reference duty ratio is shorter than a section period for which the duty ratio is lower than the reference duty ratio.