A method for generating light is provided. The method further includes measuring a period of time during which one of targets from a fuel target generator passes through two detection positions. The method also includes exciting the targets with a laser generator so as to generate plasma that emits light. In addition, the method includes adjusting at least one parameter of the laser generator according to the measured period of time, when the measured period of time is different from a predetermined value, wherein the parameter of the laser generator which is adjusted according to the measured period of time includes a frequency for generating a laser for illuminating the targets.