A light source error detection system and method including monitoring a plurality of characteristics of a light source and controlling the light source based on a plurality of monitored results. The system and method control the light source differently for a first case when only one of the monitored characteristics is abnormal than a second case when more than two of the monitored characteristics are abnormal. In controlling the light source, an output power of the light source is decreased with different time constant for the first case and the second case.