A radiation source apparatus includes a vessel, a laser source, a collector, a horizontal obscuration bar, and a reflective mirror. The vessel has an exit aperture. The laser source is configured to emit a laser beam to excite a target material to form a plasma. The collector is disposed in the vessel and configured to collect a radiation emitted by the plasma and to reflect the collected radiation to the exit aperture of the vessel. The horizontal obscuration bar extends from a sidewall of the vessel at least to a position between the laser source and the exit aperture of the vessel. The reflective mirror is in the vessel and connected to the horizontal obscuration bar.