A radiation source is configured to generate radiation. The radiation source includes a fuel droplet generator constructed and arranged to generate a stream of droplets of fuel that are directed to a plasma generation site; a laser constructed and arranged to generate a laser beam that is directed to the plasma generation site, an angle between the direction of movement of the stream of droplets and the direction of the laser beam being less than about 90°; and a collector constructed and arranged to collect radiation generated by a plasma formed at the plasma formation site when the beam of radiation and a droplet collide. The collector is configured to reflect the radiation substantially along an optical axis of the radiation source. The laser beam is directed to the plasma generation site through an aperture provided in the collector.