A radiation source includes a fuel supply configured to deliver fuel to a plasma emission location for vaporization by a laser beam to form a plasma, and a collector configured to collect EUV radiation emitted by the plasma and direct the EUV radiation towards an intermediate focus. The collector includes a diffraction grating configured to diffract infrared radiation emitted by the plasma. The radiation source includes a radiation conduit located in between the collector and the intermediate focus. The radiation conduit includes an entrance aperture connected by an inwardly tapering body to an exit aperture. The radiation conduit includes an inner portion and an outer portion, the inner portion being closer to the intermediate focus than the outer portion. The inner portion is configured to reflect incident diffracted infrared radiation towards the outer portion.