Disclosed is a system configured to project a beam of radiation onto a target portion of a substrate within a lithographic apparatus. The system includes a radiation source. The radiation source includes a grating structure operable to suppress the zeroth order of reflected radiation for at least a first component wavelength. The grating structure has a periodic profile including regularly spaced structures providing three surface levels, such that radiation diffracted by the grating structure includes radiation of three phases which destructively interfere for at least the zeroth order of the reflected radiation for the first component wavelength. The grating structure is on a radiation collector within the source.