A radiation system is configured tp generate a radiation beam. The radiation system comprising a chamber that includes a radiation source configured to generate radiation, a radiation beam emission aperture, and a radiation collector configured to collect radiation generated by the source, and to transmit the collected radiation to the radiation beam emission aperture. The radiation collector includes a spectral purity filter configured to enhance a spectral purity of the radiation to be emitted via the aperture. The spectral purity filter comprising: a base substrate; a multilayer stack on the base substrate, the multilayer stack comprising a plurality of alternating layers; and a plurality of recesses in a top side of the multilayer stack, the recesses being configured to allow the radiation having the first wavelength to be reflected in the first direction and to reflect the radiation having the second wavelength in the second direction; wherein the recesses are configured such that, in cross-section, they have a symmetric profile.