A method of manufacturing a reflective diffraction grating for specularly reflecting and diffracting a grazing-incidence beam of radiation incident on the grating , the grating having a periodic structure with a grating period comprising first and second substructures either side of a sidewall facing the incident beam , wherein the first substructure comprises a ridge and the second substructure comprises a trench , and that the ridge comprises a flat top and the trench comprises a flat floor parallel to the flat top of the ridge, wherein the method comprising the steps: - determining a configuration of the first substructure to specularly reflect the beam , incident at a grazing angle of incidence, with respect to the grating's direction of periodicity, on grating periods including the second substructures, from the first substructure into a specularly reflected beam in a zeroth-order direction; - determining a fixed or varying pitch configuration of grating periods including the second substructure to diffract the beam from the grating periods in one or more non-zero-diffraction-order direction; - determining, based on the grazing angle of incidence, a configuration of the second substructure and the sidewall of its grating period such that any rays of the beam that reflect once from the second substructure into the zeroth-order direction are obscured by the sidewall ; and - manufacturing the grating using the determined configurations of the first and second substructures and sidewall. wherein the first substructure comprises a ridge and the second substructure comprises a trench , and that the ridge comprises a flat top and the trench comprises a flat floor parallel to the flat top of the ridge, wherein the method comprising the steps: - determining a configuration of the first substructure to specularly reflect the beam , incident at a grazing angle of incidence, with respect to the grating's direction of periodicity, on grating periods including the second substructures, from the first substructure into a specularly reflected beam in a zeroth-order direction; - determining a fixed or varying pitch configuration of grating periods including the second substructure to diffract the beam from the grating periods in one or more non-zero-diffraction-order direction; - determining, based on the grazing angle of incidence, a configuration of the second substructure and the sidewall of its grating period such that any rays of the beam that reflect once from the second substructure into the zeroth-order direction are obscured by the sidewall ; and - manufacturing the grating using the determined configurations of the first and second substructures and sidewall. - determining a configuration of the first substructure to specularly reflect the beam , incident at a grazing angle of incidence, with respect to the grating's direction of periodicity, on grating periods including the second substructures, from the first substructure into a specularly reflected beam in a zeroth-order direction; - determining a fixed or varying pitch configuration of grating periods including the second substructure to diffract the beam from the grating periods in one or more non-zero-diffraction-order direction; - determining, based on the grazing angle of incidence, a configuration of the second substructure and the sidewall of its grating period such that any rays of the beam that reflect once from the second substructure into the zeroth-order direction are obscured by the sidewall ; and - manufacturing the grating using the determined configurations of the first and second substructures and sidewall. <EOS>