Abstract: An apparatus and method pertaining to an optical grating are disclosed herein. In one embodiment, in response to a light incident on the optical grating, a first component of the light at a first wavelength is selectively reflected while a second component of the light at a second wavelength is selectively rejected. A reflectance efficiency corresponding to the selective reflection of the first component of the light being a function of a surface roughness of an intermediate layer included in the optical grating. And outputting the selective reflection of the first component of the light at the first wavelength to an optical component included in an extreme ultra violet (EUV) lithography system. The first wavelength being an EUV wavelength and the reflectance efficiency maximized at the first wavelength.