A method for forming a surface-relief grating with a desired spatial variation of duty cycle in a layer of photoresist includes: providing a first mask bearing a high-resolution grating of linear features, arranging the first mask at a first distance from a substrate, providing a second mask bearing a variable-transmission grating of opaque and transparent linear features that has a designed spatial variation of duty cycle, arranging the second mask at a distance before the first mask such that the linear features of the variable-transmission grating are orthogonal to the linear features of the high-resolution grating, illuminating the second mask while varying the first distance according to displacement Talbot lithography and also displacing the second mask at an angle to its linear features such that there is substantially no component of modulation with the period of the variable-transmission grating in the energy density distribution that exposes the photoresist.