A measurement apparatus including an optical system to provide illumination radiation into a spot on a periodic structure and to receive radiation redirected by the periodic structure, the optical system including a first stop to block zero order radiation from the periodic structure and allow non-zero order radiation to pass, and a second stop to block zero order radiation passing the first stop and to allow the non-zero order radiation to pass, and a radiation detector, downstream of the optical system, to receive the non-zero order radiation.