Apparatus for optical assessment of a sample includes a radiation source, adapted to generate a beam of coherent radiation, and traveling lens optics, adapted to focus the beam so as to generate first and second spots on a surface of the sample and to scan the spots together over the surface. The distance between the first and second spots is responsive to a pitch of a repetitive pattern of the sample. Collection optics are positioned to collect the radiation scattered from the first and second spots and to focus the collected radiation so as to generate an interference pattern. A detector detects a change in the interference pattern.