Systems are disclosed that include an interferometer configured to direct test light to an overlay test pad and subsequently combine it with reference light, the test and reference light being derived from a common source, one or more optics configured to direct at least a portion of the combined light to a multi-element detector so that different regions of the detector correspond to different illumination angles of the overlay test pad by the test light, the detector being configured to produce an interference signal based on the combined light, and an electronic processor in communication with the multi-element detector. The overlay test pad comprises a first patterned structure and a second patterned structure and the electronic processor is configured to determine information about the relative alignment between the first and second patterned structures based on the interference signal.