1. A method comprising: forming a target structure on a substrate, the target structure including a periodic structure; detecting an image of the target structure while illuminating the target structure with a beam of radiation, the image being formed using a first part of non-zero order diffracted radiation while excluding zero order diffracted radiation; and determining a property of the periodic structure using an intensity value extracted from a region of interest within the image of the target structure, wherein the region of interest is identified in the detected image by recognizing locations of a plurality of boundary features in the image of the target structure and calculating, from the recognized locations, features a location of the region of interest, and wherein, in at least one direction, the number of the boundary features is at least twice a number of boundaries of periodic structures within the target structure.