The invention relates to a method for optical measurement of an OPC structure (306), having a pre-determined structure (302) on a photo-mask, in order to determine a measurement of the structure in at least one direction, whereby, firstly, a region (300) is determined on the photo-mask, which comprises the OPC structure (306) to be measured. The intensity of the determined region (300) is then scanned in a first direction and the region in which the intensity passes a threshold is determined for each scan. The maximum separation between an edge (308) of the structure (302) and an edge (312) of the corresponding OPC structure (306) is determined, based on the difference of the determined regions.