According to a first aspect of the present invention, there is provided a method of determining a unique identifier for a security element, the method comprising: optically reading the security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system, and determining data indicative of an optical property of the security element at a second, different, configuration of the filter system; and the unique identifier being determined from a map of the variation in determined data indicative of an optical property with respect to the configuration of the filter system, wherein the reading is undertaken for multiple locations across the security element at the or each configuration of the filter system, such that the map is a map of the variation in determined data indicative of an optical property across the security element with respect to the configuration of the filter system, and wherein the reading for multiple locations across the security element is undertaken in a single reading step, using a reader with a two-dimensional sensor.