A method of determining a unique identifier for a security element 20, the method comprising: optically reading the security element, where the security element comprising a number of emitters 22 that are each capable of exhibiting a different emission response to excitation; the reading comprising determining data indicative of an optical property of the security element using first emission electromagnetic (EM) radiation, emitted in response to a first excitation, and similarly determining data using second emission electromagnetic radiation, emitted in response to second, different, excitation. The unique identifier is determined from a map of a variation in determined data indicative of an optical property with respect to the excitation of the number of emitters. The excitations could be undertaken electrically or using electromagnetic radiation (76) at different intensities. The data indicative of an optical property could include the emitted spectrum and/or a physical location of an element. The security element could comprise components exhibiting quantum mechanical confinement.