Disclosed herein are optical devices, suitable for use as authentication devices for security items and documents including banknotes. The devices are generally static and yet in some embodiments have perceived dynamic optical properties due to their apparent capacity to cause output luminescent radiation to “flash” as the devices are progressively tilted relative to a user or detector of the emitted luminescent radiation. Also disclosed are items and documents comprising such devices, methods for manufacture and use of the devices, and authentication devices suitable to check whether the disclosed authentication device are legitimate or counterfeit.