Abstract:
Secure documents and banknotes use multiple visual authentication indicators. Among them, 'colour' and 'shape' are visually easy to recognize but are the easiest ones to duplicate or simulate via printing or embossing. Adding a ‘movement’ feature typically makes it more secure, but requires either tilting or turning it by the user to see the specific effect, which makes it less user-friendly. For these reasons, ‘depth’ is arguably the most formidable visual effect to be employed.