Holographic or diffraction device

摘要

A holographic or diffraction device responsive to interrogation, the device comprising an image-former and an image-concealer; wherein the image-former is adapted such that light reflected from or transmitted through it forms a combination of overt and covert holographic images; wherein the image-concealer acts to attenuate the overt holographic image of the combination of images, making the overt holographic image become not visible and a covert holographic image become visible, and wherein the attenuation of the overt holographic image by the image-concealer is alterable responsive to interrogation.