1. An optical device comprising a holographic diffractive structure comprising at least two discrete linear regions arranged to replay in response to white light illumination a visual holographic optically variable image comprising a plurality of visually discernable linear graphical elements corresponding to respective linear regions, wherein the graphical elements are located at or near the physical plane containing the holographic diffractive structure, and arranged to replay in response to coherent light illumination a plurality of secondary covert images corresponding to the respective linear regions wherein the secondary covert images are visible to an observer when imaged at a separate image plane away from the physical plane containing the holographic diffractive structure, the plurality of secondary covert images being arranged to be non-visible under white light illumination, and wherein the secondary covert images are reconstructed at different positions at their image plane.