| 摘要 |
An optical device for security and anti-counterfeit applications comprises a holographic diffractive structure (8) that generates a holographic optically variable image by diffraction that consists of at least two separate substantially co-located linear regions that generate in response to white light illumination a visually observable holographic image consisting of at least two defined graphical elements and which generates in response to coherent illumination (1) at least two covert images (5, 6, 7) focussed at a separate image plane a distance away from the real physical plane of the device, each covert image (5, 6, 7) corresponding to the replay from one of the linear regions, each covert image (5, 6, 7) being separated on its image plane from the adjacent covert image (5, 6, 7) by at least its own dimensions.
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