Method for the Characterisationof Grating Based Holographic Paper
Abstract: Grating based holographic images and patterns are widely used in packaging to catch the eye of the consumer. The characteristic rainbow-like reflection of light from holographic papers can be used either by itself or with a print to create interesting contrasts and optical effects. Holographic patterns are also used for purposes of authentication to protect products and documentsfrom counterfeiting. A grating based holographic paper consists of separate areas of reflective diffraction gratings, which disperse polychromatic light into its constituent monochromatic components depending on both illumination and observation angles, and the physical structure of the grating. Due to the angle-dependent nature of the holographic patterns, conventional measurement methods for paper products cannot be used. As of now, quality control is mainly done through visual inspection and comparison using reference samples, and occasionally with light optical microscope (LOM). Thesubjective method is unreliable as it is difficult to compare and verify results over time, while the LOM method is very slow and ineffective. An objective measurement method which is robust and fast is thus highly desirable. In this work we propose a method for fast characterisation of large area holographic images using a commercial flatbed scanner. Analysis of scanners with different optical designs has been conducted. A model that describes propagation of dispersed lighti nside the scanner and its impact on image acquisition is proposed and verified. The model is then used to extract data about the holographic paper such as grating orientation, periodicity anddistribution.
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