Optically variable watermark (OVW) microstructures for transparent substrates |
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Authors: | Robert A Lee |
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Affiliation: | CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, Clayton South MDC, 3169, Victoria, Australia |
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Abstract: | We report on the novel fabrication of optically variable watermarks (OVW) in a transparent substrate. The OVW is incorporated into the substrate via an embossing process. The master embossing die is produced by a combination of electron beam lithography, greytone photolithography, and precision electroplating techniques. The micro-embossing into polypropylene film is performed at 130 °C and 100 kN, a similar pressure to that used in intaglio printing. The optical effect produced by the device corresponds to a switch from one image to another as the device is observed in transmission when held in front of a light source and moved up and down. The image switching mechanism is a result of the refraction of the incident light through the embossed interlaced rows of micro-prisms, with the two different images corresponding to opposite prism slopes. Specific results reported here include an OVW image switch from a “$” to a “50” symbol and a second test image corresponding to an image switch from a portrait to a logo style image. Applications of the OVW technology include anti-counterfeiting protection for polymer banknotes, identity cards with transparent regions and labels attached to brand name products. |
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Keywords: | Optically variable watermark Micro-embossing Security |
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