Optically variable watermark (OVW) microstructures for transparent substrates

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.