TABLE OF CONTENTS The high security foil microstructures described in the previous section can be characterised as belonging to a class of optically variable devices ideally suited to high volume banknote type applications. High volume means volumes greater than 40 million units; all formed using the same OVD image. For these application volumes the high origination costs of the master plate, typically of the order of US$40,000, can be amortised over the production run resulting in a unit foil price of much less than 1 US cent. For lower volume applications the Kinegram or Exelgram solution is much less cost effective and there is therefore a need for much lower cost solutions outside of the security compromised dot matrix hologram technologies. Three different approaches to this low volume and high security requirement are described.
One method for increasing flexibility, while retaining security through optical variability, involves the development of liquid media or inks containing microscopic flakes or pigment particles, which are in themselves individually optically variable.
The US company Flex Products has a long history in the development of products of this type; in particular products based on microscopic thin film flakes which change colour (e.g. from red to green) with changing angle of view due to the cancellation of particular wavelengths and the reinforcement of other wavelengths as the incoming white light is partially reflected and partially transmitted at the boundaries of different layers of different refractive index...
