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Defense & Security

Patent review: In the money--
Optical security for the world's currencies

Keeping money secure from counterfeiting requires constantly developing new techniques to stay ahead of the criminals.
14 March 2008, SPIE Newsroom. DOI: 10.1117/2.2200803.0001

Almost as soon as banks and governments printed paper money, people tried to counterfeit it, and it didn't take long for those issuing currency to seek ways to distinguish it from imitation. Because money needs to be quickly and easily authenticated wherever it is exchanged, real money needs to be at least partly distinguishable by the human eye. As a result, most security methods for paper currency are optical.

U.S. Patent 17,473, issued in 1857 and titled "Means Of Preventing Counterfeiting Bank Notes &c.," may be the earliest iteration of an optical method of manufacturing secure paper money. It involved using paper tinted to absorb light, and printing ink that also absorbed light rather than reflecting it so that clear photographic copies could not be made.

Since then, thousands of methods have been invented to manufacture secure currency -- approximately 1,200 in the U.S .patent office alone since 1975. In the current catalog (IPC8), the International Class Code B41M00314, listed as "Security Printing," contains over 700 entries in the U.S., EP, and PCT offices. These use magnetic properties, electrical conductivity, thermochromic inks, IR-responsive or luminescent substances, x-ray absorption, optically variable effects, or inclusions such as thread, tape, or other shapes of metal or plastic foils.

The majority of these methods are optical: thermochromic inks that change color (wavelength reflection), luminescent or fluorescent inks that emit light in response to the right stimulus, and even inclusions that are usually interrogated by the reflection and transmission of light. The most interesting of these, however, are the optically variable effects.

"Optically variable" papers show different characteristics in different light, or at different angles of light incidence. Many show a color shift or color change. Others use multiple sets of coatings to produce a holographic effect.

The United States $20 bill that came out in 2003 (the $20 bill is the most counterfeited bill in the U.S.) contains a security thread, an embedded watermark, high-detail engraving, special background colors, and optically variable pigment (OVP) for a corner printing of "20" that shifts from black to green. (The OVP comes from Flex Products, Inc. in California, a company with a notable presence in secure currency patenting. Much of their patenting involves special pigments.)

A new $5 bill was released in March 2008 with many of these same features. While originally there were no plans to introduce a more-secure five, counterfeiters were taking advantage of the location of the security strip in the older bills, bleaching them and turning them into counterfeit 20s. In the new five, the security strip is in a different location.

A new $100 bill design, planned for release late in 2008, will incorporate a new security thread from Crane & Co. that uses micro-lenses to shift an image according to the viewing angle. Crane has applied for patents in both the U.S. and World (PCT) offices. The patents, US20070273143, and WO2007133613, share the title "Micro-Optic Film Structure That Alone Or Together With A Security Document Or Label Projects Images Spatially Coordinated With Static Images And/Or Other Projected Images."


The patent describes micro-lenses on a printed plastic film. The micro-lenses cause sets of images to appear to float above the surface of the film, and are designed to interact with each other, or with images printed on the paper, completing or canceling other images. This micro-lens film can be included in the paper of the currency as security threads, or as a variety of shaped inclusions.

But security for the $100 bill certainly won't stop there! Other technologies will play a part as well, most of which will be optical in nature. The trend is toward the combination of multiple mechanisms. Combined effects are of course much more complex - and much more difficult to reproduce. While the government is likely to use technologies from multiple sources, many of the leading "Secure Currency" companies are patenting combined methods, particularly optically variable effects.

A recent World filing, 2008003949/WO-A1 "Security Device," just published this January, provides a good example of optically variable effects. It involves a transparent area with one or more optically variable security effects. It was filed by De La Rue International, Ltd., a leading company in the field of secure currency.

The optically variable effects that appear in the transparent window under one set of lighting conditions and disappear or appear differently under others, can include shifting images and/or colors. The dependent claims of the patent provide for single or combined effects that can occupy different areas of the transparent region, or be combined in the same space. They can be produced by diffraction gratings, by metalized prismatic facets, or by diffractive or holographic devices. The patent covers the use of a single optical mechanism or multiple mechanisms, separate or combined.

Many recent patents from the best-known currency manufacturers involve a subtle interplay of multiple factors. An excellent example of this is US 7,316,422, "Safety element and a valuable object", granted January 8 of this year (2008) to Giesecke & Devrient GmbH. Giesecke & Devrient are inarguably the heaviest patenters in the field of secure currency in the major offices - that is, in the PCT (World), EP (European), and US.


This patent combines color shift over temperature with color shift over angle of incidence as well. It also involves a metallic layer with measurable electrical properties - that would also block the transmission of light. The patent combines at least four methods of protection. Among the dependent claims, most of which provide greater detail on the appearance of the effects, one claim (Claim 13) also provides for the additional properties of magnetism or luminescence to make the product more machine testable.

This patent demonstrates the current trend in secure currency. Multiple types of effects are already the norm, with magnetic inks side by side with optically variable inclusions, and other types of effects as well. But the newest patenting, and the newest solutions, will contain interplays between multiple effects that will provide authentication that cannot be detected even through careful investigation without prior knowledge of the mechanisms. In other words, if examiners do not know at just what angle to hold the bill, or what light to use, and just what to look for, they will not be able to see the effects. This is the frontier of optical solutions for safe and secure paper money.

Margaret Fiore

Nerac Analyst Margaret Fiore, a member of the Nerac intellectual property group, is particularly experienced in patent investigations, specializing in reporting on topics related to optics, lasers, imaging, aerospace, military projects and contracting, and telecommunications. She brings Nerac clients a strong interest in many scientific disciplines, enabling her to be highly effective in researching unusual topics involving multiple fields of study. Ms. Fiore has 18 years of industry experience in electronic aviation controls with United Technologies Corp., and in fiber-optic transmissions of analog and digital video with the Broadband division of ADC Telecommunications. She was a technical writer and engineering liaison for telecommunications and aerospace companies in Connecticut. Ms. Fiore, who holds a bachelor's degree in English and journalism from the University of Massachusetts, is also a certified electronics technician. She is expert in commercial and military technical documentation for aerospace, and Bellcore technical documentation for the telecommunications industry.

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