Take a close look at one of the newest Canadian $5 bills. If you hold it against a strong light you will see it contains a narrow strip of plastic that is embedded within the paper. This strip carries a special optical thin film coating that is exposed on the back of the bank note only in small rectangular patches where it touches the surface of the paper. The strip changes colour from gold to green when viewed at an increasing angle and it is inscribed with a repeating message "CAN 5."

This unique colour-changing strip, now common on all Canadian currency, is designed to foil counterfeiters. It cannot be reproduced through normal printing or photographic processes, nor can it be peeled away because it is not stuck onto the surface.

George Dobrowolski displays Canadian $50 bills with NRC-developed optical thin-film security features.

The strip is an optical security device – a state-of-the-art anti-counterfeiting feature that incorporates a principle developed by the National Research Council for the protection of Canada's money.

Putting a stop to bogus bills

In 1968, the Bank of Canada was worried that counterfeiters would soon be armed with high-resolution photocopiers, printers and computer production techniques. It approached NRC for help in making it harder for criminals to produce fake Canadian money. The NRC suggested incorporating optical thin films into bank notes that would change colour with the angle of viewing.

NRC researchers had been working on new thin film optical filters, and the research of physicist George Dobrowolski and his team on thin-film multilayer devices was attracting a great deal of interest. But the technology for currency protection did not come quickly. It took 10 years of work, but eventually it was ready to be used for security applications. In the late 1980s, Toronto company Identicard Ltd. licensed the technology to make fool-proof ID cards and driver's licenses.

The beauty of the technology was that it was relatively inexpensive for the Bank of Canada to produce the security patches for its bills. But because the machines required for their cost-effective manufacture required millions of dollars to build, counterfeiters simply did not have the resources to build their own.

As NRC technology continued to evolve, so did the security protection built into Canada's bank notes. In the second generation of the optical security devices, the colour-changing material is introduced not onto the surface, but into the bank note paper itself in the form of a thin, coated plastic thread. The Bank of Canada continues to upgrade its security features to make it even tougher to produce counterfeit Canadian bills.

Globally-recognized expertise

Today, researchers at the NRC Institute for Microstructural Sciences continue to develop their world-renowned design and fabrication methods for multilayer coatings for various applications. In addition to anti-counterfeiting, thin films are used in scientific instruments, solar cells, computers, visual displays, architectural and automotive glass and telecommunication devices. NRC holds many patents on technology related to optical thin films.

Optical thin-film technology also helps protect identification cards and driver's licenses.

For his contributions to the field of optical engineering, George Dobrowolski received two medals from the Optical Society of America and was named a Member of the Order of Canada.

Although they can be quite attractive, the optical security devices on bank notes are much more than mere decoration. Thanks to NRC's pioneering optical thin-film research, optically-variable devices based on thin films can be found protecting passports, bank notes and other documents all over the world.