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Note: All the content in this month’s article is publicly accessible. The Federal Reserve is not shy about its methods to ward off counterfeiters.

The $100 bill, the highest denomination of U.S. currency in general circulation, is a marvel of modern security printing. As one of the most counterfeited notes globally, it incorporates a complex array of anti-counterfeiting features that blend cutting-edge technology with traditional techniques.

The evolution of the $100 bill exemplifies the ongoing arms race between currency designers and counterfeiters. The most recent redesign, introduced in 2013, marks a significant leap forward from its predecessor. While the iconic portrait of Benjamin Franklin remains, the new bill boasts enhanced security features that were absent or less sophisticated in the older version. These updates include a 3D security ribbon, color-shifting ink that changes from copper to green, and a watermark visible from both sides of the note.

This month’s Tech Corner article will delve into the scientific principles and innovative techniques behind some of these security measures, both old and new. We'll examine the materials science, optics and advanced printing methods that make these features possible, and understand how they work together to create a banknote that's increasingly difficult to replicate illegally.

3D Security Ribbon

The 3D security ribbon is one of the most advanced and visually striking anti-counterfeiting features on the new $100 bill. This blue ribbon, woven vertically into the paper rather than printed on its surface, contains microscopic lenses. These lenses create a remarkable visual effect: As the bill is tilted, the images on the thread appear to move with a holographic effect. Specifically, the ribbon showcases alternating images of bells and the number "100" that shift and change as the viewing angle changes. This movement is the result of moiré patterns created by the interaction between the micro-lenses and the printed images beneath them. The complexity of this feature lies not just in its visual effect, but in the precision manufacturing required to create and embed the ribbon. The ribbon is composed of highly specialized micro-optics and cannot be replicated by traditional printing methods, making it exceptionally difficult for counterfeiters to recreate. This blend of visual appeal and technical sophistication makes the 3D security ribbon a cornerstone of the modern $100 bill's anti-counterfeiting measures.

Color Shifting Ink  

Color-shifting ink represents a triumph of materials science in currency design. This optically variable ink, applied to the numeral 100 in the lower right corner of the bill, appears to change color from copper to green when the note is tilted. The effect relies on specially formulated inks containing metallic flakes or liquid crystals. These microscopic structures selectively reflect different wavelengths of light depending on the viewing angle, creating the illusion of a color change. The precise composition of the ink is a closely guarded secret, involving complex chemical formulations and production processes. What makes this feature particularly effective against counterfeiting is that it cannot be replicated by standard printing techniques or photocopying. The color shift is not just a visual trick, but a result of the physical properties of the ink itself, requiring sophisticated technology to produce. Image courtesy of US Currency Education Program

Bell in the Inkwell

The bell in the inkwell is a security feature that combines intricate design with color-changing ink. Located on the front of the $100 bill, this element depicts a copper-colored inkwell containing the Liberty Bell. When the bill is tilted, the bell changes color from copper to green, creating a distinctive visual effect. This color-shifting property utilizes similar technology to the “100” printed in the bottom left hand corner, but with an added layer of complexity in its design. The bell appears to emerge from the inkwell, giving a subtle 3D effect that further enhances its security value. This feature employs a combination of intaglio printing for the fine details of the inkwell and specialized color-shifting ink for the bell. The intricate artwork and precise registration required to align these elements make it extremely challenging to replicate accurately. Together the raised printing and color changing ink encourage closer inspection by users, thereby increasing the likelihood of detecting counterfeit notes.

Benjamin Franklin Watermark

Image courtesy of US Currency Education Program

Security Thread

Images courtesy of US Currency Education Program

All U.S. currency denominations, with the exception of $1 and $2 notes, have a security thread woven into the paper of the bill. On the $100 note, this thread is positioned to the left of Benjamin Franklin's portrait. The thread is embedded within the paper rather than printed on the surface, making it impossible to replicate through standard printing or photocopying. When viewed normally, the thread appears as a thin dark line. However, when the bill is held up to light, the thread becomes clearly visible as a solid line and reveals the text "USA 100" repeated along its length. This text is printed in such fine detail that it's difficult to reproduce accurately. The thread is also fluorescent, appearing as a bright pink line when exposed to ultraviolet light. (You can see the colors and positions of the security thread for each denomination in the above image). This combination of visibility under different lighting conditions, microprinting and fluorescence makes the security thread a powerful tool in authenticating the bill.

Currency Paper 

Image courtesy of WGBH Educational Foundation / PBS

The science behind the specialized paper used for $100 bills is a blend of materials engineering and security design. The 75% cotton and 25% linen composition is carefully chosen for its durability, distinctive texture, and optimal ink interaction. This blend creates a robust, tear-resistant material that withstands repeated folding while providing a unique tactile sensation due to the microscopic characteristics of the natural fibers. The paper's specific porosity ensures ideal absorption and adhesion of specialized inks, crucial for the longevity of printed elements and the effectiveness of security features. The integration of red and blue security fibers adds further complexity. These fibers, incorporated during manufacturing, become an integral part of the paper's structure, randomly distributed throughout the sheet. They possess specific optical properties, being visible under normal light and reactive to certain wavelengths, including ultraviolet. Chemically treated to resist degradation and alteration attempts, these fibers contribute to the paper's durability and security. This combination of material science, manufacturing techniques, and security design results in a substrate that is both suitable for everyday use and highly resistant to counterfeiting attempts.

Conclusion

From the color-shifting ink and 3D security ribbon to the intricate watermarks and embedded security thread, each feature on the $100 note represents a triumph of technological innovation. These elements not only create formidable obstacles for counterfeiters, but also empower citizens to easily authenticate currency. The evolution from older to newer versions of the bill showcases the ongoing technological arms race in financial security. As we've showcased, understanding the science behind these measures provides a deeper appreciation for the complexity of modern currency and highlights the critical intersection of technology and finance in our daily lives. The humble $100 bill, with its layers of hidden complexity, serves as a powerful reminder of human ingenuity and the relentless pursuit of security in our financial systems, even in an increasingly digital world.

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