X-ray Microscopy of Paper Currency Security Features

As Economic Impact Payments released by Congress in response to the current global pandemic begin rolling out across the country, we focused our imaging on paper currency and the many layers of security built into our paper notes. From special inks, textures, materials, and embedded components, the Treasury Department incorporates many security features to minimize counterfeiting attempts. Utilizing our SkyScan 1273 desktop micro-CT, we imaged a $100 US Note taking a virtual look within the bill while resolving some of the internal security features.

X-Ray Microscopy Imaging of Security Features Within Paper Currency

Our imaging of this $100 note demonstrates the capability to resolve small features within samples based on differences in X-ray attenuation through the sample. This work demonstrates that even high-aspect ratio samples can be effectively imaged using our desktop systems. As can be seen in Figure 1 above, the inks jump out of the image from the background signal provided by the cellulose fibers used in the paper the notes are printed on. Even among the inks used, different inks produce different signals based on their unique composition, which is highlighted in the image by differences in signal intensity. 

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Figure 2: Volumetric MIP renderings of watermark front (top) and back (bottom)

The first security feature we highlight is shown in Figure 2, the watermark portrait of Benjamin Franklin present at the edge of the note. While the printed image is present at the center of the note makes it visually invisible from the front and back edges of the note when viewed without sufficient backlighting, the ink used attenuates more X-ray signal than the cellulose comprising the bill and we can observe the watermark portrait from both sides of the note.

Figure 3: Volumetric MIP rendering of the embedded polymeric security thread within the note

In addition to the watermark, we also can identify the position of the embedded polymeric security thread within the note (Figure 3). In this view, we digitally sliced the edges of the note to focus on the thread itself. While writing is visible on the strip when viewing the note optically, the writing is not present within these images due to a lack of contrast between the polymeric thread and the organic pigments used to print onto the thread.

Figure 4: Volumetric MIP renderings of the raised printing texture on the front portrait of the note

Using enhanced intaglio printing, the Treasury creates a raised texture for some inks to provide tactile validation of the integrity of the document1. One area where this is especially prominent is the left side of the bill within Benjamin Franklin’s shoulder, as shown in two views in Figure 4. When looking at a view through the note, the raised ink features are clearly visible extending from the surface of the note.


Figure 5: Volumetric MIP rendering of the damage in the print integrity arising from repeated creasing

While the rest of the article focused on examining security features within notes, we also observed the effects on the ink of repeated creasing of the note along the horizontal midline resulting in degradation of the ink (Figure 5). This degradation can be seen through the cracks and gaps present in the printed image surface visible within the MIP rendering. While these notes are designed to be durable for around 15 years, normal damage occurs during routine circulation1.

Conclusion

The high-resolution micro-CT imaging with the SkyScan 1273 is well suited for this application studying non-destructive imaging of currency. We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our image of the month series next month.

Scan Specifications

Sample $100 Note
Voltage (kV) 40
Current (µA) 300
Pixel Size (µm) 60
Rotation Step 0.3
Scan Time (HH:MM:SS) 01:33:12

This scan was completed on our desktop SkyScan 1273 system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using Dataviewer and CTVox.

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com.

References

1      https://www.uscurrency.gov/denominations/100

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