Spectroscopy’s New Role in Document Security
It is estimated that over $1 billion was lost due to fraudulent checks in the U.S. in 2008.
Even in today’s digital world, some of our most valuable documents are still printed on paper. Think of banknotes, passports and even professional licenses – all high value and at risk for counterfeiting or tampering and forgery. The accessibility of advanced printers, inks and graphic design programs to the average consumer has made counterfeiting much easier in recent decades. In fact, the percentage of counterfeit U.S. bills coming from ink jet printers grew from 1% to 60% between 1995 and 20081.
The security printing industry has responded to the challenge by developing a multitude of new anti-counterfeiting and protection techniques, many of which are based on incorporation of novel optical features enabled by spectroscopy. Ocean Optics modular and portable spectroscopy systems stand to play a key role in the development and authentication of these new countermeasures.
High value documents
- Currency (banknotes)
- Checks and money orders
- Stock and bond certificates
- Event tickets
- Tamper-proof labels
- ID cards and licenses
- Birth certificates
- Prescription pads
Security Printing – Much More Than Meets the Eye
Well over 50 specific techniques are now in use to facilitate authentication of printed documents and deter tampering, ranging from overt features like embossed patterns to covert features perceptible only to specialized equipment. The higher the document value, the more features are used concurrently to provide layers of protection2.
“Impressive” Early Anti-counterfeiting Measure
Colonial bills printed by Benjamin Franklin’s firm in 1739 had raised impressions from leaves – unique and difficult to reproduce.
Security Printing Inks:
- magnetic inks
- penetrating inks
- photochromic inks
- metameric inks (color dependent on lighting)
- luminescent and iridescent (optically variable) inks
- fugitive inks (soluble in water or certain solvents)
- thermochromatic ink (affected by heat or rubbing)
- chemically sensitive ink
Specialized Printing Methods:
- impressed or printed watermarks
- intaglio (etched printing)
- prismatic printing
- registration of features on both sides
- paper toner fusion
- pressure-release scents
- chemically reactive paper
- polymer “paper” with transparent windows
- use of colored or UV fluorescent security threads
Security Patterns and Elements:
- guilloches (fine-line printing)
- serial numbers
- void pantographs or verification grids
- latent images
- inclusion of biometric information
- scratch-off labels
- metallic foil strips or logos
- custom barcodes
- needle perforation
- optical stripes (laser-readable memory devices)
- optically variable devices (OVDs)
Spectrally Unique Pigments and Inks Stand Out
Inks and pigments with optically unique properties can provide quick visual verification of authenticity, and can be difficult to replicate. These range from iridescent or color-shifting inks containing tiny flakes of mica to cause a change in color with viewing angle to inks that are poorly read by the illumination sources in traditional scanners used for copying3.
Fluorescent inks are often used to overlay documents with special words or images that appear only when illuminated by UV or light of another specific wavelength. Fluorescent pigments may also appear as fibers incorporated into the paper itself. In fact, researchers at Northwestern University have developed tunable fluorescent inks in which the color of emission can be finely tuned by altering the chemical composition of the ink, allowing it to act as an optical signature that is extremely difficult to replicate4. With multiple emission peaks, the only way to identify these tunable inks in the field would be using a compact, portable spectroscopy system.
To test this, we quickly assembled an Ocean Optics system to look at fluorescence from the European Union flag security feature on a 20 Euro note. When viewed with UV light, fluorescence from the security feature should cause the blue background on the flag to appear green, while the yellow stars should appear orange.
Our modular fluorescence system consisted of a QR600-7-VIS-NIR reflection probe, an LLS-LED-385 light source for UV excitation and a FLAME-S-UV-VIS-ES spectrometer for detection. With the probe pointed at the flag, a broad fluorescence peak was seen spanning ~490-560 nm, corresponding to the expected green color for the background when viewed with UV light. Fluorescence from the stars was visible in the same spectrum as a strong peak from 610-620 nm.
While a counterfeit bill might be able to replicate this feature accurately enough to fool the eye at a glance, a full spectral measurement compared against an authentic Euro fluorescence spectrum could very quickly discern a fake.
Spectroscopy also offers other opportunities for definitive authentication of fluorescent inks, even when fairly successfully employed by counterfeiters. This is exemplified in work from Yale University on intrinsic fluorescent lifetime measurements of U.S. Federal Reserve notes against counterfeit samples5. They measured the intrinsic fluorescence of bills using a USB2000 spectrometer connected to a 2-photon microscope, followed by fluorescence lifetime measurements with a multichannel plate PMT to find significant differences in the fluorescence lifetime(s) of inks on counterfeit bills versus the genuine issue.
Upconverting nanoparticles and nanocrystals are another hot new topic in document security, as advances in synthesis techniques are resulting in better control over their optical properties, manufacturing and durability6. Upconverting inks would allow infrared light to be used for illumination and detection using spectrophotometric systems operating at visible wavelengths (see inset). Optical tags based on Raman, SERS (Surface Enhanced Raman Spectroscopy) and other micro- or nanoparticles also show promise in this area.
Looking Twice at Optically Variable Devices
Have you seen an image on a passport or other form of identification change color, pattern or appear to move when viewed from a different angle? If so, you were probably looking at an optically variable device (OVD). OVDs are near-impossible to replicate or reproduce, thanks to the use of complex diffractive elements or interference films. Their appearance can vary with angle of illumination, rotation, or with the amount of incident light. Holograms are a type of OVD, as are the moving images that were used on the tickets to Barack Obama’s presidential inauguration in 20097. Even surface plasmon resonance is being used to create OVDs.
Though most OVDs are designed to provide immediate visual authentication of documents, Ocean Optics spectroscopy systems play a key role in their development and characterization, as can be seen upon reviewing patents in the area. As counterfeiters begin to attempt replication of OVDs, field-portable systems from Ocean Optics are likely to be a critical tool in authentication and fraud prevention.
Taking Spectroscopy to the Page – Forensic Document Authentication
Complex security features now protect many of the typical high-value documents like passports and banknotes, but everyday printed documents can still be routinely forged, copied or altered. When this happens, authentication depends on forensic analysis to determine origin and age of the document or features therein. In addition to microscopic evaluation, the chemical composition of the ink(s) used is often determined using chromatographic analytical techniques like TLC, HPTLC, GC-MS and HPLC, generally at the expense of preserving the document.
Spectroscopy offers an effective and nondestructive alternative, as demonstrated by a team at the Slovak University of Technology in Bratislava8. The team used Vis-NIR reflectance spectroscopy to perform principal component analysis (PCA) on the reflectance spectra of prints from different inkjet printers. Their system included an HR4000-CG spectrometer, DH-2000-BAL UV-Vis-NIR light source and standard reflection/backscattering probe in a 45° holder to measure samples from 450-1000 nm. Blank paper was used as a reference to subtract background effects like paper inhomogeneity.
In comparing spectra from various models of Canon and Epson printers, differences in the spectral range 600-1050 nm allowed inks from different brands and models within the same brand to be uniquely characterized using a three-component PCA model. Measurement of a print obtained from an Epson SX420 inkjet printer could then be assigned correctly to the appropriate group, indicating that it should be possible to correlate individual marks of inks to their printing origin using reflectance spectroscopy.
The power in this conclusion is that an entire document could be quickly and nondestructively mapped for consistency and printing origin using a portable Vis-NIR spectrometer, allowing evidence of alteration to be uncovered, and comparisons in print origin to other documents or pages to be made. Other optical techniques like LIBS (laser induced breakdown spectroscopy)9 and Raman spectroscopy10 are also being used in this area of forensics for nondestructive authentication of documents, often using Ocean Optics systems.
Riding the Wave(length) in Document Authentication
Optical security features and analysis techniques are an increasing part of safeguarding and evaluating document security. Compact spectroscopy systems are an important part of every step in that process, from development to analysis in the field. As optical security features become even more complex, Ocean Optics systems will surely play a key role in the development of more sophisticated authentication methods. Our technologies can be adapted for integration into robust OEM analysis systems and complemented by user-friendly pass/fail software to help professionals in this burgeoning field that protects our economic systems, our possessions and even our identities.
1 “Counterfeit Cash – Fake Money: Stats Infographic.” Money Quotes Daily.
2 Van Renesse, Rudolf L., ed. Optical document security. Artech House Publishers, 1998.
3 Optical Document Security Conference (www.opticaldocumentsecurity.com), 2014 programme
4 Hou, Xisen, et al. “Tunable solid-state fluorescent materials for supramolecular encryption.” Nature Communications 6 (2015).
5 Chia, Thomas H., and Michael J. Levene. “Detection of counterfeit US paper money using intrinsic fluorescence lifetime.” Optics Express 17.24 (2009): 22054-22061.
6 Vennerberg, Danny, and Zhiqun Lin. “Upconversion nanocrystals: synthesis, properties, assembly and applications.” Science of Advanced Materials 3.1 (2011): 26-40.
7 “Bringing a New Business Into Fold.” WSJ. N.p., n.d. Web. 21 July 2015.
8 Gál, L., et al. “Principal component analysis of VIS-NIR reflectance spectra as the tool for resolution and identification of inkjet inks for forensic analysis.”
9 Lennard, Chris, Moteaa M. El-Deftar, and James Robertson. “Forensic Application of Laser-Induced Breakdown Spectroscopy for the Discrimination of Questioned Documents.” Forensic Science International (2015).
10 Braz, André, Maria López-López, and Carmen García-Ruiz. “Raman spectroscopy for forensic analysis of inks in questioned documents.” Forensic Science International 232.1 (2013): 206-212.