Alicia Mealha, Goyalab
Rodrigo Sánchez González, Axiom Optics
INTRODUCTION
Counterfeiting of banknotes and other documents has become increasingly prevalent, posing significant economic and security threats. The proliferation of counterfeit documents in circulation escalates annually, underscoring the urgent need for robust and effective security measures. Banknote and document security aims to restrict counterfeit documents, and consequently, their circulation. This is important to maintain public trust in banknotes and other documents.
While a quick visual inspection can sometimes identify counterfeits, employing spectroscopy tools offers a more reliable way of detecting counterfeit banknotes and passports. Specifically, a portable fluorescence spectrometer allows a quick and reliable analysis of security features, such as those only visible under UV light. Fluorescence spectroscopy has many advantages because it is simple, quick, and potentially portable. Furthermore, it has no impact on the integrity of the sample under study. There are several methods to incorporate a fluorescent substance into a banknote. The use of fluorescent ink, obtained by mixing a binder with a luminescent pigment, is commonly employed to print the security threads of banknotes. There can also be randomly dispersed fluorescent fibers in the paper, which are produced using a polymer that fixes the fluorophores. Another option is the incorporation of luminescent grains mixed with the paper. The choice of a luminescent pigment is crucial since it must be durable over time. It must also have a relatively small difference between the excitation and emission wavelengths to facilitate the measurement, it must be difficult to obtain on the market, and hard to replicate. To bolster security, various elements such as watermarks, holograms, security threads, iridescent stripes, color-shifting ink, and intricate printing patterns are also integrated into these documents.
We can classify banknote security measures into three levels. The first level comprises elements visible to the naked eye and known to the public, such as the shape and texture of the banknote, the presence of intaglio printing, a watermark, a holographic patch, and a security thread. The second level encompasses colored fluorescent security elements visible under UV light. Finally, the last level includes security features detectable only by the banknote issuer. These security measures must meet certain criteria: they must be durable, relatively easy to integrate into the banknote, and they must be cost-effective.
MEASUREMENT METHODOLOGY AND RESULTS
Banknotes
Fluorescence tests were conducted using a GoyaLab portable fluorescence spectrometer equipped with an LED excitation source at 365 nm and a detection range from 380 nm to 780 nm. All measurements were obtained by approaching this handheld spectrometer to the different fluorescent features on the documents under study (Figure 1).
Figure 1. GoyaLab Fluorescence LED spectrometer measurement on a 10-dollar banknote.
We conducted fluorescence measurements on four different banknotes and observed the presence of multiple security features, such as the vertically positioned security strip, which varies in physical width and fluorescence wavelength. This marker was present on the American, Turkish, and Israeli banknotes, as seen in Figure 2.
Figure 2. American (a), Turkish (b), Israeli (c), and English (d) banknotes under UV excitation.
The resulting fluorescence spectra of the vertical strip reveal clear differences in the American banknote when compared to the Turkish and Israeli banknotes, as seen in Figure 3. This difference is due to the type of fluorescent ink used. For the American banknote, we observe an orange fluorescence feature around 600 nm with narrow peaks, while for the other two banknotes, we observe a blue fluorescence appearing around 450 nm in the form of broad features. Even though the fluorescence of the Turkish and Israeli banknotes is visually similar, we notice that their spectra have slight differences due to the composition of the fluorescent pigment, its concentration, and the width of the fluorescent band.
Figure 3. Emission spectra of the fluorescent vertical stripes on various banknotes resulting from UV excitation.
We also analyzed other security components present in the banknotes that are only visible under UV light. On the English (Figure 2d), Turkish, and Israeli banknotes, we observe fluorescence originating from a printed number that corresponds to the value of the banknote. These components are in different places depending on the origin of the note and may exhibit different emission colors.
We notice that the spectra of these other features in the banknotes also exhibit significant differences. On the Turkish banknote, this fluorescence appears in the red-orange part of the spectrum, and its spectrum is similar to that of the American banknote vertical strip, with emission near 600 nm. On the Israeli banknote, the denomination symbol is printed with a mixture of pigments, creating a wide fluorescent spectrum ranging from green to yellow. For the English banknote, the number 10 shows both yellow and red fluorescence, resulting in a spectrum that combines aspects of the other two. We observe a wide spectrum between green and yellow, although less intense than that of the Israeli banknote, with a peak also in the red region, like the Turkish banknote.
Figure 4. Emission spectra of other fluorescent features on various banknotes resulting from UV excitation.
It is interesting to visually compare the fluorescence of different banknotes. We notice that each country uses fluorescent compounds to varying extents. As mentioned before, all banknotes, except the English one, have a vertical fluorescent strip whose color and bandwidth vary. The American and English banknotes are quite simple in their use of fluorescent elements, unlike the Turkish and Israeli banknotes. These latter two countries have chosen to incorporate randomly placed fluorescent fibers. On the Turkish banknote, these markers are blue, while on the Israeli banknote, they are yellow and red.
Visual inspection of the banknotes allows observation of the complexity of the security features with the presence of numerous holograms and thin films on the right-hand strip in addition to fluorescence. Some of these features are observed in Figure 5. A watermark is sometimes visible, and for some banknotes, emerald illustrations produce different effects under light. All these mentioned components can vary in color or reveal additional details depending on the incident light.
Figure 5. Photos American (a), Turkish (b), Israeli (c), and English (d) banknotes.
Passports
Passports utilize the same fluorescent security techniques as banknotes, such as fluorescent inks, randomly dispersed fluorescent fibers in the paper, as well as fluorescent other symbols visible under UV light. Other elements are also often incorporated, including fluorescent stitching, a bright thread, and multicolored fluorescence. Other security features are integrated in addition to those fluorescent, such as laminated films, fine line patterns or guilloches, as well as holograms.
We have examined two fluorescent security elements, in a Spanish and French passports. In Figure 6a we can observe a page of a Spanish passport, featuring well-known security elements such as randomly arranged fluorescent fibers, in blue and yellow. We also notice the appearance of the word “España” in fluorescent yellow, as well as the country’s coat of arms and vertical red and yellow bands on the side of the page, representing the national colors. The second element that we analyzed is the sewing thread connecting the pages in a French passport (Figure 6b). This thread has been blended with an intense red fluorescent dye. Additionally, fluorescent ink is used to show a map of France in two different colors. This page alone shows how fluorescence can be used to encode documents with diverse and complex features.
Figure 6. Spanish (a) and French (b) passports under UV excitation.
Figure 7 shows the fluorescence spectra of some of these fluorescence features in both passports. The two elements shown in the graph have very distinct spectral signatures. Each country employs different fluorescent dyes and markings to differentiate their documents and to act as a deterrent to counterfeiting. Fluorescence spectra can thus allow authentication of the origin of such documents.
Figure 7. Emission spectra of some fluorescent features present in a Spanish and French passports under UV excitation.
OTHER INFORMATION
European banknotes are less resistant to physical stress and are more vulnerable to liquids compared to English banknotes because their composition is different. Sterling notes are made of a polymer substrate. Polymer banknotes are more expensive than paper banknotes, but they have a longer lifetime, are more difficult to destroy, and remain intact when exposed to water or liquids due to their smoother surface. Ise of polymers in banknotes can also help counterfeiting.
CONCLUSION
In conclusion, the integration of fluorescence inks into important documents such as banknotes and passports represents a significant effort in the fight against counterfeiting and the assurance of the authenticity of official documents. This paper shows that portable and easy-to-use fluorescence spectroscopy tools can provide a precise and reliable method for analyzing the security features of these documents, thereby enhancing public trust in these essential documents. Other security methods, combined with fluorescence, make counterfeiting more difficult to achieve, thus contributing to the protection of the economy and the preservation of users’ trust. Counterfeiting banknotes has become increasingly difficult due to the advancement of security features and the diverse ways these elements are used in banknotes from different countries.