## 3 Results and Discussion
Figure 4: **(a)** The two analyzed zones in 1-dollar banknote; **(b)** MEES spectra of authentic and counterfeit 1-dollar banknotes, measured in Zone A (left) and Zone B (right). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Peak/nm** & **Authentic** & **Counterfeit** \\ \hline
215.6 & - & + \\ \hline
219.3 & - & + \\ \hline
220.3 & - & + \\ \hline
228 & + & - \\ \hline
233 & + & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: List of main peaks which are missing in Zone B of the authentic 1 USD banknote and present in the counterfeit (and vice versa). Inkjet printing became more popular among counterfeiters in the last decade [4]. This is related to the better stability of these inks and the possibility to add metallic colors (which is not possible using laser printers). See, for example, the distribution of counterfeit printing methods in Canada for two decades (Figure 5) [29]. While the criminals take good care of obtaining excellent reproduction of the optical emission spectra, their product cannot reproduce the right MEES spectra. New Israeli Shekel (NIS) banknotes are also forged. According to unofficial estimates, about 250M NIS are forged each year. We analyzed authentic and counterfeit 200 NIS banknotes (which is the most popular banknote in Israel, with ca. 290 million bills in 2018) [29]. The off-white zones on both sides of the bill, marked in blue in Figure 6a, were analyzed by MEES. Contrary to foreign currency bills, this time, we obtained better information on the counterfeit banknotes: some of them were printed using inkjet, and others were laser printed bills. The comparison between the MEES spectra of the authentic, laser-printed, and inkjet-printed banknotes is presented in Figure 6b. Figure 5: Share of counterfeits by printing method—Canadian dollar. Figure 6: **(a)** The two analyzed zones in 200 NIS banknotes; **(b)** MEES spectra of 200 NIS authentic, inkjet, and laser printed banknotes. In this case, a clear spectral distinction between the three banknotes is also evident. Another observation is related to the overall MEES intensities: unlike the previous cases, the authentic 200 NIS bills possess the highest MEES signals in most of the analyzed wavelength range. This is probably related to special composition of these bills, but this hypothesis could not be validated, since all information (chemistry and processes) on Israeli banknotes is kept secret. In addition to the significant differences in the peaks, differences in the overall intensities are noted. The intensities from the authentic bills are closer to those obtained from laser printed ones and much higher than those from inkjet-printed ones. Information on the printing mechanism of authentic Israeli banknotes is not in the public domain; however, it could be that they are printed in a process closer to that of a laser printer (namely, fusing of toners). Another interesting case is the 100 NIS banknotes that we received from the Israel Police (Division of Identification and Forensic Science), for which the forgery method was unknown. MEES spectra of these banknotes were compared to those of authentic ones (at the same location), and the results are shown in Figure 7.  
Again, a clear-cut distinction is evident. Based on our previous measurements of various inks, it is possible that a laser printer was used to produce these counterfeit bills. The list of the main peaks which are missing in the authentic banknotes and present in the counterfeit (and vice versa) is presented in Table 6.  
Figure 7: (**a**) The sampling location on a 100 NIS bill; MEES spectra of authentic and counterfeit 100 NIS banknotes; (**b**) MEES spectra of authentic and counterfeit 100 NIS banknotes.