## 3 Results and Discussion
### Photoluminescence (PL)  
The VSC is capable of measuring the PL intensities using different preset excitation optical filters [26; 37]. We further studied the PL of writing ink strokes, reference crystal violet, and dyes on TLC plates (crystal violet, D1, D2, and D3) using the ten preset optical filters of the VSC for excitation. Figure 11 presents the PL spectra of the ink strokes of the writing BP pens of subgroup I, namely PE1, TO1, RX1, and RO4 pens under excitation of the ten different preset optical filters of the VSC. The PL spectra of the subgroup I samples are characterized by a broad emission in the range of 705-740 nm upon excitation using the first 400-485 nm band-pass filter. When the ink stroke samples were excited using the second to fourth excitation band-pass filters, a narrower emission band was observed at around 730 nm. Upon excitation with the fifth to seventh band-pass filters of longer wavelengths (\(<\)585 nm), a slight gradual red shift of the PL maxima up to 750 nm was observed. In the case of excitation using the eighth 585-720 nm band-pass filter, the emission red shift was greatly increased to around 850 nm, with relatively low PL intensities. The PL characteristics of the ink strokes of the subgroup are in good agreement with their IRL images shown in Figure 1, as discussed earlier. Interestingly, the intercomparison of the four PL spectra of the four samples of subgroup I reveal that the emission maxima of the PE1 and TO1 pair are very similar. Similarly, the RX1 and RO4 pair express comparable emission maxima. The similar PL characteristics of the two pairs, namely (PE1 and TO1) and (RX1 and RO4), further confirm the similar dye composition present in their corresponding ink formulas in agreement with the TLC results shown above in Figure 7a. These results indicate that PL measurements using the VSC can be used to differentiate different ink samples. Figure 12 displays the PL spectra of the ink strokes of the writing BP pens of subgroup II, namely RE1a, PR2, and VN1 pens under excitation of the ten different preset optical filters of the VSC. The PL spectra of the subgroup II samples were characterized by a weak emission in the range of 705 nm-740 nm upon excitation using the first 400-485 nm band-pass filter. This observation is in good agreement with the IRL images of the same samples shown above in Figure 2. When the ink stroke samples were excited using the second excitation band-pass filters, both the RE1 and PR2 samples (Figure 12a,b) showed a relatively high emission intensity with maxima centered around 736 nm. The emission maxima were then slightly red shifted to 764 nm up to the seventh excitation filter. In addition, the RE1a and PR2 samples continued to express enhanced emission around 745 nm up to the eighth excitation filter. The VN1 sample behaved differently and its PL spectrum (Figure 12c) indicates different emission maxima relative to the RE1 and PR2. Moreover, the VN1 sample did not show appreciable emission upon excitation using the  
eighth (585-720 nm) to the tenth (645-800 nm) band-pass filters. These results confirm the findings obtained from the TLC studies of those samples (Figure 7b). Figure 13 displays the PL spectra of the ink strokes of the writing BP pens of subgroup III, namely PK1 and BV1 pens under excitation of the ten different preset optical filters of the VSC. The spectra indicate that both the PK1 and BV1 samples express weak PL under the excitation of the first (400-485 nm) band-pass filter in an alignment with their faint dark appearance as discussed in the IRL images shown above in Figure 3. When the ink stroke samples were excited using the second excitation band-pass filters, the PK1 featured emission peaks centered around 666 nm and 732 nm, while BV1 showed emission bands with maxima at 670 nm and 742 nm, as can be seen in Figure 13a,b. Upon excitation with band-pass filters of longer wavelength, the PK1 sample expressed relatively higher PL intensity at around 666 nm, compared to 742 nm in the case of the BV1.