## 3 Results and Discussion
### Spectral Reflectance of Prints  
The results of spectral reflectance measurements of the printed UVF ink and hybrid ink systems are presented in Figure 4a,b. Peaks detected in the range of 580-660 nm correspond to the spectral reflectance of the used UVF red, Eu3+ -based pigment [26]. In Figure 4a, spectral reflectance of the UVF ink systems on UT paper can be seen. It can be seen that that the UVFp in TLC displays the highest spectral reflectance in the wavelength area of interest compared to the other ink systems. This occurrence can be attributed to the influence of the transparent base on the decrease in reflectance in UVF and UVF on TLC ink systems. Furthermore, it can be concluded that TLC ink does not cause any changes in the reflectance spectra of the UVF pigment. A similar situation is observable for the reflectance spectra of UVF and hybrid ink systems on CB paper, presented in Figure 4b. Previous research [30] has shown that CB paper is a highly suitable printing substrate for TLC ink, and the same can be concluded for UVF ink/pigment printed on CB paper. Reflectance of the UVFp in TLC is higher compared to the same hybrid ink on UT paper (31.5% vs. 29% of maximal reflectance). The purpose of the spectral reflectance measurements of UVF ink in hybrid systems was to evaluate the performance of the UV fluorescent pigment when combined with TLC ink. Since the reflectance spectra showed no significant decrease in intensity or changes in UVF pigment peaks, it can be concluded that used TLC ink is suitable for the combination with used UVF pigment and applied on the appropriate substrates. The reflectance spectra of the TLC ink printed on UT and CB paper substates are shown in Figures 5 and 6. When the functional material inside the \"pigments\", i.e., microcapsules, is in the chiral nematic/cholesterol phase, a single reflection peak occurs, representing the  
Figure 4: Spectral reflectance of UVF pigment/ink printed on: (**a**) UT paper; (**b**) CB paper. spectral characteristic of the colour play effect. This peak moves across the visible range as a function of the temperature. Inside temperature activation range of TLC printing ink, temperature-dependent colours are clearly observable. As the samples of TLC ink printed on UT and CB paper substates show, the effect starts at 26 \({}^{\circ}\)C, which peaks at 626 nm for CB, showing red colour. At the same temperature, the TLC sample with UT substrate peaks at 595 nm, where the colour becomes orange. With further increase in temperature, the peak of reflectance spectra shifts towards shorter wavelengths and narrows. As the temperature rises over 32 \({}^{\circ}\)C, the peak shifts outside of the visible region (below 400 nm) and disappears completely above 44 \({}^{\circ}\)C, when TLC turns into isotropic liquid. This temperature is commonly referred to as the clearing point ([PERSON] et al., 2017). Although reflectance spectra of TLC ink are very similar on both paper substates, UT shows more statics, especially at higher temperatures. Figure 5: Reflectance spectra of TLC ink printed on UT paper substrate. Figure 6: Reflectance spectra of TLC ink printed on CB paper substrate. Reflectance spectra of all temperature-dependant printed samples at 27 \({}^{\circ}\)C for UT and CB paper substrates are shown in Figure 7a,b. Spectral reflectance of TLC on UT substrate peaks at about 550 nm, resulting in green colour of the sample, UVFp in TLC peaks at 572 nm (yellow), and UVF on TLC at 615 nm (orange). UVF pigment in TLC ink changes the effect of pure TLC ink, shifting the reflectance peaks towards higher wavelengths, which are more pronounced for UVF on the TLC sample. For CB substrate, UVF on TLC and UVFp in TLC have peaks at approximately the same wavelengths as for UT, but TLC peaks at 580 nm, differ from UT substrate by 30 nm and show another colour (yellow). The reason for this effect could be thickness of the UT paper substrate, which affects temperature-dependant measurements.