## 3 Results and Discussion
### Microscopy of Printed Inks  
Microscopic images of the printed samples are presented in Figures 12 and 13. The images were taken under same conditions at a room temperature of 25 \(\pm\) 1 \({}^{\circ}\)C and 50-55% relative humidity. Images present the surfaces of prints produced in hybrid printing ink systems, printed on UT and CB paper substrates, captured without UV and exposed to UV radiation to ensure the visibility of the fluorescence effect. Figure 11: CIELAB colour values of TLC, UVF on TLC, and UVFp in TLC inks printed on CB paper substrate, presented as (a) (\(a^{*}\))+); (b) \(L^{*}\)(T) graphs. Figure 12: Microscopic images of: (**a**) UVF on TLC without UV; (**b**) UVF on TLC exposed to UV radiation; (**c**) UVFp in TLC without UV; (**d**) UVFp in TLC exposed to UV radiation; printed on UT paper substrate (mag 200\(\times\)). Figure 13: Microscopic images of: (**a**) UVF on TLC without UV; (**b**) UVF on TLC exposed to UV radiation; (**c**) UVFp in TLC without UV; (**d**) UVFp in TLC exposed to UV radiation; printed on CB paper substrate (mag 200\(\times\)). Figure 12a-d presents images of hybrid ink systems printed on UT paper substrate. Observing Figure 12a,c, captured without exposure to UV radiation, one can conclude that visual observation of images corresponds to the results of spectral reflectance and the CIELAB colour values presented in Figures 5, 7 and 10. It is obvious that in a UVFp in TLC hybrid system the mixing of the UVF pigment in the TLC printing ink causes the forming of a complex structure with properly distributed thermochromic and fluorescent pigments particles in the layer. By overlaying the TLC ink with UVF printing ink the distribution of particles is less visible due to the layer of UVF pigment mixed in the transparent base that diminishes the thermochromic effect to a certain extent. By exposing the printed hybrid systems to UV radiation (Figure 12b,d), one can see that the fluorescence effect is more expressed in UVF on the TLC printed layer. The reason for this is that overlaying the TLC ink with UVF printing ink enables a stronger visual response of fluorescent printing ink. In Figure 13a-d one can see the microscopic images of hybrid ink systems printed on CB paper substrate. Since CB paper substrate has an extremely complex surface structure, with a more pronounced roughness than UT paper (Figures 1 and 2; Table 2), the microscopic images show a different visual response of the observed samples in comparison to UT paper. On the sample which is not exposed to UV radiation (Figure 13a), almost no response of either thermochromic or UVF effect is visible. Relatively uniform distribution of the ink layer on the surface can be seen. The sample exposed to UV radiation clearly shows the effect of fluorescence; although, the fibres and the structure of the substrate are notably expressed despite the double-layer application of inks (Figure 13b). In the hybrid system of UVFp in TLC (Figure 13c), the microcapsules in TLC ink are correctly distributed and the structure of the substrate is visible. On the sample exposed to UV radiation, the ink particles are very nicely expressed, and the fluorescent effect is visible with properly distributed thermochromic ink microcapsules. Comparing visual effects visible on microscopic images of UT and CB paper substrates, one can say that CB substrate gives a higher visual effect of fluorescence and a more pronounced thermochromic effect in the observed hybrid ink systems. Probably, due to its surface characteristics, it expressed more roughness and higher values of printing ink thickness in comparison to UT paper, and better interaction in the printing process was achieved causing the positive dual-functional effect, as was confirmed by the presented measurement results. Nevertheless, both effects are visible in their respective ranges (temperature or wavelength) on both substrates.