## 3 Results and Discussion
### Spectral Reflectance of Prints
Both hybrid ink systems, UVF on TLC and UVFp in TLC, show peaks at 620 nm because of fluorescent pigment that is detected during measurements, even without UV light. Results are similar for TLC and hybrid ink systems printed on UT and CB substrates at 29 \({}^{\circ}\)C (Figure 8a,b). Reflectance spectra of pure TLC ink peaks at 494 nm for UT, and for CB at 500 nm, which occurs as a green colour of the printed sample; here the difference between the UT and CB samples is only 6 nm, which is almost imperceptible compared to the difference between UT and CB at 27 \({}^{\circ}\)C (Figure 7a,b). UVFp in TLC peaks at about 503 nm for both UT and CB but has a higher peak for CB substrate, which also applies to the samples measured at 27 \({}^{\circ}\)C. UVF on TLC peaks at about 530 nm for both UT and CB substrates. Further increase in temperature resulted in fewer differences between spectral reflectance peaks for TLC and hybrid ink systems. The samples printed on UT and CB paper substrates were measured at 31 \({}^{\circ}\)C and the results are shown in Figure 9a,b. TLC and UVFp in TLC peak are about 460 nm for both UT and CB paper substrate. UVF on TLC peaks at 480 nm for UT, while the peak for CB cannot be determined exactly because of high device statics. Colorimetric analysis (Tables S2 and S3) was used to describe thermochromic colour play effect of liquid crystals in pure TLC printing ink, as well as hybrid ink systems with TLCs and UVF pigment. The CIELAB colour values of all samples printed on UT substrate were calculated from the corresponding reflectance spectra shown in Figure 10.  
Both pure TLC and hybrid ink systems show full colour play effect caused by the chiral nematic/cholesteric phase of TLCs (Figure 10a). This effect implies the red-green (\(a^{*}\)) and yellow-blue (\(b^{*}\)) values start and end at almost the same points of the (\(a^{*}\),\(b^{*}\)) graph,  
Figure 7: Reflectance spectra of TLC, UVF on TLC, and UVFp in TLC, at 27 \({}^{\circ}\)C: (**a**) on UT paper substrate; (**b**) on CB paper substrate. showing the colour of the sample in an isotropic phase of the TLC, where no colour has developed. At intermediate temperatures, where the chiral nematic/cholesteric phase produces the single reflection peak, the entire loop is formed, exhibiting a thermochromic effect [11]. All three measurements form a similar loop, but some differences do occur. Pure TLC results in the most intense colour play effect, forming the widest loop in (\(a^{*}\),\(b^{*}\)) graph. The loops of UVF on TLC and UVFp in TLC (Figure 10a) cover a smaller area in comparison with pure TLC, showing less intense thermochromic effect, especially in some parts of the spectrum, for example, UVF on TLC--in the blue part. The temperature-dependent diagram shows these differences in more detail, including lightness \(L^{*}\) measured at each individual temperature of the printed sample (Figure 10b). Each curve extends from 20 to 47 \({}^{\circ}\)C, where the reflection peak appears invisible. A single maximum occurs in each \(L^{*}\) (T) curve in the yellow-green region, where the colour has the highest lightness \(L^{*}\). Inside the temperature activation region, \(L^{*}_{max}\) occurs at almost the same temperature for all samples (Table 6). The intensity of lightness \(\Delta L^{*}_{max}\) is the highest for TLC, then follows UVF on TLC and UVFp in TLC (Table 6; Figure 10). Pure TLC on CB shows similar results to that of the UT paper substrate, experiencing the most intense colour play effect. UVFp in TLC shows a slightly less diminished effect within the same curve limits of pure TLC, while UVF on TLC has the weakest thermochromic colour play effect. The reason for this less intense thermochromic effect of hybrid ink systems printed on both substrates could be the composition of the UVF printing ink, i.e., UVF layer printed over TLCs, which most probably decreases selective reflection of liquid crystals by partially absorbing light through the UVF layer.