Colorimetric analysis of thermochromic samples in different forms employing a digital camera

The calibrated and characterised digital camera was used for the recording of a dynamic colour change of thermally treated three-component leuco dye based thermochromic samples in various forms - bulk system, dry pigment, water-and solvent-based ink formulations, and their dry layers coated on different paper substrates. The study has proved that the proposed approach enables to examine the thermochromic behaviour through the individual stages of the thermochromic coating preparation process. The measured path in CIELAB colour space and derived characteristics of the dynamic colour change were obtained with higher time-temperature resolution than with previously employed methods, enabling to observe even the less distinct features. The most significant differences were related to the length and complexity of the colour path. Dry layers of the water-based thermochromic ink coated on papers had higher values of the total length of the path and the total colour contrast than the layers of the solvent-based ink. The colour of water-based coatings had higher chroma and also it was less influenced by paper substrate. When compared to the bulk thermochromic system, the dry pigment and dry ink layers on paper substrates exhibited a larger width of a hysteresis loop at the decolouration limit, mainly due to the decrease of the temperature at which the colouration starts on cooling.