Thermochromic Ink
An ink which changes colour or becomes transparent due to a change in temperature. The thermochromic effect is caused by changes in the chemical composition or physical properties of thermosensitive pigments. Thermochromic ink may have more than two color changes at different temperatures.
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Fig. 1. Ireland. Passport card issued in 2015:
a — front side. Polymer substrate; b — the same. Image printed with thermochromic ink. View at a temperature of about 20 ˚С; c — the same. View when heated to 40 ˚С. Change of transparency
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Fig. 2. Ireland. Passport issued in 2013:
a — page 6. Paper substrate; b, c, d — the same. Image printed with thermochromic ink. Change of color (grey – green – yellow) as the temperature increases up to 40 ˚С; e — the same. Zoomed fragment. Thermosensitive ink pigments. View at a temperature of about 20 ˚С; f — the same. Zoomed fragment. Thermosensitive ink pigments at a temperature of 40 ˚С