Thermal Ink
An ink which changes its colour or optical density, when the image is exposed to heat. The images may change colour, become visualized or decolourized. Usually reversible thermal inks are used, which return the initial colour after heat exposure (fig. 1).
a
b
Fig. 1. Thermal Ink. 500 Latvian Lats (2008):
a – general view; b – decolourization of the image after its exposure to heat