Method and device for the secure protection of an official document and official document thus protected

A device for security protection of a surface portion of a layer of an official document bearing protected inscriptions, said device comprising a transparent security layer which extends in a format adapted to that of said surface portion,

• wherein:
• the transparent security layer comprises at least one semi-transparent printed image applied as a printing composition, the printing composition comprising 9% to 22% by weight thermochromic capsules, 30% to 45% by weight transparent binder and 25% to 50% by weight solvent, the printed image comprising at least one transparent binder and at least one thermochromic pigment adapted to change color when there is a predetermined temperature variation,
• in a first, non-activated state, at a first temperature, said semi-transparent printed image reveals at least one semi-transparent visible motif which allows the protected inscriptions to be seen through the image, said motif not being visible in a second, activated state, at a second temperature, the second temperature being different from the first temperature, wherein
• a proportion of thermochromic pigments in the binder is such that the semi-transparent image allows the protected inscriptions to be seen through the image when the thermochromic pigments are in the non-activated state.