Photochromic inks are invisible inks which, gradually under exposure to Ultra-Violet rays, become coloured, in blue, or violet. This reaction of coloration is reversible, once the document is no more exposed to UV rays.
UV rays are emitted by the sun, a U.V. lamp or a photographic flash.
Instead of being invisible, those inks can be also coloured and some variations are possible:a yellow ink will switch to green, a red ink will switch to violet.
Printing process can be dry offset, flexography, silk-screen and gravure, but in any case the deposit must be important. Wet offset canoot be used.