Photosensitive color-changing printing, also known as photochromic printing, is a fascinating process where dyes change color under ultraviolet (UV) light. Initially, these dyes are colorless. For instance, dye A is invisible until UV light hits it, causing a molecular reaction that opens its structure into a cyanine form (dye B), which is blue. When dye B is later exposed to visible light, it reverts to the colorless dye A.To enhance durability, we now encapsulate these photochromic dyes using microcapsule technology. The dyes are sealed within tiny capsules, usually 1-10 microns in size. This not only improves the lightfastness of the dye but also protects it from external factors.Currently, we have developed four primary photochromic dye colors: purple, blue, yellow, and red. These dyes have a closed-loop structure, meaning they are colorless when applied to a fabric but change to their respective colors under UV light.The term 'photochromic' refers to the transition of one color to another. This is achieved by mixing the photochromic dye with a conventional colorant during the printing process. For example, if you mix the photochromic dye red with the paint blue, the fabric will initially appear blue. However, once exposed to UV light, it will change to a purple hue.