Photochromic inks are special types of inks that change color when exposed to different levels of light. They contain compounds that react to ultraviolet (UV) light, allowing them to darken in sunlight and return to their original color in the absence of UV light. This unique property makes them useful for various applications, including printing and packaging. These inks are often used in products like sunglasses and smartphone cases, where they can provide visual effects and enhance user experience. Additionally, photochromic inks can be employed in security printing to create features that are difficult to replicate, adding an extra layer of protection against counterfeiting.