Permanent marker with invisible blue ink. The writing or mark created with this marker is invisible under normal light and appears only under UV light, revealing a fluorescent blue color. It works perfectly with any UV light source (UV keychain, UV lamp, UV light, or UV flashlight). It is ideal for notes, secret messages, marking personal and valuable items, suitable for both industrial and personal use. It is designed for surfaces such as metal, glass, stainless steel, plastic, fabrics, and wood.