A UV pen and light in one convenient tool. Not a marker, but an actual UV ballpoint pen! We tested it and found it works really well on most, but no all, colors of paper. Best to try it on different options before you decide on what you are going to use. We found it worked best on manilla folders, brown envelopes, cardboard, etc. to really see the high contrast of the ink from the background. Bright white paper, neon green and neon orange paper it didn't show up as well. Neon pink paper worked well though. Click on the comparison images for detailed explanation of the respective comparison.
Making use of ultraviolet light can be an excellent way to do stages of a multi, leave clues to a puzzle or a great addition to a night cache.
The ink is made from special compounds that are invisible to the naked eye, but illuminate when exposed to ultraviolet light. A blacklight is conveniently located on the opposite end of the pen making this perfect both for geocachers who want to build geocaches with a "UV Light Required" attribute or for those who want to find them!