Highlighter Most highlighters show different effects in different light. There is a kind of highlighter called "cheat pen", which can't be seen when written on paper, and traces will only appear when illuminated by purple light; However, it seems that this kind of pen has not been produced and sold yet.

Or a developing pen. You can't see what you write on the paper, but you can see what you write with the light behind the pen. That light is ultraviolet light.

Hope to adopt.