How to Make Glasses That Can See Invisible Ink

Since centuries, spies, clandestine lovers and others have used invisible ink as a way to send messages. This week’s Wonder of the Day describes how to make glasses to see invisible ink, and other ways scientists are developing to reveal secret written messages.

Invisible ink can be any substance which dries clear, or is outside the range of vision of humans. It requires the application of a chemical to make it visible. There are many ways to reveal ink, but heat activation is by far the easiest. Lemon juice, for example, is a common heat-activated invisible ink that can be revealed when the paper is ironed or placed over a radiator or 100-watt light bulb. Other kinds of invisible ink require more complicated chemical concoctions to be made visible.

There are also light activated invisible inks that contain substances which glow under certain types lights but remain undetectable to the naked eye. For example, when an amusement park stamps your hand as you leave, the stamp image appears under a black light, but it’s invisible to the naked eye when viewed in normal lighting. Light-activated invisible inks can also be used to write messages on the back of a deck of playing cards.

Scientists are also working on “sympathetic” invisible inks, which are mixtures of chemicals that respond to a specific reagent or set of reagents. These are similar inks to the oxidation/reduction or acid/base inks many printers use for recording serial numbers, the date and the time of printing. Examples of sympathetic inks include the yellow dots that appear on prints from some color laser printers, which can be read with a special pair of glasses.