When we were kids, to write “important” secret messages for our friends, we were using an age-old trick: just use lemon juice instead of normal ink, and the message receiver would only have to run the piece of paper over the flame of a candle to read it.
It was undoubtedly a romantic solution, and causes a sweet tear of tenderness now when looking back. Nowadays, the dear old lemon juice has been replaced by special markers which use invisible ink, while instead of the candle one can use a much more advanced infrared video camera.
The infrared camera, in fact, can pick up light at frequencies below what the human eye can see. Special inks used by this kind of markers can only be seen within that range, and thus can be detected only by devices which can see that light.
So, using an infrared camera to take video footage of a piece of paper with apparently nothing written on, it will be possible to see the invisibly written secret message. The ink, based on alcohol, can be used for a wide range of applications, such as for example marking banknotes, which could become traceable in case of a bank robbery, or maps and papers with a confidential content.
Furthermore, connecting the camera to a normal monitor, the contents of papers written with invisible ink can be viewed in real time, using it as an authentication protocol to access restricted areas.
For more details and advice on all possible applications of infrared cameras in your work or any other way, we recommend that you consult our experts, visiting the Endoacustica website.