Making and writing with "invisible" ink was always a fascinating wet Saturday afternoon for us as kids. It usually involved lemon juice and iron, or was it wax and watercolors? Now, scientists at Xerox have taken invisible ink to the next level, not to create secret, childish codes, but to enable a quick and easy way to security-protect valuable documents using readily available technology.
Their method means that digital printing equipment can be used to quickly add a layer of security to checks, tickets, coupons, and other high-value documents with hidden lettering. The lettering shows up only under ultraviolet light. But, the really fascinating aspect of the research is that although conventional invisible inks usually require fluorescent dyes, the Xerox team has side-stepped these materials.
Reiner Eschbach and Raja Bala have exploited the unique properties of dry ink, or color toner, and the way we perceive printed images to create an effectively hidden printed layer.
"We can create fluorescent writing on a digital printer without using fluorescent ink," explains Eschbach, "That means a four-color digital printer can print everything it normally would, and it can simultaneously individualize a document with a fluorescent identifier."
The key to the new technology lies in the paper itself. Paper manufacturers normally add optical brighteners to their products to make them appear "white" to our eyes, even though paper without them would be a duller gray-brown. Eschbach and Bala realized that certain combinations of toner could be printed to allow the paper's fluorescence to shine through when exposed to UV light. Based on this insight, the team has developed a special printer font that uses the contrast to "write" fluorescent letters and numbers.
Fausto Intilla's web site: www.oloscience.com