Japanese firm Fujitsu has demonstrated technology that can hide information inside images so that it can not be seen by the human eye. The process produces pictures that look normal, but reveal secrets when viewed through any camera, such as the one on a phone.

In principle, the technique is similar to steganography, a method used by the ancient Greeks which saw them taking wax tablets and hiding secret messages underneath the top layer of wax.

"The concept is to be able to link the printed page into the digital domain," said Mike Nelson at Fujitsu Europe. "The key is to take the yellow hue in the picture, which we skew ever so slightly to create a pattern. A camera is perfectly sensitive to that yellow hue but the human eye doesn't see it very well.

"Any camera, even those in mobile phones, can decode it very easily."

Fujitsu says that most modern mobiles and PDAs could be used to decode the hidden messages, providing they can be installed with a small Java program for decoding the picture.

One drawback is that at the moment the process can only hide very small amounts of data. The limit is currently just 12 bytes, but Fujitsu says they will raise the figure to 24 bytes before long, allowing the process to encode the same amount of data as in a standard barcode.

Apparently any colour printer can be used to print off the special images, and Nelson said that Fujitsu is currently in the process of licensing the technology to publishers.