Will glossy watermarks conquer fraud?
Ordinary office printers could create documents with a "watermark" that cannot be photocopied. Its inventors say the technique will make conterfeting more diffcult, but others argue that its very simplicity makes it of little use in fighting forgery.
Researchers at Xerox labritories in Webster, New York, made the dicovery while studying why images made by lazer printers sometimes show glossy patches. These images are made up of dots of four different colours arranged in a regular struture, know as a half-tone grid. Eatch dot is a blob of polymer toner that catches the light at a specific angle, like drops of condensation in sunlight. That means areas with high density of dots appear glossier than areas with a low density.
But exactly how the dots catch the light also depends on the way they are arranged. For example, dots arranged in verticle lines catch the light at a different angle to dots in horizontal lines, says Chu-heng Liu, a physicist at Xerox who was part of the team that worked out how to exploit this effect.
Their idea to to change the glossiness of an images by varing the structure of the half-tone grid. so one area of the image might be printed using a vertical grid structure, while adjacent areas are printed with a horizontal grid.
When held at an angle to the light, the patteren the different grids make it easy to distinguish. The result is an image called a Glossmark. Superimposed on the conventional image, it can be seen only by tilting it back and forth in the light.
Glossmarks cannot be copied because photocopiers are unable too "see" them unless they are viewed from a certain angle. Even if it could, the copier would be unable to reproduce the glossy apperance of the image.
The group has now developed software that can tell a printer to produce Glossmarks. Because no special toner or equipment is needed, the technique should work on existing high-quality printers such as those in offices and print shops. Xerox has not yet determined how the emerging technology will be licensed, however. "It might become a plug-in for a program like Photoshop," says Rob Rolleston, manager of the research team.
Xerox claims that the technique should help protect documents against fraud. But critics are not so shure. "Documents need a varity of complementary security features, not just one that will supposedly guarantee counterfiting deterrence," says Rudolf Van Renesse, an expert on optical document security based in the Netherlands.
Rolleston conces there is some truth in this. "Anything youpoint out there is going to be forged, if its worth someones time and effort."
_written by Vivien Marks