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Cambridge boffins build laser 'unprinter' to burn pages clean

Works with standard paper and toners

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A team of scientists from the University of Cambridge has published plans for a laser "unprinter", which vaporizes toner used in printing to leave a clean sheet of paper.

The idea of reusing printed paper has been around for a while, and it's not too difficult provided you use expensive coated paper. Toshiba has recently been showing off a printer that can erase its own single color toner off normal paper, but the technique has limitations. With Toshiba's system the ink is heated to make it disappear, but the technique leaves a residue and the same paper can only be reused five times.

"Toshiba have been selling the 'e-blue' toner for a while - which, like old thermal fax paper, fades under the right type of light. However that - of course - applies only if you buy their magic toner," the Cambridge project's supervisor Julian Allwood told New Scientist. "Our ambition was to develop a method that would remove conventional toner from conventional paper in order to allow re-use of the paper. Toshiba's is a different approach to the same problem."

The Cambridge team figures it has cracked the problem, and its unprinter can work with any laser printed or photocopied paper, with no need for special toner that's more expensive gram-for-gram than cocaine. It works by tracing the outline of the toner and then flash-frying it off using the device's laser.

"The key idea was to find a laser energy level that is high enough to ablate - or vaporise - the toner that at the same time is lower than the destruction threshold of the paper substrate. It turns out the best wavelength is 532 nanometres - that's green visible light - with a pulse length of 4 nanoseconds, which is quite long," said team member David Leal-Ayala.

The team used the technique to wipe a standard sheet of A4 three times without appreciable damage, but have warned that using the system too much could result in damage to the substrate, causing a yellowing effect. They have found some papers hardier than others, and are continuing experiments while considering whether to file patents or commercialize the technology. ®

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