A revolutionary way to print pictures without ink has been invented by a US company called Zink Imaging.

The company, a spin-off of Polaroid, says it will use the technology to make hand-held printers that can be integrated into mobile phones and digital cameras, with the first products available at the end of 2007. The key to creating the devices is doing away with ink, using a new type of digital printing that changes colour of paper when heat is applied.

"Unlike ink jet and thermal transfer printing, which need liquid ink cartridges and ink ribbon cassettes, with Zink all you need is the paper," says Steve Herchen, chief technology officer of the company. "All the colour-forming chemistry is embedded within the paper."

Zink paper is made from a white plastic sheet covered with micro-thin layers containing three types of colour-forming dye crystals – yellow on top, magenta in the middle and cyan on the bottom. The paper is initially colourless, looks and feels like ordinary white photograph paper and is not light sensitive.

Amorphous glasses

During printing, the paper passes through a thermal print head, which contains a thin row of tiny heater elements – 118 spanning 1 centimetre. The applied heat causes the dye molecules to change from crystals to amorphous glasses, a process that also releases colour.

To bring out the correct colour at each pixel, the temperature and exposure to the heater element is precisely controlled. The crystals in the top yellow layer require the highest temperature and shortest time to melt and become coloured; the magenta middle layer a lower temperature and longer time; and the bottom cyan layer the lowest temperatures and longest exposure time.

Because most pixels have combinations of all three colours, full colour images are obtained by each heater element delivering a series of rapid, time and temperature controlled pulses.

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