To make ZINK photo paper, which Herchen says feels exactly like regular photo paper, the researchers start with a white plastic sheet as a base material, then add very thin layers of dye crystals. The dye molecules that make up these crystals are structured in such a way that the crystal is transparent. When heat is applied, the molecules change their physical orientation from a crystal to an amorphous glass, a process that releases color.
The configuration of the crystal layers–yellow on top, magenta in the middle, and cyan on the bottom–is a crucial element in the printing process. When these layers pass through the thermal printhead, says Herchen, they are heated by 300 tiny heaters per square inch. And in order to bring out the appropriate color at each pixel, the temperature and amount of time each heater is on is precisely controlled. The crystals in the yellow top layer require the highest temperature to show their color but the shortest amount of time. To turn on the cyan bottom layer, the heaters operate at their lowest temperature for the longest amount of time. Bringing color out of the magenta middle layer requires heating times and temperatures somewhere in between.
“[The printer] is doing combinations of these pulses for every single pixel,” says Herchen. To produce a green pixel, for instance, the heating element would turn on some yellow layers with a quick, high-temperature pulse, cool back down, and then turn on cyan with a longer, low-temperature pulse. It takes only tens of microseconds to deliver these pulses. In a typical two-by-three-inch picture, which takes less than a minute to print, there are approximately two hundred million heat pulses.A technology based on thermal heating begs the question: how easy is it to ruin the paper before and after one has printed a picture? Herchen says that laboratory tests have shown that the paper–both before and after printing–doesn’t change colors at temperatures as high as 70 °C (158 °F). And if placed in sunlight, the picture will fade at a rate similar to that of other thermal printings and many inkjet printings: about 5 to 15 years.
Some analysts think that ZINK’s technology has the capability to change the way people think about photos. “Right now, we’re still relatively formal about photos,” says Chris Shipley, executive producer of DEMO and cofounder of Guidewire Group, a technology research firm. People take pictures, collect them, and put them in books, but if they’re taken with a cell phone, often they stay there, she says. “The idea that a photo can be a note, a moment captured and shared quickly, is something that ZINK enables,” Shipley says. In the short run, she says, the technology could make photo sharing more casual.
Herchen says ZINK has plans for two products by the end of the year: a stand-alone portable printer and a printer integrated into a digital camera, both producing two-by-three-inch pictures. While the printer can be designed into tiny gadgets, it can also be integrated into larger electronics, and Herchen expects that within the next few years the printing technology could also show up in computer towers, laptop computers, and even home television sets.