Imagine you're sitting in a movie theater, large popcorn perched on your lap, when the lights go down for the show to begin. But before the previews roll, the screen turns bright red and the whole room is cast in cherry light. Suddenly, you notice that an image of Harry Potter on his broomstick has appeared on the side of your popcorn bucket. Wasn't a random design there a minute ago? Then, as quickly as the apparition emerges, it's gone, disappearing in a flash of blue as the screen changes colors. Wait a second--now the bucket shows a picture of Dumbledore, Harry's headmaster! What sort of black magic is this?

Sorry, Potter fans, it's not an enchantment--just colorful science. Researchers at Xerox have just unveiled this new color printing technology, which lets users print one image on top of another, then view the images one at a time by exposing the document to specific colors of light.

The technique, called Switch-A-View printing, developed from research in which scientists studied how to make printed images look best in different kinds of light, such as under sunlight or the glow of florescent lamps. The researchers eventually started to think about twisting their work around in funny ways, says Robert Loce, a principal scientist at Xerox. The resulting work produced images printed in specific shades of red, blue, and yellow; shine red light on the image and anything printed in red absorbs the light and disappears, leaving only the other colors to be reflected and picked up by your eye.

Initially, the Xerox team thought about using the images in advertising--on the side of popcorn boxes, or as premiums in nightclubs--deploying them in any location where the lighting can be controlled. "It's got a very strong, dynamic impact if you set it up properly," Loce says. "It gets people excited."

The researchers also realized that computer screens could be used to produce the proper color of light; imagine a kid armed with a candy wrapper or a box of cereal who has to go to the manufacturer's Web site to produce the light needed to find out if he's won a prize or solved a puzzle.

Eventually, Xerox recognized that the printing technique is useful for more than just fun and games, says Loce. Paper documents can be printed with images hidden on them as a sort of watermark, establishing their authenticity. Faked coupons cost businesses over $700 million a year, he says; imagine the savings if cashiers could run coupons under a red light to make sure they haven't been photocopied. "It's useful for low-level anti-counterfeiting," Loce says. "I probably wouldn't use it on the euro or the dollar bill, but I'd use it on coupons and tickets."

The images are fairly easy and cheap to produce, says Loce, and can be made on standard office printers, as long as they're properly calibrated. The technology is ready for licensing now, and will probably start popping up on products soon.