Researchers have made a major step toward a holographic videoconferencing system that would let people communicate with one another almost as if they were in the same room. They have developed a full-color, 3-D display that refreshes every two seconds, and they've used it to send live images of a researcher in California to collaborators in Arizona. In the coming years, the researchers hope to develop a system that refreshes at standard video rates and can compete with other 3-D displays.

"Holography makes for the best 3-D displays because it's closest to how we see our surroundings," says Nasser Peyghambarian, chair of photonics and lasers at the University of Arizona. A hologram is a display that uses an optical effect called diffraction to produce the light that would have come from an object in the image if the physical object were in front of the viewer. Holographic images appear to project out into the space in front of the display. By walking around a holographic image, it's possible to see objects in it from different angles.

Holograms don't require glasses to view, and unlike other glasses-free 3-D systems, multiple people can use them simultaneously without having to stand in a particular place. But the development of holographic displays has lagged behind that of other 3-D systems because of the difficulty in creating holographic materials that can be rapidly rewritten to refresh the image.

The first video holographic display was made at MIT's Media Lab in 1989. The volume of the hologram was just 25 cubic millimeters, smaller than a thimble. Since then, researchers have been trying to develop practical holographic systems but have come up against limitations in scaling these displays up to larger sizes. A big challenge has been the attempt to eliminate expensive optical components without sacrificing the refresh rate.

A few companies sell 3-D displays for medical and design applications, but many of these systems don't produce true holograms, and they tend to be expensive, not least because they're produced in small amounts. "Some need lasers, some need powerful computers to operate, or many displays stacked together," says Jennifer Colegrove, director of display technologies at industry research firm DisplaySearch. She notes that in 2010, such "volumetric" displays will generate $5 million in revenue, a small sliver of the $1 billion 3-D display market. Despite their expense, she says, "these displays are still primitive," and lack a combination of image quality, speed, and display size.