Today's 3-D movies are far more spectacular than the first ones screened more than 50 years ago, but watching them--both at the movie theater and at home--still means donning a pair of dorky, oversized glasses. Now a new type of lens developed by researchers in Microsoft's Applied Sciences Group could help make glasses-free 3-D displays more practical.

The new lens, which is thinner at the bottom than at the top, steers light to a viewer's eyes by switching light-emitting diodes along its bottom edge on and off. Combined with a backlight, this makes it possible to show different images to different viewers, or to create a stereoscopic (3-D) effect by presenting different images to a person's left and right eye. "What's so special about this lens is that it allows us to control where the light goes," says Steven Bathiche, director of Microsoft's Applied Sciences Group.

3-D technology has seen a renaissance recently. Thanks to the success of movies like Coraline, Up, and Avatar, Hollywood is spending more money than ever to give audiences a stereoscopic experience. And electronics manufacturers are racing to replicate the 3-D theater experience in the home. The market for 3-D-capable televisions is expected to grow from 2.5 million sets shipped in 2010 to 27 million in 2013, according to the research firm DisplaySearch. However, the glasses required to watch 3-D video is a turnoff for many would-be early adopters.

At the Society for Information Display International Symposium in Seattle last month, companies showed off 3-D displays that don't require glasses. These sets often use lenticular lenses, which are integrated into the display and project different images in two fixed directions. But a viewer needs to stand in designated zones to experience a 3-D effect; otherwise the screen becomes an out-of-focus blur.

Microsoft's prototype display can deliver 3-D video to two viewers at the same time (one video for each individual eye), regardless of where they are positioned. It can also shows ordinary 2-D video to up to four people simultaneously (one video for each person). The 3-D display uses a camera to track viewers so that it knows where to steer light toward them. The lens is also thin, which means it could be incorporated into a standard liquid crystal display, says Bathiche.

The idea of tracking viewers to make the glasses-free 3-D easier has been around for decades. One of the big challenges, explains Ken Perlin, professor of computer science at New York University, is that the computers used for eye-tracking were too expensive and too slow to make such a system practical. As computers have become faster and cheaper, viewer-tracking systems have gotten up to speed; other components, particularly those needed to target viewers, have remained bulky and impractical to manufacture on a large scale.

Microsoft's wedge lens is about 11 millimeters thick at its top, tapering down to about six millimeters at the bottom. A traditional lens, found in a projector, sits between a point of light and its focal point--the spot where the light is focused. This is the reason why viewer-tracking 3-D systems are often so bulky. The design of the wedge lens bypasses this problem, explains Bathiche. "Instead of having light travel in air, it travels within the lens," he says. "It allows us to compress the distance between the projector and the screen."