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."
The focal point in the new screen is the flat surface of the wedge. An optical trick means that light enters through the edge, bounces around inside the lens (much as if it were in a fiber-optic cable), and, when the light has bounced enough times to reach a specific angle (known as the "critical angle"), it exits through the front of the lens. Bathiche says that the specialized lens design, which includes a rounded, thicker end, dictates how the light bounces around and when and where it can escape.
The direction the light comes out depends on the position and angle that the light as it enters the bottom edge of the lens. This is controlled using an array of light-emitting diodes at the bottom of the screen. The viewer-tracking cameras are also positioned at the bottom edge of the lens; these collect light traveling the other way through the lens. Bathiche says that system's viewing angle is about 20 degrees, but hopes that with tweaks to the lens design, this can be increased to 40 degrees.
Bathiche says the 3-D lens can replace the traditional backlight in a liquid crystal display (LCD) to create a glasses-free 3-D display. Light from the lens will shine through the liquid crystals, projecting images at the viewers. The quality of the resulting picture is limited by the screen's refresh rate. A normal 240 Hertz LCD can accommodate two 3-D views, meaning that each viewer's eye receives a video that refreshes at a rate of 60 Hertz. Any slower, and the frames the video would be jerky. Alternatively, four viewers could watch their own 2-D video using the same display at a refresh rate of 60 Hertz. If the video were split again, then the frames would become jerkier. The technology is to some degree "at the mercy of what the LCD panel in front of the backlight can do," says Michael Bove, director of the consumer electronics laboratory at MIT. To address this, Bathiche says Microsoft is pushing display manufacturers to make faster LCDs. Bathiche's group is also exploring other ways to use the 3-D lens. If integrated into a backlight of a laptop, he says, it could provide a way to instantly toggle between a private view, in which the backlight steers the images from the screen toward a single person's eyes, and a shared view, in which the backlight shines the images out in all directions.
