Anybody who has ever seen the movie Star Wars remembers the scene in which the robot R2-D2 projects a 3D movie of Princess Leia imploring Obi-Wan Kenobi for help. Now, a new technique can conjure similar rudimentary 3D images out of thin air, a team of engineers reports today in Nature. Although far from as refined as the fictional Princess Leia projection, the new technique might one day help guide surgeons in delicate procedures.
Scientists have tried to create 3D projections, or volumetric displays, for more than a century with varying degrees of success. Some displays work by rapidly projecting a sequence of sliced-up images onto a rotating piece of glass, giving the impression of a complete 3D object. Others beam images onto clouds of fog or dust. However, very few displays can be precisely controlled while still floating freely in the air. “It is easy to make a volumetric display that works, [but] it’s very difficult to make a volumetric display that works well—hence 100 years of research,” says Barry Blundell, a physicist and engineer at the University of Derby in the United Kingdom who was not involved in the new work.
Now, a team of engineers has come one step closer to the ideal by exploiting the phenomenon of photophoresis, in which small, airborne particles can be manipulated with an intense beam of light. Daniel Smalley, an electrical engineer at Brigham Young University in Provo, Utah, and colleagues used a special combination of lenses to create a laser beam with both bright and dark regions. The dark areas trap the tiny particles, because heat from the surrounding light pushes them back in if they try to escape.
Using a barely visible violet laser controlled by mirrors, the researchers trapped a cellulose particle and moved it rapidly through space. The quickly moving mote was illuminated by other, colored lasers, making it visible. By moving the trapped particle fast enough, the researchers were able to trace out patterns in the air that, to an observer, appeared as a single image. “It’s not unlike when you have a sparkler at nighttime and you draw your name in the air,” Smalley says. “We know intellectually that it’s just one spot, but our eyes will integrate if it goes too fast.”
Smalley and colleagues have dubbed their device an Optical Trap Display, and with it they produced simple, free-floating outlines of objects including a butterfly and the university’s logo. Unlike holograms—like the ones on your credit cards—these images can be viewed from any angle, no matter where you are standing. Just like the projections of sci-fi films, they actually take up 3D space.
The team also created detailed, high-resolution images, but these required more movement of the laser—and so needed more time to produce. A picture of Earth, for example, took almost 20 seconds to create, so it could only be captured by a camera with a long exposure time. And all the detailed images were less than 3 centimeters wide, tiny enough to hover over the tip of a single finger. These limitations could be overcome by using a plane of particles moving simultaneously, rather than relying on a single particle to do all the work, Smalley says. Blundell agrees: “That is where the scaling up of the technology will either do well or fail.”
But even with further improvements, volumetric displays will never provide the photorealism we might expect from the movies, Blundell says. The real benefits, he says, will come from the ability to display spatial relationships in a way we can’t do today: Using 3D visualizations instead of 2D ones, for example, could help physicians facing complicated procedures. “If you’re threading a catheter through the vasculature of the brain or the heart—those are some complicated 3D paths,” Smalley says. “If you could get a high-resolution MRI image of the data before the procedure … you could use this little display to give you the cross section of the artery,” and avoid harming patients.