The Princess Leia project: ‘volumetric’ 3D images that float in ‘thin air’

Making the 3D displays of science fiction real
January 31, 2018

Inspired by the iconic Stars Wars scene with Princess Leia in distress, Brigham Young University engineers and physicists have created the “Princess Leia project” — a new technology for creating 3D “volumetric images” that float in the air and that you can walk all around and see from almost any angle.*

“Our group has a mission to take the 3D displays of science fiction and make them real,” said electrical and computer engineering professor and holography expert Daniel Smalley, lead author of a Jan. 25 Nature paper on the discovery.

The image of Princess Leia portrayed in the movie is actually not a hologram, he explains. A holographic display scatters light only on a 2D surface. So you have to be looking at a limited range of angles to see the image, which is also normally static. Instead, a moving volumetric display can be seen from any angle and you can even reach your hand into it. Examples include the 3D displays Tony Stark interacts with in Ironman and the massive image-projecting table in Avatar.*

How to create a 3D volumetric image from a single moving particle

BYU student Erich Nygaard, depicted as a moving 3D image, mimicks the Princess Leia projection in the iconic Star Wars scene (“Help me Obi Wan Kenobi, you’re my only hope”). (credit: Dan Smalley Lab)

The team’s free-space volumetric display technology, called “Optical Trap Display,” is based on photophoretic** optical trapping (controlled by a laser beam) of a rapidly moving particle (of a plant fiber called cellulose in this case). This technique takes advantage of human persistence of vision (at more than 10 images per second we don’t see a moving point of light, just the pattern it traces in space — the same phenomenon that makes movies and video work).

As the laser beam moves the trapped particle around, three more laser beams illuminate the particle with RGB (red-green-blue) light. The resulting fast-moving dot traces out a color image in three dimensions (you can see the vertical scan lines in one vertical slice in the Princess Leia image above) — producing a full-color, volumetric (3D) still image in air with 10-micrometer resolution, which allows for fine detail. The technology also features low noticeable speckle (the annoying specks seen in holograms).***

Applications in the real (and virtual) world

So far, Smalley and his student researchers have 3D light-printed a butterfly, a prism, the stretch-Y BYU logo, rings that wrap around an arm, and an individual in a lab coat crouched in a position similar to Princess Leia as she begins her projected message. The images in this proof-of-concept prototype are still in the range of millimeters. But in the Nature paper, the researchers say they anticipate that the device “can readily be scaled using parallelism and [they] consider this platform to be a viable method for creating 3D images that share the same space as the user, as physical objects would.”

What about augmented and virtual-reality uses? “While I think this technology is not really AR or VR but just ‘R,’  there are a lot of interesting ways volumetric images can enhance and augment the world around us,” Smalley told KurzweilAI in an email. “A very-near-term application could be the use of levitated particles as ‘streamers’ to show the expected flow of air over actual physical objects. That is, instead of looking at a computer screen to see fluid flow over a turbine blade, you could set a volumetric projector next to the actual turbine blade and see particles form ribbons to shown expected fluid flow juxtaposed on the real object.

“In a scaled-up version of the display, a projector could place a superimposed image of a part on an engine showing a technician the exact location and orientation of that part. An even more refined version could create a magic portal in your home where you could see the size of shoes you just ordered and set your foot inside to (visually) check the fit. Other applications would included sparse telepresence, satellite tracking, command and control surveillance, surgical planning, tissue tagging, catheter guidance and other medical visualization applications.”

How soon? “I won’t make a prediction on exact timing but if we make as much progress in the next four years as we did in the last four years (a big ‘if’), then we would have a display of usable size by the end of that period. We have had a number of interested parties from a variety of fields. We are open to an exclusive agreement, given the right partner.”

* Smalley says he has long dreamed of building the kind of 3D holograms that pepper science-fiction films. But watching inventor Tony Stark thrust his hands through ghostly 3D body armor in the 2008 film Iron Man, Smalley realized that he could never achieve that using holography, the current standard for high-tech 3D display, because Stark’s hand would block the hologram’s light source. “That irritated me,” he says. He immediately tried to work out how to get around that.

** “Photophoresis denotes the phenomenon that small particles suspended in gas (aerosols) or liquids (hydrocolloids) start to migrate when illuminated by a sufficiently intense beam of light.” — Wikipedia

*** Previous researchers have created volumetric imagery, but the Smalley team says it’s the first to use optical trapping and color effectively. “Among volumetric systems, we are aware of only three such displays that have been successfully demonstrated in free space: induced plasma displays, modified air displays, and acoustic levitation displays. Plasma displays have yet to demonstrate RGB color or occlusion in free space. Modified air displays and acoustic levitation displays rely on mechanisms that are too coarse or too inertial to compete directly with holography at present.” — D.E. Smalley et al./Nature

Nature video | Pictures in the air: 3D printing with light

Abstract of A photophoretic-trap volumetric display

Free-space volumetric displays, or displays that create luminous image points in space, are the technology that most closely resembles the three-dimensional displays of popular fiction. Such displays are capable of producing images in ‘thin air’ that are visible from almost any direction and are not subject to clipping. Clipping restricts the utility of all three-dimensional displays that modulate light at a two-dimensional surface with an edge boundary; these include holographic displays, nanophotonic arrays, plasmonic displays, lenticular or lenslet displays and all technologies in which the light scattering surface and the image point are physically separate. Here we present a free-space volumetric display based on photophoretic optical trapping that produces full-colour graphics in free space with ten-micrometre image points using persistence of vision. This display works by first isolating a cellulose particle in a photophoretic trap created by spherical and astigmatic aberrations. The trap and particle are then scanned through a display volume while being illuminated with red, green and blue light. The result is a three-dimensional image in free space with a large colour gamut, fine detail and low apparent speckle. This platform, named the Optical Trap Display, is capable of producing image geometries that are currently unobtainable with holographic and light-field technologies, such as long-throw projections, tall sandtables and ‘wrap-around’ displays.