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age to a volume the size of a ping-pong ball, and the results demonstrated so far are based on long-exposure images that took up to a minute to generate.

Says Barry Blundell, senior lecturer in computing at the University of Derby in the U.K. and a researcher into volumetric displays since the late 1980s, “With the optical-trap display, I would have to see images generated a lot faster. The only way to do that is parallelism; you’ve got to have more lasers surrounding the display, and more particles. The problem could be that you need to have so much physical apparatus that you lose the viewing freedom.” Smalley claims the technology exists to drive and illuminate a collection of particles in the shape of the spatial light modulator, the same kind of device as that used to research holographic displays and optical computers. Bove argues the laser and light-modulator components needed for scaled-up displays are now relatively cheap.

Still, expectations may be set too high.

“The general public has for 40 years been seeing cinematic depictions of physically impossible things, and when they do see what’s possible, they are disappointed,” says Bove.

Smalley concedes, “At this stage, you
don’t have to be an expert to realize that
this isn’t the Princess Leia display you
are looking for. But, if given the oppor-
tunity to be developed further, I don’t
think you would be disappointed.”
Researchers may be trying too hard
to make fact out of fiction. “What some
of the people working on volumetrics
haven’t realized is that the key ele-
best material. It seems unlikely that it
is,” he says.

It is possible to produce freestanding volumetric images without injecting particles into the air. More than a decade ago, Hidei Kimura, founder and CEO of Japanese company Burton Inc., and Taro Uchiyama of Keio University found that when focused on specific points, microsecond bursts of high-intensity infrared light could cause air molecules to become glowing plasma. Kimura envisaged the technology being used to create levitating signs above head height for use in emergencies; the bursts would be intense enough to burn the hand of a user foolish enough to try to touch the glowing voxels.

Much shorter pulses could yield a safer system. Yoichi Ochiai of the University of Tsukuba and Kota Kumagai of the University of Utsunomiya in Japan showed at the ACM SIGGRAPH conference in 2015 the results of a prototype based on lasers that fire bursts no more than 100 femtoseconds long. According to Ochiai, users would simply get a tingling sensation from touching the plasma voxels, though users would need to be careful to not let their eyes get too close to the images, as retinal damage is a distinct possibility. Robert Stone, professor of interactive multimedia systems at the University of Birmingham in the U.K., says he has concerns over the eye forming strong afterimages because of the brightness of the plasma.

The plasma projector has the advantage of being far more resistant to disturbance by moving hands than the particle-based option. However, all volumetric displays to date have a common problem, Smalley says: “It is like taking a bunch of fireflies and organizing them into patterns. Everything looks like a ghost. You don’t have the self-occlusion to make objects that look realistic.

“We want to be able to take a point
and have it shine light in only one di-
rection. That would mean it begins to
look solid.”
The lack of self-occlusion in the op-
tical-trap display is, for the moment, a
secondary issue. It is difficult to move
the single particle that flies around the
Brigham Young display any faster than
is possible today; that limits its cover-

“The general public
has for 40 years
been seeing
cinematic depictions
of physically
impossible things,
and when they do
see what’s possible,
they’re disappointed.”

ments are complex movement and dynamics, not super-high resolution,” Blundell argues.

Smalley envisages applications where the user needs to inspect the shape closely and move around it. The ability to produce mid-air streamers in fluid-dynamics simulations and models of organs to help with planning medical operations seem good examples. “A lot of 3D technologies can’t give you a strong spatial sense when you get up close. With ours, you can,” he says.

Bove says by looking closely at requirements for target applications and working with user-interface designers, the developers of volumetric displays can move from experiment to market more easily. “Can it be behind a transparent barrier? Is it important that it be viewable from any angle or is 90 degrees OK? Is it acceptable for it to have moving parts?” he suggests as questions to be asked.

Developing volumetric technologies for specific applications may lead to the problem of no individual market being large enough to support research and development, but such displays look more technologically feasible, Bove says. “The problem with the Leia display is that it needs all of the boxes to be ticked.”

Further Reading

Smalley, D.E. et al

A Photophoretic-Trap Volumetric Display,

Nature, 553, pp486–490 ( 25 January 2018), doi: 10.1038/nature25176

Ochiai, Y., Kumagai, K., Hoshi, T., Rekimoto, J., Hasegawa, S., and Hayasaki, Y.

Fairy Lights in Femtoseconds: Aerial and Volumetric Graphics Rendered by Focused Femtosecond Laser Combined with Computational Holographic Fields, ACM Transactions on Graphics, Volume 35, Issue