IEEE Spectrum November, 2014 - 56

ghost or ghoul? The man with upraised arms [top] has
disappeared into the scenery-a trick known as optical camouflage.
He first had to don retroreflective material; the observer had to put
the projection system on his head. The man in the bottom photo is
wearing the reflective material under his shirt, so his bones seem
to be bursting from his chest.

To do this, you could give a projector and the associated half mirror to each person, to be worn on the head
or installed over it. The other method is to make an
autostereoscopic 3-D display that produces a hologramlike image in the round by using a large number of
viewpoints, created by an array of RPT projectors. To
present motion as smooth as a movie's panning shot,
you would need several dozen projectors. However,
because commercially available projectors are bulky,
it would be hard to pack them closely, and so their
various viewpoints would necessarily be very different from one another. When you viewed the resulting
3-D image in the round, it would lurch from one perspective to the next in herky-jerky fashion instead of
moving smoothly.
We therefore developed a workaround that consists
of a projector, a lens array, a half mirror, and a retroreflective screen. We also included an additional lens to ensure
that each lens in the array is equally well illuminated. Projected light forms an image on the field lens. Each lens of
the array works as a single projector, and they are placed
so that their projected areas overlap. As a result, the number of lenses in the array is equivalent to the number of
projection sources and, in turn, the number of viewpoints.
We implemented our basic, one-observer system in a
car by applying retroreflector screens to the passengers'
doors and to the dashboard. In our initial experiment we
used only one camera and fixed the projector to the ceiling, just above the driver's head. Even though we hadn't

more prActicAl ApplicAtions include retroreflective
gloves that allow assembly workers to see fine work that their
own hands would otherwise obscure.
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corrected the image to accurately reflect the distance of the
camera from the driver's eye, this stripped-down system
made blind spots clearly visible and the car's structures virtually transparent. Furthermore, the displayed image was
sufficiently bright to see even in the daytime.
Next we built a setup specifically for a Toyota Prius that
made the backseat virtually transparent, so that the driver
could see everything behind the car when driving in reverse.
In this application, the system had one projector and six lenses,
allowing the driver to act naturally without the system's
having to track the movements of the driver's head. The
result is a panoramic view that the driver can use intuitively-
quite unlike the current, counterintuitive system, which
sends output from a rearview camera to a display mounted
on the dashboard. We are now collaborating with several
automakers and automotive electronics companies in turning our concept into a commercial system.
Besides making cars safer and driving more enjoyable, this
technology could be used in the cockpit of an airplane to make
the floor transparent during landing, enabling the pilot to see
the runway in full view. In a cruise ship, it could provide an outdoor view for windowless rooms by making the walls appear
invisible. That way, even the innermost cabins could have
virtual portholes. It could work in office buildings, too, offering the denizen of every cubicle a wonderful wide-angle view.
With our RPT system, these forms of augmented reality are practical right now. As engineers continue to
improve active displays, such as OLEDs, by making them
brighter and by manipulating them so that they conform to complex surfaces, our technology may find wider
application-wherever we need to see through things. n
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Table of Contents for the Digital Edition of IEEE Spectrum November, 2014