Scientists have found a way to photograph people in 3D through walls using WiFi



amplifi meshpoint covers home in wifiYouTube/AmpliFi

Wi-Fi can pass through walls.

This fact is easy to take for granted, yet it’s the reason we can
surf the web using a wireless router located in another room.

But not all of that microwave radiation makes it to (or
from) our phones, tablets, and laptops. Routers scatter and
bounce their signal off objects, illuminating our homes and
offices like invisible light bulbs.

Now, German scientists have found a way to exploit this property
to take holograms, or 3D photographs, of objects inside of a room
— from outside it.

“It can basically scan a room with someone’s Wi-Fi transmission,”
Philipp Holl, a 23-year-old undergraduate physics student at the
Technical University of Munich, told Business Insider.

Holl initially built the device as part of his bachelor thesis
with the help of his academic supervisor, Friedemann Reinhard.
The two later submitted a study about their technique to the
journal Physical Review Letters, which
published their paper
in early May.

Holl says the technology is only in its prototype stage and has
limited resolution, but he is excited about its promise.

“If there’s a cup of coffee on a table, you may see something is
there, but you couldn’t see the shape,” Holl says. “But you could
make out the shape of a person, or a dog on a couch. Really any
object that’s more than 4 centimeters in size.”

How to see through walls with Wi-Fi

The ability to see through walls using Wi-Fi has been around for
years.

Some setups can detect home intruders or track moving objects
with one or two Wi-Fi antennas. Others use an array of antennas
to build 2D images. But Holl says no one has used Wi-Fi to
make a 3D hologram of an entire room and the stuff inside of it.

“Our method gives you much better images, since we record much
more signal. We scan the whole plane of a room,” he says.

Holl’s method differs from the others in few significant ways.


wifi holography antenna take pictures through walls holl reinhard prl 1Philip
Holls and Friedemann Reinhard/Physical Review Letters (CC BY
3.0)

First, it uses two antennas: one fixed in place, and another that
moves. The fixed antenna records a Wi-Fi field’s background, or
reference, for the spot it’s placed in. Meanwhile, the other
antenna is moved by hand to record the same Wi-Fi field from many
different points.

“These antennas don’t need to be big. They can be very small,
like the ones in a smartphone,” Holl says.

Second, both antennas not only record the intensity (or
brightness) of a WiFi signal, but also its phase: a property of
light that comes from the fact it’s a wave. Laser light is all
one phase, for example (which makes it an excellent method to
create
holograms
), while an incandescent bulb puts out a mix of
different phases of light. Similar to lasers, Wi-Fi routers emit
microwave radiation in discrete frequencies and phases.

Finally, the signals from both antennas are simultaneously fed
into a computer, and software teases out the differences of
intensity and phase “more or less in real-time,” says Holl.

This is where the magic happens: The software builds many
two-dimensional images as one antenna is waved around, then
stacks them together in a 3D hologram. And because Wi-Fi travels
through most walls, those holograms are of objects inside a room.

Holl and Reinhard’s first holograms are of a shiny metal cross
placed in front of a Wi-Fi router:


wifi holography antenna take pictures through walls holl reinhard prl 2Philip
Holls and Friedemann Reinhard/Physical Review Letters (CC BY
3.0)

The resulting images may not look like much, but they prove the
concept works: The moving antenna can capture Wi-Fi shadows and
reflections of objects in 3D, right through a wall.


wifi holography antenna take pictures through walls holl reinhard prl 3
A
Wi-Fi hologram of a cross. Holl’s technique can capture the WiFi
shadow cast by the object (left) through a
wall.

Philip Holls and Friedemann
Reinhard/Physical Review Letters (CC BY 3.0)


The applications for Holl’s Wi-Fi holography, he says, are pretty
expansive. Adding an array of reference antennas, say, inside of
a truck, might help rescue workers detect people in rubble left
by an earthquake — or spy agencies see if anyone is home.

“You could probably use a drone to map out the inside of an
entire building in 20 to 30 seconds,” he said.

Holl created the video below to show how his team’s technology
works.

__

This article and images was originally posted on [Business Insider] May 22, 2017 at 05:09AM

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