In Real Time Catching a Glimpse of a Moving Target Around Corner

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Camera technology | Round the Bend; Efforts to see around corners get a boost from better detectors. Economist, Dec 12, 2015.
http://www.economist.com/news/sc ... etectors-round-bend

Note:
(a) For laypersons, this retells

Gariepy G et al, Detection and Tracking of Moving Objects Hidden from View. Nature Photonics, _ : _ (online publication Dec 7, 2015)
www.nature.com/nphoton/journal/v ... hoton.2015.234.html

, where Daniele Faccio, a male, is the last author.
(i) At the introduction, the Nature report said, "Techniques for imaging static objects that are hidden from view * * * have been recently demonstrated" since around 2009.
(ii) The same introduction pointed out that the achievement in this report is:

"In the following, we first show that we can locate the position of an object hidden behind a wall with centimetre precision, without the need for pre-acquiring a background in the absence of the object. We then show that real-time acquisition is possible for an object moving at a few centimetres per second.

(iii) The report described methodology:

"As in many real-life situations, there is not always a conveniently placed wall, door or window that can be used as a reflective surface to send and collect light, so we rely only on the floor, in this case, a piece of white cardboard. The camera is therefore imaging a patch of the floor that is just beyond the edge of the obscuring wall. We then send a train of femtosecond laser pulses to the floor, 15 cm to the left of the field of view of the camera.


(b) "Dr Faccio’s group is one of several trying to do this. All use a method similar to that employed in radar, except that it substitutes light for radio waves and is thus known as LIDAR."
(i) "The term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging."  Wikipedia
(ii) Lidar
https://en.wikipedia.org/wiki/Lidar
(Although thought by some to be an acronym of Light Detection And Ranging, the term lidar was actually created as a portmanteau of "light" and "radar")
(iii) Both visible light and radio wave are electromagnetic radiation
https://en.wikipedia.org/wiki/Electromagnetic_radiation
(section 3 Electromagnetic spectrum: graphic)

(c) Economist: "Unlike conventional LIDAR, though, the pulses sent out from looking-around-the-corner equipment have to bounce off three things before they make it back to the detector. First, they must be reflected from something in the line of sight of their source. This pushes some of them in the direction of the hidden target. Then, they have to be reflected from that target back in the direction of the first reflector so that they can undergo a third reflection, towards the device that sent them out."

Why "bounce off three things" instead of one (target to something at the corner to the equipment)?   

This is because, obviously, a laser beam is imperative, shining near the equipment and then onto the floor 15cm to the left of the around-the-corner focus of the equipment.