http://optics.org/articles/news/8/8/39/1
"The photodetectors on the rear surface are
used to record the intensity and color of a
source of illumination behind the object.
The light emitters on the front surface then
generate light beams that exactly mimic the
same measured intensity, color and trajectory.
The result is that an observer looking at the
front of the object appears to see straight
through it."