So, the problem is to make an invisibility suit, based on
long-known idea of cameras on one side linked to
on the opposite side.
For the sake of simplicity, we could aim first for an
invisibility cube. You want to be able to put this down
somewhere, and have it "invisible"
no matter what
direction it was viewed from.
The first problem with the camera/screen system is that
the cameras themselves will be visible, meaning that you
can have front-to-back visibility but not vice versa.
However, modern cameras can be almost arbitrarily
so that you could design a screen with a camera peering
through an almost-invisible dot in the middle of it. (The
cameras on the back are linked to the screens on the
and vice versa).
Nevertheless, this still leaves the invisible cube
like a collection of small black dots (the camera holes),
which is less than ideal.
(this is the important bit)
LEDs act as photodiodes as well (just not all that
efficiently). You can try this yourself by putting a
voltmeter on a single LED and shining bright light on it.
It should be possible to create an LED-based display
can also work as a camera. The LEDs would be activated
(emitting light) for perhaps 1 millisecond, and would
be turned off and instead used to detect incoming light
a further millisecond. The eye will see continuous
(end of the important bit)
This won't work, because the individual LED elements
detect light coming from all directions - so they won't
as a camera.
The answer is to construct the device like an insect eye,
only more so. Put each pixel (each LED) at the bottom
of a tiny tube (the tubes are orthogonal to the screen -
they point outward). Now it will only see light coming
it square-on to the screen. Collectively, all the pixels on
the screen act (when used in "sensing" mode) as camera,
with a "parallel focus" looking straight out of the screen.
If you now link two of these compound-eye-
cameras/displays back to back, you will have a cube
is completely invisible when viewed from directly in
or directly behind (another two pairs of screens make it
invisible from the sides, top and bottom also).
But however nevertheless.
All of this will only work for viewers standing squarely in
front of (or behind etc) the cube. Anyone looking from
angle will see a black cube: the LEDs are buried at the
bottoms of their tubes. Even if this were not the case,
image they saw would be "off" - at the wrong angle.
But so nevertheless however.
Now we make it a bit more complex. So far, each of our
pixels is just a small tube with an LED at the bottom
(acting as both camera element and display element).
Now imagine that each pixel is actually a little cluster of
tubes, all radiating out like the spines on a sea-urchin.
Call each of these tubes a "sub-pixel". If each pixel
consists of 20 subpixels (pointing in 20 different
directions), then the "invisibility" will work perfectly
each of those 20 angles. If the viewer is standing at, say
degrees to the face of the cube, then they will see the
image which is seen by the sub-pixels which are looking
45 degrees away from the opposite face. Which means
cube will be properly invisible. The more sub-pixels
are to each pixel, the more angles the cube can be
from and remain perfectly invisible.
I hear you say "if each pixel is like a sea-urchin, you
be able to pack them together, and the image will be
dotty" (did you just say that? I think you did).
Aha! Say I. Not so. If you design your sea-urchins
enough, they will pack very closely, with their spines
Of course, if each pixel has 20 sub-pixels, this means
light from any one direction will only be detected with
1/20th the efficiency; but no problem - that's good
(the electronics are sensitive). More worryingly, the
projected image will only be 1/20th as strong, and
on only 1/20th of the surface. But again this is not a
problem. Each sub-pixel can be made arbitrarily small
you don't notice the dottiness), and the brightness can be
racked up 20-fold to compensate. After all, the monitor
you're reading this on has significant black gaps between
the pixels, and you are not aware of those.
(You could also do the same sort of thing with a very
dense flat array of LEDs and a series of micro-lenses, as I
think are used in some displays which give an illusion of
3D, or like those embossed plastic cards which show a
moving image as they're tilted; but the sea-urchin
business is perhaps easier to understand. Either way, the
resolution of the image will be 1/20th the pixel density,
if each pixel looks in 20 directions. Current pixel
densities are maybe 100 per inch, giving a displayed
resolution of 5 per inch - good enough to hide a big box
when looked at from a few yards away.)
So - Gadulka! - a video-based all-round unvisibility shield