New 3-D Projection System (The Economist)

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Here is another article from The Economist (Aug31,96) about the
3-D Projection System, DBrockway posted from LA Times Sept10,96.
The article is on page 67 and starts along the same lines as 
the LA Times piece about the long time ambition to recreate
3-D from ancient times and the tricks and illusions attempted so 
far. I didn't include this lame part of the article and took
excerpts from the most interesting parts.

One of technology's long-held ambitions has been to create truly 3-D 
moving images not in order to send technophobes mad, but for more 
practical purposes. A group led by Elizabeth Downing at Stanford
University, in California, has now created, albeit crudely, just
that. It is an image you can walk around and that really is exactly
where it seems to be.
The prototype display, announced by Dr.Downing and her group in this
weeks Science, is among the first to produce a genuinely 3-D image 
one which observers can see from any position with the standard
equipement provided by nature.
It is rather like a 3-D version of a cathode-ray tube(CRT) the thing
behind most televisions and computer monitors. Whereas a CRT traces 
pictures on a glass sheet, the Stanford device draws them inside a 
glass block. Mixed into the glass are small amounts of certain "rare-
earth" metals-praseodymium, erbium, and thulium-in the form of their 
ions (atoms with some electrons missing).
Shine a light on an atom or ion, and some of it may be absorbed. Each
kind of atom absorbs light of the same frequency, or split into less
energetic bits. Each of the ions used by Dr.Downing's group, though,
can absorb the energy from two successive bursts of infra-red(and 
therefore invisible) light at different frequencies, before getting
rid of it in one go. The combined energy of two lots of infra-red
produces visible light of a certain color. Red for praseodymium,
green for erbium and blue for thulium.
The reserchers have sandwiched together three slices of glass, each
doped with different rare-earth ion, into a block one and a half
centimeters along each side. At each slice they point a pair of infra-
red lasers operating at frequencies chosen to titilate the ions in that
slice. Where the laser beams intersect, they pump their energies into
the ions, which respond to produce a point of colored light 1/10 of a mm
wide. Using tiny computer controlled mirrors to move the laser beams 
around rapidly, a picture (so far, only a geometrical shape) can be
traced out within the glass.
The first attempts to weave this kind of magic, in the early 70's,
were hampered by the absence of materials good enough at converting
infra-red into visible light, and of lasers of the right frequencies.
But the glasses and lasers available today are still far from perfect.
More serious challenges await. The researchers are pleased to have
found subtances that emit red, green , and blue light, since those are
the colors that CRT displays mix to create the full range of hues. 
However ,they cannot yet combine them into a single glass block. When
mixed, the ions tend to release their energy by dissipating it among
their neighbors vibrations, instead of projecting light. Hence the current
three layered cube, which appears to contain something more like a dancing
neapolitan ice-cream than a shimmering full color phantasm.
The researchers have a solution: make the layers extremly thin and build
up the block of alternating red, green, and blue emitting sheaves. This
could work like a CRT screen, which blends each full color dot. Sticking
the sheaves together, though, will require a glue with exactly the same 
optical properties as the glass, so that light can pass smoothly through
the block. Such glues exist but it may take some work to make the joins
seamless.
A more fundamental question is how to make a solid looking object. Real
things are usually opaque, so the front surface hides the back. This helps
you to see them properly. The 3-D display, by contrast, conjures things out
of pure light, and light is not opaque. Watching transparent 3-D tv could
be a strain on the eyes. Dr.Downing is instead working on uses for the
gadget where transparency would be a bonus such as medical imaging, where
the whole point is to see inside.
If technical problems can be cracked, the device may have a future. The
nearest competitor that it has is something created by the American navy:
a tall cylinder containing a spinning, helical surface, which reflects 
colored light carefully aimed at by lasers.




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