How it works
- By George I. Seffers
- Jun 19, 2000
Researchers use multiple laser beams and a crystal of cerium-doped, strontium
barium niobate crystal to create and read a 3-dimensional holographic image
of an object.
The first laser beam is split into two more beams with a laser splitter
or mirror. The two offshoot beams, known as the reference beam and objective
beam pass over the object, intersect at the crystal, creating the image.
The image forms because exposure to light or heat causes the electrons within
the crystal to become excited and to gather together in darker places, such
as the shadow created by object being fixed.
The reference and objective beams require nearly identical light waves
for the best results.
Once the image is stored on the crystal, another beam known as a reader
beam is aimed at the crystal from the opposite direction as the first two
beams. The light goes through the crystal, disperses around the room and
is intersected by a viewer's eye, causing the holographic image to appear
to be floating in space. Because the electrons are once again being exposed
to light, however, the image fades substantially and will disappear altogether
in a relatively short period of time.
In addition, even if the crystal is stored in a dark, cold environment,
the crystal itself generates miniscule amounts of infrared light, and the
image still will disappear over time.
Researchers have stalled the process somewhat by applying an electric
field to the crystal prior to use, thereby causing entire regions of positive
and negative electrons to polarize. The image is then fixed using entire
regions of electrons, rather than individual electrons, creating a limited
immunity to light exposure. This polarizing process has allowed images to remain
fixed for 24 hours despite constant exposure to light. Furthermore, researchers
say they might be capable of using the process to store the image for longer
periods but have not yet tried.
The advantage to having the image disappear is that stored images or
information can be deliberately erased and the crystal reused.
The distance to which an image can be beamed depends only on the power
of the reader laser being used, but because the reader beam is essentially
aimed right at the viewer's eye, only very low-level beams can be used without
causing harm. Projecting the beam directly to the eye so far is the most
promising method of reading an image, but also has some other drawbacks,
including limits on the number of people who might view the image simultaneously.
Large crowds might be problematic.