How it works

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.