By Jason Sapan
Holographic Studios
So what are holograms? The best way to think of a hologram is to envision
them as impressions on light waves.
Light is a wave. All waves behave more or less the same. For one thing they
tend to echo. They reflect off of many surfaces. Sort of like sound waves
echoing to make SONAR or microwaves in RADAR. The wave is sent out; it hits
an object; it bounces back. Pretty simple idea. But what you don't think
about is that when a wave bounces up against an object it takes its shape.
Like pressing a piece of clay up against a key. The key leaves a three dimensional
impression in the clay. Well if you imagine the clay as a light wave, basically
the idea of holography is throwing the clay up against the key, having the
key make an impression on the front of the clay, letting it bounce off,
and finally storing the shape of the clay permanently.
Now with SONAR or RADAR we are dealing with waves that are
not visible. You can't see sound waves or microwaves. However with light
waves we are working in the visible spectrum and consequently things that
are visible are things that tend to record on photographic film. So in effect,
a hologram is a photograph of the impression left on the surface of a light
wave after it has bounced off an object.
Film Emulsion
Lets look at a hologram recorded on silver halide film. What is film?
Well, first there is a base material of clear plastic or glass. Then there
is a very important layer that contains the photoreactive chemistry. They
call it the emulsion. Its a very special composition. And there's always
room for it. Its jello. Plain old gelatin, without any flavor or color of
course. Inside the gelatin there are two chemicals joined in a molecule.
The are suspended like fruit in jello. In an emulsion, each chemical retains
its own identity. Just like each piece of fruit floating in the jello does.
First, there's silver. As we all know silver has a very unique property.
It tarnishes when it combines with oxygen. And it turns black. Next, there's
iodine. The stuff you use to kill the germs on a cut. Its a very reactive
chemical. So reactive that when it mixes with the silver to make silver
iodide, it results in a silver that will tarnish very quickly.
When a light wave goes into this layer of jello its energy is transferred
to the silver iodide molecule. Remember how a light wave looks like a corkscrew?
Well, try to imagine this corkscrew winding up its energy into the silver
iodide molecule just like a wind-up toy. You give it a good twist and the
energy goes into making the toy run. In the case of the silver iodide molecule
you give it a good wind up of light energy. Its sort of like setting a bear
trap. Your are putting your energy into pulling the trap open. Now its set
to snap shut. The same thing is happening in the silver iodide molecule.
Light gives it energy to be ready to snap onto another atom. When you put
it in a bath of photographic developer, it grabs hold of the oxygen in the
bath, and tarnishes. That's why black and white negatives are black. And
so are holograms before they are bleached. So, you can think of photography
or by extension holography as the art of selectively tarnishing silver in
jello where light has energized it.
Now in the case of a hologram, the patterns of light wave impressions are
what is being photographed in the layer of emulsion. Generally film emulsion
in holography runs about 10 microns thick. A micron being a millionth of
a meter (a meter is about a yard) in size. That's pretty small, but a photon
measures about a half of a single micron in size. That's smaller than an
ants asshole. So the emulsion seems pretty large to a photon. That's how
we are able to photograph this microscopic wave impression in film and make
holograms. Holograms are photographs of the three dimensional impressions
stored on light waves. Sort of like fossils.
When you pour plaster into a fossil, let it harden, and then remove it,
you have a three dimensional sculpture of the impression that was left in
the stone. Similarly, when you pour jello into a jello mold, let it set,
and then remove it, you have a three dimensional sculpture of the shape
of the jello mold. And when you put light into a hologram, you get a three
dimensional sculpture in light of the object that left its impressions on
a photon and was captured within the thickness of a photographic emulsion.
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