From: jlevine@rd.hydro.on.ca (Jody Levine)
Subject: holography?
Organization: Ontario Hydro - Research Division
Date: Thu, 17 Dec 92 23:16:59 GMT

Someone was asking about how holography works. I was hoping that someone
else would answer, but since I haven't seen anything, I'll take the
task on. I'll talk about regular, monochrome laser holograms. Feel free 
to flame me, it's not my field.

Take an ordinary transparent piece of film, stick it in front of an object,
and illuminate the object with a laser. The light will reflect off the
object in all directions, and some of it will be absorbed, so you get
a light/dark pattern from the object. Now sit on a grain in the film.
You'll receive rays of light from all over the object, add up the amplitudes
and react accordingly. If someone looked at the result, he'd see an
unfocussed mess becuase we didn't have a lens :-) and all of the grains saw
pretty much the same thing (a ray from everywhere on the object).

Now get a new piece of film. This time split the laser beam with a mirror
so that the film receives some light from the object and some from directly
from the laser (reference). Now you still see light coming from all over
the object, but wait! It's interfering with the reference and you're getting
a different pattern depending on the difference in distance travelled between
the reference and the object beam. Object rays coming from directly in front
of you are closer than those from way off to the side, so some cancel out with
the reference and some add up. This way, all of the grains get a different 
view depending on where on the film they are. We don't have a picture,
but an interference pattern.

To play the hologram back, you must use the same laser to send light
through the hologram. The light DIFFRACTS around each of the grains, and
since you get a different amount of diffraction from each grain, you get
another interference pattern, percieved as light and dark, in your eye.
This is the picture. The kicker is that you get a different pattern 
depending on your view angle, becuase each grain spreads the light out
in all directions and the light from the far side of the hologram
with the light coming from directly in front of you. Before you were the
film that saw an interference pattern between the reference and the object,
now you're an observer seeing and interference pattern caused by the
diffraction of the reference around the film.

What's neat about this, is that you can use either positive or negative film
and it will still look positive because your eye doesn't notice the 180
degree phase shift, only light and dark. You can also, using thick film
and three light sources, get colour holograms on black and white film,
but that's another story.

As I mentioned before, since were not focussing the light, one
point on the film sees many points on the object, and the direction 
information (where the light came from) is preserved. If you have a
small piece of a hologram, you can look at it as if you were looking
through a window and, depending on where you look from, you can see
various points on the object. You can't see the whole object at once,
but you can adjust your viewing angle to select which part you'd like
to see. 

That's how a small piece of a hologram can have information
about the whole object. Note that all of the pieces have slightly different
information about it, because they each saw the object from a different 
angle.

I hope that this helps, and that the simplifications are not too inaccurate.

J