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From: halazar@mit-amt.MEDIA.MIT.EDU (Michael Halle)
Newsgroups: comp.graphics
Subject: Re: What's a hologram?
Message-ID: <591@mit-amt.MEDIA.MIT.EDU>
Date: 25 Aug 89 07:07:40 GMT
References: <4791@portia.Stanford.EDU> <1746@dover.sps.mot.com> <2330@ucsfcca.ucsf.edu>
Organization: MIT Media Lab, Cambridge, MA
Lines: 90
In-reply-to: root@cca.ucsf.edu's message of 25 Aug 89 02:02:03 GMT


Most generally, I guess, a hologram has the ability to record intensity
and phase of light.  That permits the holographic medium to bend light.
Very commonly, this bending is done with a pattern of fringes recorded
in an emulsion.  In the type of hologram with which most people are
familiar, a laser transmission hologram, laser light bouncing off an object
interferes with beam coming straight from the source and produces an
interference pattern (a bunch of fringes, 300 or so per mm) recorded 
on a photographic plate.  

When lit with an illumination beam of laser light coming from the same
direction as the reference beam, the developed holographic plate sends
out rays of light that exactly duplicate those that came from the original
object.  So you can think of the hologram as a window frozen at exposure,
endlessly replaying light from the object.

A laser transmission hologram is just a blur in white light because
many vertical perspectives of the object, each perpective a different
color, all smear together, the smear proportional to distance from
object to hologram.  To be able to be seen in white light, the object
must be as close as possible to the holographic plate.  In fact, a
laser transmission hologram is often used as a master hologram,
illuminated so as to project an image of the the object out into
space.  The plate is masked so that only one vertical perspective view
is recorded.  A second, or transfer, hologram is placed to straddle
some plane of the projected object.  When this plate is lit with white
light, a spectrally colored image of the object is presented to the
viewer.  This type of hologram is called the rainbow or Benton
hologram.  Using similar techniques, achromatic holograms or full
color holograms can be made.

Real image holograms can only be made of objects stable enough to hold
still during exposure and small enough to fit on a holographic plate.
(The National Geographic globe-o-gram and holographic portraits of
people are made with a pulsed laser, so even relatively instable
objects can be imaged because they can't move very much during the
extremely short exposure.)  Holographic stereograms, on the other
hand, can be made of anything that can be imaged from different
directions.  Holographic stereograms are made by replacing the object
with a projection screen.  A single, 2D perspective view of the object
is projected with laser light onto the screen.  This view can be made
with a movie camera, a scanning electron microscope, or a computer,
for example.  A vertical slit of the holographic plate is exposed to
the projection screen and to a reference beam.  The vertical slit is
then moved and a new perspective view is projected.  This procedure is
repeated a hundred times or so, producing a bunch of eye-pupil-width
"windows" when illuminated, each one a window onto the view projected
when the slit was made.  A two eyed viewer sees one view with one eye,
another view with the other.  The original views are computed so that
these two views are a stereo pair.  As the viewer moves left or right,
different pairs are presented, precisely the views the eyes would see
when viewing the original object.  When this hologram is made white
light viewable, these windows are projected out into space at a
convenient viewing distance.  Holographic stereograms can also be made in
full color.

CGHs, or "computer generated holograms" (sort of a misnomer, as
computers can do much towards making stereograms), are made by
calculating the fringe pattern necessary to bend light to image points
of light in space.  The pattern is typically written onto a substrate
with an electron beam writer.  Because the pattern is so fine,
computation takes a very long time.  As a result, the technology is
not really practical for display holographic purposes right now.  They
are most often used in optical patten matching and other forms of
optical computing.  Dynamic holography, or holographic video, is based
on some aspects of CGH.  The state of the art in holographic video
(last night, or so) is thirty-two scanlines lines of a static, 3D
triangle.

The "surface molded" prints that you are talking about are called
lenticulars.  Lenticulars are much older than holograms, so they
aren't really rip offs of holographic technology.  These images are
recorded photographically, not holographically.  The pattern on the
surface is a whole bunch of cylindrical lenses.  These lenses work
similar to a holographic stereogram in that they project discrete
images of the object out to different positions in space.  The image
is recorded on a print behind the lenses.  Only a small number of
views can be recorded, so the image repeats as you move from side to
side.  In fact, at some positions, the left and right views may be
reversed, producing an inside out, or pseudoscopic, image.
Lenticulars are thus cheaper to make but have a lower image quality
than holographic stereograms.

It's great to see curiosity about 3D, especially multi-perspective
technologies.  After all, people don't have two eyes just to fill
their faces.

						--Michael Halle
						  Spatial Imaging Group
						  MIT Media Laboratory