How Holograhy Works

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• How Holograhy Works
• Outline
• What is a hologram?
• How is a hologram made?
• How is a hologram viewed?
• How are Holograms differed from Photographs?
• Various types of holograms
• Mass-Produced Holograms
• Applications of Holography

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• What is a hologram?
• It is a light wave interference pattern recorded
  on photographic film (or
• other suitable materials)
• 2. How is a hologram made?

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A laser beam is split into two beams see
schematic diagram above The reference beam U1
is spread by a lens or curved mirror and aimed
directly at the film plate. The object beam is
spread and aimed at the object. The object
reflects/diffracts some of the light U2 on the
holographic film-plate. The two beams interact
forming an interference pattern on the film. This
is the hologram. Laser light is needed because
hologram is made by interfering coherent waves
(of same wavelength and phase). The principle
of holography was discovered in Britain by Dennis
Gabor in 1948. He was awarded the Nobel price for
this discovery in the early 70's.
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To gain some insight into the optical
interference pattern on the film, let us first
consider the object being a single point and the
reference beam being a spherical wave. (In
practice, an object consists normally of many
points. Then, the hologram contains the
interference patterns from the reference beam and
the diffracted waves from all object points on
the film.) The interference between the reference
spherical wave and the diffracted spherical wave
from one point object will be similar to that
shown in the following figure.
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Let U1 and U2 be the reference and the object
wave, respectively.
The intensity of interference I on the film (z
d) will be
I U1 U22 (U1 U2) (U1 U2 )
This equation shows that the fringe spacing (l/q)
of the interference pattern is inversely
proportional to the separation between the two
points.
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Next, the hologram is to be properly developed so
that its transmittance is proportional to the
incident intensity that was recorded.

Transfer characteristics of a photographic
transparency near the linear region of the
transmittance vs. exposure curve.
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3. How is a hologram viewed? When the hologram
is illuminated by the reference beam from its
original direction, a 3-dimensional image of the
object appears where the object was originally.
Most holograms must be viewed with laser of the
same color. (Some holograms recorded with
multi-color lasers can be viewed with the same
multi-colors lasers.)
Holographic recording
Holographic reconstruction
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Mathematically, when the hologram is illuminated
with the reference wave Ur, the wave U is
transmitted through the hologram.
The third term on the right-hand side of this
equation is the original wave multiplied by the
intensity Ir of the reference wave, the virtual
image. If Ir is uniform (independent of x and
y), e.g. a plane wave Ir ½ exp(-jkz), this third
term constitutes the desired reconstructed
wave. The fourth term is a conjugated version of
the original wave modulated by Ur2, the real
image. If the reference wave is a uniform plane
wave Ur2 (x,y) Ir ½ is a constant
independent of x and y. The first two terms
represent the reference wave, modulated by the
sum of the intensities of the two waves. The
wave associated with the third term will
physically be separated from the waves
associated with the other terms for hologram
viewing, when the angle q between the reference
and the object beams is sufficiently large.
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4. How are Holograms differed from
Photographs? Holograms (recording light wave
interference patterns) do NOT look like images
on photographs. The recording medium needs to
accommodate higher resolution interference fringe
patterns. Holograms can reconstruct
3-dimensional images. Part of a hologram can
reconstruct the same a 3-dimensional image
because the object wave spreads over the hologram
during recording. Holograms can be multiplexed,
i.e. recording multiple holograms on the same
recording medium.
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• 5. Various types of holograms
• There are many ways to classify holograms. For
  examples, they can be
• classified according to the various ways a
  hologram or multiple holograms
• are recorded.
• 5.1 Different geometry of recording a hologram
  
• Transmission, thin holograms They are made with
  both beams approaching
• the film from the same side.

Holographic recording
Holographic reconstruction Thin holograms
are recorded when the fringe spacings are large
compared with the thickness of the recording
medium.
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• Transmission, thick (volume) holograms They are
  made with both beams
• approaching the film from the same side at large
  angles q between them.
• Large angle recordings reduce fringe spacings.
  Thick holograms are recorded
• when the fringe spacings are small compared with
  the thickness of the recording medium.

Holographic Recording
Holographic Reconstruction
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• Reflection, thick (volume) holograms They are
  made with both beams
• approaching the film from opposite sides.

Holographic Recording
Holographic Reconstruction
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• 5.2 Different ways to record multiple holograms
  (a composite or a
• Multiplexed hologram)
• Spatial multiplexing multiple hologram elements
  are made using different
• parts of the recording medium (a composite
  hologram).

Each hologram element is formed with the
identical reference wave and with standard
holographic procedure. However, the object,
mounted on a turntable, is rotated through a
small angle between exposures. A mask, an opaque
plate with a single transparent slit placed over
the recording medium during its exposure, defines
the width of the hologram element. Between
exposures the mask is translated the width of an
element in a horizontal direction opposing the
direction of object rotation. The full width of
the recording medium is thus exposed strip by
strip in sequence.
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Illumination of the entire multiplexed hologram
with a beam duplicating the original reference
allows the viewer to see the image rotate in the
same direction as did the object during
formation of the multiplexed hologram, when he
scans across the composite hologram.
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Angular multiplexing multiple holograms are made
using different angles q between the two
recording beams to expose the same area of the
recording medium (a multiplex hologram). For
example, one of multiple hologram elements is
recorded, when the angle of the reference beam is
changed incrementally and the object is rotated
incrementally. Then, to view the reconstructed
images from different rotational angles, the
angle of the reference beam can be scanned
incrementally.
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5.3 Color holography Only light of the
same color can interfere. (Light of different
colors cannot interfere.) Holograms are
recording of interference patterns.
Reconstructing holograms with a different
color than that used in recording give false
images.
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To overcome these problems of false images in
reconstruction, one can either code the reference
beams or make a composite holograms
Coded reference beams (a multiplexed
hologram) The reference beams are coded by a
diffusing screen. Different color (wavelength)
sees the diffusing screen differently.
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• A composite color holograms
• If the hologram is recorded as a composite of
  many small areas, each of which records
  holographic fringes formed with one wavelength
  only, false
• reconstruction and false images can be eliminated
  entirely. For example, one can overlay the
  panchromatic recording material with a mosaic of
  color filters. Each filter allows only one
  wavelength component to expose the material
  behind it. After processing the holographic
  plate, the hologram must be placed back in
  register with the mosaic.

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• Mass-Produced Holograms
• Copying with Photographic films

Polymer Instead of films, use polymer made
from light sensitive plastic. The Polaroid
Corporation mass produced holograms by this
method. Dichromates Made on a light sensitive
coating of gel that contains dichromate. Very
vivid holograms on jewelry, watches,
etc. Embossed Made by stamping a computer
synthesized hologram with surface profile on
foiled backed mylar film using a metal master
(most common method)
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• Applications of Holography
• Holographic Art Holography museums (Museum of
  Holography
• Chicago http//www.holographiccenter.com/museum1.h
  tm A great exhibit
• of artistic and medical holography), advertising
  (holographic T-shirts
• http//hmt.com/holography/index.html), jewelry,
  postage stamps, etc.
• Security from Forgery Credit cards, currency (
  http//www.holo.com/),
• tickets, etc. Credit cards become more secure,
  since holograms are more difficult to replicate
  (fewer people can replicate holograms well.)
• Also, since hologram reconstructions require
  read-out beams the same as the reference beams
  used in hologram recordings, one can control the
  security of hologram reconstruction by
  controlling the complexity of the reference and
  read-out beams.

Optical devices Diffraction gratings,
holographic lenses, etc. These are holograms in
which the object is a mirror or a lens. A flat
mirror as an object produces a diffraction
grating. A lens or a concave mirror as the
object produces a hologram that behaves like a
lens. These holographic optical devices can be
lighter or less expensive than traditional
mirrors and lenses.
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Holographic Interferometry A very precise
technique for measuring changes in the dimensions
of an object. Useful in industrial stress
analysis (non-destructive testing), etc. The
technique involves interference between two
waves, one of the two waves is the
reconstruction wave from the hologram and the
other is the wave from a changed object. Or, the
two waves can be the reconstruction waves from a
double exposure hologram (one exposure before
and another after the object is changed).
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The two wave can also be the reconstruction waves
from a double Exposed hologram, one exposure
before and another after the object is changed.
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Medical Applications Combining CAT scans into a
3-dimensional image Ultrasound holography, etc.
Holographyic microscopy The attraction of the
hologram is the large depth of field available.
In using a conventional microscope, a series of
photographs could be taken with the microscope
focused at different planes. But, if the object
is in motion, e.g. in studying the growth of gas
bubbles in a liquid under pressure, the state of
the object may have changed before the series of
photographs is completed. If a hologram is made,
the investigation of the various planes of the
object can be made at leisure.
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Web- site References Liti3D a division of Liti
Holographics (http//www.litiholographics.com/) A
new, creative company that can convert your photo
album or video clip into a stunning hologram at
a very reasonable cost. Hypermedia Technologies
(http//hmt.com/holography/index.html ) Multiple
commercial site for several companies. AD2000,
Cherry Optical, Holaxis, ...etc. Shop for
holographic T-shirts, holograms, etc.. Museum
of Holography--Chicago (http//www.holographiccent
er.com/museum1.htm ) A great exhibit of artistic
and medical holography. Hologram bargains and
holography classes available. Token admission
cost.