HOLOGRAPHY

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• HOLOGRAPHY
• One of the most interesting
• applications of laser light is the
• production of three-dimensional images
• called holograms. In an ordinary
• photograph, the film simply records the
• intensity of light reaching it at each
• point. When the photograph or
• transparency is viewed, light
• reflecting from it or passing through
• it gives us a two-dimensional picture.

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• In holography, the images are formed
• by Interference, without lenses. When
• a laser Hologram is made on film, a
• broadened laser Beam is split into two
• parts by a half-silvered mirror, Fig.1.

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• One part goes directly to the film
• the rest passes to the object to be
• photographed, from which it is
• reflected to the film, and the
• interference of the two beams allows
• the film to record both the intensity
• and relative phase of the light at
• each point. After the film is
• developed, it is placed again in a
• laser beam and a three-dimensional
• image of the object is created.

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• The details of how the image is
• formed are quite complicated. But we
• can get the basic idea by considering
• one single point on the object . In
• Fig.2(a), the rays OA and OB have
• reflected from one point on our object.
• The rays CA and OB come
• directly from the
• source And interfere
• with OA and OB at
• points A and B

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• On the film. A set of interference
• fringes is produced as shown in
• Fig.2(b). The spacing between the
• fringes changes from top to bottom as
• shown. Why this happens is explained
• in Fig.3. Thus the hologram of a
• single point object would have the
• pattern shown in Fig.2(b). The film in
• this case looks like a diffraction
• grating with variable spacing.

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• Hence, when coherent laser light is
• passed back through the developed film,
• the diffracted rays in the first-order
• maxima occur at slightly different
• angles because the spacing changes.

O
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• (remember Eq. sin?m?/d so where
• the spacing d is greater, the angle
• ?is smaller.) Hence, the rays
• diffracted upward (in first order)
• seem to diverge from a single point,
• Fig.4.

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• This is a virtual image of the
• original object, which can be seen
• with the eye. Rays diffracted in
• first order downward converge to make
• a real image, which can be seen and
• also photographed. (Noted that the
• straight-through undiffracted rays
• are of no interest.) Of course real
• objects consist of many points, so a
• hologram will be a complex

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• interference pattern which when
• laser light is incident on it, will
• reproduce an image of the object. Each
• image point will be at the correct
• (three-dimensional) position with
• respect to-other points, so the image
• accurately represents the original
• object. The image can be viewed from
• different angles as if viewing the
• original object.

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• THE THICK, OR VOLUME, HOLOGRAM
• The holograms discussed above have
• been assumed to have negligible
• thickness and are referred to as plane
• holograms. If the recording medium is
• thick with respect to the spatial
• frequency, the interference fringes
• act as a series of ribbons, somewhat
• similar to a Venetia blind. The
• reconstructing beam will generally
• pass through several constraint on the

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• diffraction pattern produced in a way
• similar to Bragg scattering of X rays
• from crystals. In the Bragg-scattering
• experiments, the regularly spaced
• atoms in the crystal act Like partially
• reflecting planes, scattering the waves
• in definite preferred directions see
• Fig.5. In these preferred directions
• the waves reflected from adjacent
• planes differ from each other by

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• Exactly one wavelength and, being in
• phase with each other, produce
• constructive interference.

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• The Bragg-scattering relationship for
• these directions is given by?2dsin?
• where d is the distance between
• reflecting planes, ?is the wavelength
• of the waves, and ? is the reflection
• angle shown in Fig.6.

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• MULTIPLEX HOLOGRAMS
• One of the remarkable features of the
• hologram is its ability to produce
• multiple scenes from the same
• photographic emulsion. If the distance
• between the fringes is smaller than
• the emulsion thickness, each ray of
• the reconstruction light originating
• from the direction of the reference
• beam will pass through several
• partially reflecting planes. see Fig.7.

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• The reflection rays from each of these
• planes must be an integral number of
• wavelengths apart.

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• If the reillumination beam forms an
• angle significantly different from
• the reference beam, the light
• reflected from the adjacent planes
• will no longer be in phase and the
• virtual image will no longer be
• visible. It is therefore possible to
• produce many Holograms in the same
• photosensitive medium, each with the
• reference beam at a different angle.

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• When viewed later, each of these
• images can be separately viewed simply
• by varying the angle of the reference
• beam. This technique has been used to
• store hundreds of images in a single
• crystal of lithium niobate. The
• process is capable of storing an
• entire book in an appropriate medium
• by slightly changing the direction of
• the reference beam with each exposure.

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• When viewing the finished hologram,
• one can turn the page by merely
• moving the reconstructing beam.
• Alternatively, a multiplex hologram
• can be produced by appropriately
• moving the reference beam angle with
• time, thereby producing holographic
• motion picture.

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• WHITE-LIGHT-REFLECTION HOLOGRAMS
• One of the possible arrangements for
• producing white-light holograms is to
• place the photosensitive film between
• the reference beam and the object, see
• Fig.8.

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• Such a hologram is produced simply by
• illuminating the object through the
• photosensitive medium, thus avoiding
• beam splitters, mirrors, etc. In
• practice, the reference intensity is
• so high relative to the scattered
• intensity that the technique is
• limited to shiny objects located close
• to the recording medium. Better
• reflection holograms can be made by
• separating the object and reference

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• Beams. Since the reference and object
• beams are oppositely directed, the
• spatial frequency is extremely high. A
• large number reflecting planes are
• thereby produced, separated by about a
• half wavelength of light. As a result,
• the reconstructing light must be of
• the same wavelength or the reflections
• from adjacent planes will not be in
• phase for constructive interference.
• Alternatively, if the hologram is

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• Viewed in white light, the appropriate
• wavelength will be selected to produce
• the reflected image. Ordinary
• photographic emulsions are of limited
• use as they tend to shrink during
• development.

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• OTHER HOLOGRAMS
• One of the most impressive holographic
• image is formed by a 360 circular
• film. The technique was developed by
• T.H.Jeong using a photographic
• emulsion mounted on a cylindrical
• surface surrounding the object.

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