07-09-2013, 02:50 PM
HOLOGRAPHY
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Introduction and Background
The aesthetic appeal and commercial usefulness of holography are both related to the ability
of a hologram to store a three-dimensional image. Unlike ordinary photographs, holograms record
both phase and amplitude information. Because phase is a relative property, construction of a
hologram requires a “reference beam” in addition to the light reflected from an object’s surface.
Additional requirements include a powerful, coherent, monochromatic light source (e.g. a laser,
whose workings you should understand); a vibration-free flat surface; some common optical devices;
and photographic plates, chemicals, and a darkroom.
A hologram can be made by exposing a photographic plate to the interference pattern made by
the reference and object beams. The plate is then developed and dried. When illuminated by a
reference beam similar to the original one, it recreates a three dimensional picture of the object.
There is nothing mystical about this recreation, and you can understand how it works with the help
of some Fourier transform mathematics.
Transmission and Reflection Holograms
The first holograms were examples of transmission holography. Transmission holograms require
a monochromatic reconstructive beam. In other words, the image is only visible when the hologram
is illuminated by either a laser or other single frequency source. This limitation arises from the
way a hologram stores information. The interference patterns produced from an object and ref-
erence beam, whose formalism has been derived above, are actually three dimensional hyperbolae
of revolution[3]. The position of the photographic plate relative to the object determines how this
interference pattern is recorded onto the photographic emulsion.
Additional Details
Following certain procedures will help you to ensure success in making your holograms.
1. Recall that in the derivation above we ignored the contribution of |ψ0 |2 in comparison with
|ψ1 |2 ; that is, we assumed that the reference beam was more intense than the object beam.
However, the derivation did not tell us how much more intense the reference beam should be.
It has been found empirically that a reference/object intensity ratio between 3 and 10 yields
good results. A photocell is provided to measure the intensities of the reference and object
beams.
2. Higher intensities (consistent with a suitable reference/object ratio), will need shorter exposure
times, and, will, in general, produce better holograms.
3. A clean, uniform reference beam is needed for a high quality hologram. A pinhole apparatus
can help you achieve this goal. Set it up carefully in order to minimize the diffraction and
interference patterns from imperfect optical surfaces (mirrors, beam splitters, and lenses) in
the path of the reference beam. In addition, you should make sure that the optical surfaces
are clean and free of dust.
4. The path lengths of the reference and object beams should differ by less that the “coherence
length” of the laser. You should understand what this means about the purity of the laser
light.