A
hologram is a diffraction pattern recorded on holographic film created by the
interference pattern between an object and a reference beam. For a
comprehensive look at holography click here. For more general
description of holograms click here. For a
history of holograms and holography click here.
Holograms
are made using lasers,
this is because they are highly monochromatic and coherent which is necessary
to create a good diffraction pattern. There are two main types of hologram, reflection
and transmission. They are named after the way in which they are used.
Reflection holograms are viewed by reflecting light off of the hologram; these
can usually be viewed in white light. Transmission holograms are viewed by
transmitting light through the hologram. To view a transmission hologram it is
necessary to use laser light of the same frequency used to create the hologram.
Below
are the pictures of the basic setup for creating transmission and reflection
holograms.
Figure
1. A basic multi-beam setup that can be used to make transmission holograms.
Figure
2. Basic setup for creating a reflection hologram.
When
creating holograms it is important to make sure that the path length that the
light travels is less than the coherence length of laser being used. The
coherence length is the region of space where the light is spatially and temporally
coherent. To find the coherence length of a laser, one can set up a
Michelson interferometer as described here for a HeNe
laser.
It
is also important to have the apparatus vibration free. This can be several
ways. The easiest way is to create a reflection hologram and lean the object
you are trying to image on the photographic plate. This way if the entire
apparatus is moved, the object and the photographic plate will move together
with no relative movement between them. When creating transmission holograms
things get a little more complicated, the apparatus must be built on a platform
that helps to minimize vibrations. This can be as simple as a sandbox or as
complex as an optic bench designed for laboratory and industrial use, such as
the tables seem here.
Special film must also be used to create holograms. Holograms are not developed
like photographs, when developed the film becomes the hologram; it is not
developed on paper like photographs. The type of film used also varies
depending on the type of hologram being created. Film for transmission
holograms is usually thinner allowing more light to pass through the hologram,
while film for reflection holograms is thicker so there are more particles for
light to bounce off of. Supplies for creating holograms, including film can be
found here.
With
only a basic knowledge of holography it is possible to create holograms. It
doesn’t take an expensive laser; holography can be done with a laser pointer. But with
more advanced skills complicated holograms can be created such as those
described here.
Holographic portraits
can even be taken.
And there are many practical uses for holography
too. Holograms are used on credit cards and other things for security purposes. Digital holograms have been
developed that are truly three dimensional and can help in medical imaging and
other areas. Holograms are even being created on coins in
Holograms
are not only fun but they incorporate many area of physics and have many
practical applications!