REFLECTION HOLOGRAM
Preliminary Activity
The 30 mW He-Ne laser on the holography table
takes at least a half hour to warm up before the
intensity of the undiverged beam is large enough
(about 13 mW) for exposing holograms. So, if its not
already turned on, do so immediately at the beginning
of the period. After 90 minutes the laser power will
peak at about 23 mW. But you can begin making
holograms before that if you make correspondingly
longer exposures. Just before each exposure you will
be measuring the reference beam intensity at the film
plane with the laser power meter.
Introduction
Holography is a 3-D image-making process. It
recreates what light actually does after reflecting from
a real object. The first hologram was conceived of
and produced in 1948 by Dennis Gabor, a researcher
at the Imperial College in London, several years
before the invention of the laser. For this he received
the Nobel prize in physics in 1971.
There are a number of different types of holograms.
In a transmission hologram, light from a laser is
spread out by a lens to light up the holographic film
or plate. The viewer stands on the side of the film
opposite the laser, so that the light is "transmitted"
through the hologram, and peers into the "window" to
see the 3-D scene.
A reflection hologram, although it must be made with
a laser, can be viewed with ordinary white light. The
hologram is seen with the light source and the viewer
on the same side of the film, the light reflecting from
the hologram.
A 360o (or cylindrical) hologram allows one to see all
around an object. It is necessary to view it with a
diverged monochromatic beam of light, usually
coming from a laser. In a holographic interferogram,
the same object is used for two exposures on the same
film with the object having experienced a small
change between exposures. When viewing the
hologram, an interference pattern is observed
corresponding to the interference of the light coming
from two slightly different objects. Thus the
hologram enables one to detect minute changes in an
object.
Viewing Holograms
Before trying your hand at making your own
holograms, look at the various holograms that are set
up in lab. In your lab report, briefly describe the
images you are viewing. The reflection hologram is
best viewed with light from a desk lamp. The
transmission holograms and the 360o hologram
should be placed in a diverged laser beam. Shining a
laser on a piece of ground glass or on a plastic screen
are two ways of making such a beam. Some of the
transmission holograms are single-scene and others
are multiplex (two-scene).
Experimental Setup
To make a hologram, a laser beam is split into two
beams, one that directly illuminates the film (the
reference beam) and another that illuminates the
object (the object beam), then reflect to the film.
Since the reference and object beams originate from
the same mono-chromatic (single wavelength),
coherent (in-phase) source of light, the reference
beam interferes with the light scattered from the
object. This creates an interference pattern on the
film that is captured during exposure. Then, when
laser light of the same wavelength passes through the
developed film at the same angle as the reference
beam, the light is diffracted and produces a 3-D
image of the original object. For a reflection
hologram, the film itself serves as the beam splitter
with the direct beam being the reference beam and
the light from the object being the object beam. This
experiment explores reflection holograms.
Part of this experimental setup involves the laser
beam being reflected so as to pass through a camera
and then a spatial filter. The camera provides a
convenient shutter. Use the bulb setting and the
SPECTRAL ANALYSIS
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remote shutter release cable when you are setting up
your object.
The spatial filter helps eliminate noise from dust on
the mirrors and imperfections in the lenses and
produce large, clean beams. It contains a microscope eyepiece and a 10 m pinhole situated at the
focal point of the converging lens (when properly
adjusted).
All optical components have been carefully aligned.
During the experiment, if you do not see the object or
reference beams at their usual brightness, or if
something gets bumped out of position, seek
assistance from the instructor. Do not touch the
surfaces of any of the optical components,
especially the front-surface mirrors.
light-colored. Orient the film/object holder at about a
10o angle with respect to the incident diverged laser
beam. As indicated above, in this geometry, the film
itself serves as the beam splitter with the direct beam
being the reference beam and the light from the
object being the object beam.
Film that fits the film holder has been precut and
placed in a black envelope inside a used film can. The
film (8E75) is sensitive to red light and is available
from Integraf of Lake Forest, Illinois. Similarly, the
glass plates will be placed inside a black envelope in
a second used film can. Close the camera shutter and
cock the camera. Install the film or glass plate in the
holder. Put the cardboard box over the target area.
Everyone except the person opening the shutter of the
camera should move well away from the hologram
table. Wait for about 1 minute for vibrations to
subside.
Procedure
In your lab report you should include a sketch of the
setup for the making of the hologram. Indicate (in a
table on your data page) the x-y coordinates for each
active optical component. Adjacent screw holes are
separated by 1 inch.
Turn out the room lights and close the door to the
store room. There is a green lamp on the holography
table that may be useful when setting up your objects.
Be sure to turn the green lamp off when film is out.
Open the shutter of the camera and expose the film
for about 0.25 seconds (longer if the laser has been
on less than 90 minutes). During the exposure,
everyone in the room should freeze to avoid setting
up wind currents or vibrations. If the film moves as
little as one wavelength with respect to the object
during the exposure, the hologram probably will not
turn out. Put the exposed film and plates into a third
used film can. Take the exposed film and plates into
the darkroom.
Each person will make one reflection hologram using
film and one using a glass plate. For each hologram
in this experiment, record the parameters of its
production:
(a) laser beam intensity of the reference beam at the
film plane (use the laser power meter set on a
lower scale - probably 200 W)
(b) exposure time
(c) development time
After development, use the red wax marker to
identify each hologram in the upper right hand
corner.
Securely mount the object on the surface behind the
opening of the film holder. Best results are obtained
if the object is fairly close to the film plane and is
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SPECTRAL ANALYSIS