Holography
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 or from a slide projector with a
special filter.
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. So 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).
Also, look at the 120o rainbow hologram. Turn on
the white light bulb inside and make observations
while rotating the hologram. Move your head up and
down and note the color variation.
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). Since the reference and
object beams originate from the same monochromatic (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
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diffracted and produces a 3-D image of the original
object.
place a tray in the deep sink for washing the film in
cold running water.
An experimental layout for making a transmission
hologram is shown in Figure 1. The camera provides
a convenient shutter.
When you are finished for the day, pour the
used D-19 down the drain (while running the
water) and pour the other three back into their
respective bottles.
Use the bulb setting and the remote shutter
release when you are setting up your object.
The beam-splitter reflects about 50% of the incident
beam to become the object beam, while the
transmitted portion becomes the reference beam. The
neutral density wheel is used to vary the intensity of
the reference beam. The spatial filters help eliminate
noise from dust on the mirrors and imperfections in
the lenses and produce large, clean beams. Each one
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.
Procedure
You will be making these holograms during two
different lab periods. One lab report is sufficient for
this experiment.
The setup on the holography table may differ from
Figure 1. In your lab report you should include a
sketch of the setup for each type of 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.
In the darkroom, pour the chemicals into the glass
dishes. Pour undiluted Kodak D-19 developer into
the first glass dish, stop bath into the second, fixer
into the third, and Photoflo into the fourth. Also
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.
1. Reflection Hologram
Each person will make one reflection hologram. 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 in the hole
of the film holder. Best results are obtained if the
object is fairly close to the film plane and is lightcolored. Try using coins or buttons. Orient the
film/object holder at about a 10o angle with respect to
the incident diverged laser beam. 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, reusable packet inside a white
envelope. The film (8E75) is sensitive to red light
and is available from Integraf of Lake Forest, Illinois.
Close the camera shutter and cock the camera.
Install the film in the film holder, put the cardboard
box over the target area, and wait for about 1 minute
for vibrations to subside.
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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.
Take the exposed film into the darkroom and place it
in the developing solution until it darkens to about
70% of completely black (usually about 2 or 3
minutes). Agitate the film occasionally while it is
developing. Then rinse the film in the water tray for
1 minute. Next, put the film in the bleach until it is
clear (approximately 30 seconds), followed by
another 1 minute rinse in the water tray, 30 seconds
in the Photoflo, and another 30 seconds in the water
tray. Finally, blow it dry with the air gun.
Table 1. Making a reflection hologram.
Action
Time
1
Expose
0.25 sec
2
Develop until 70%
dark
2-3 min
3
Rinse in water
4
Bleach until clear
5
Rinse in water
1 min
6
Photoflo
30 sec
7
Rinse in water
30 sec
8
Dry with air gun
1 min
30 sec
2. Transmission Hologram
Each person will make one transmission hologram.
While one person is developing exposed film,
another can be setting up to make an exposure.
Place your object on the circular stand so that it is
completely illuminated. Small white objects work
best. Use the laser power meter to check the intensity
of both the object and reference beams at the film
holder. For best results, the intensity of the reference
beam should be about twice that of the object beam.
You can rotate the neutral density wheel to adjust the
intensity of the reference beam.
Close the camera shutter and cock the camera. Install
the film in the film holder and wait for about 1
minute for vibrations to subside. Expose the film for
2 seconds.
Develop the film in the same way as you did the 360o
hologram.
Observe your hologram in one of the diverged laser
beams. Label the side from which you observe the
image. Record your observations. Each of you
should include a good transmission hologram with
your lab report.
Analysis
Study the holograms that you have made. Discuss
your results, commenting on possible ways of
improving the procedures or on possible applications
of what you have learned.
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Figure 1. Possible layout for making a transmission hologram.
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Figure 1. Possible layout for making a transmission hologram.
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