Holography
Preliminary Activity
There are two alternative lasers you may use for this
experiment. The diode laser only need about 5
minutes to warm up to full intensity. However, 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 you plan to use it and
it is not already turned on, do so immediately at the
beginning of the period. After 90 minutes the laser
power will peak at about 20 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.
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
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.
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
11
Holography
1
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, though
you may attempt a transmission hologram also, if you
wish.
Procedure
In your lab report you should include a sketch of the
setup for the making of the hologram. This sketch
should be to scale with the scale of the drawing
indicated clearly.
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.
It should spread out horizontally in an elliptical shape
to fully illuminate the subject region.
Once you are satisfied with the illumination of the
subject, turn off the main lights, and block the laser
beam with a box, so the subject is no longer
illuminated. Remove one plate or piece of film from
its container in a dark part of the room — keep it
away from the laser light. Close the container to
protect unexposed film. Place the plate on top of the
subject, preferably with the sticky emulsion-side
down.
Wait quietly at least 10 seconds after setup to allow
the subject and plate to settle. Now slowly lift the
box off the table slightly (1 or 2 cm) while still
blocking the laser light. Wait at least 5 seconds for
table vibrations to subside, then lift the box away
slowly (don’t make a breeze). Expose the hologram
for about 10 seconds (you may want to try various
times to see which work best). Bring the box back
down to block the laser light. Place the plate in a
container marked for exposed, undeveloped film.
You may want to expose several plates before
developing. If so, be sure to keep them in order in
the container, so you remember which is which.
Reflection Hologram with HeNe Laser
Before opening the package containing the
holography film or plates, turn out the room lights
and close all doors. If you are working in the
darkroom, you may leave the safety light on. If you
are working in the lab, you may use the green lamp,
but keep it from shining directly or reflecting directly
on the film or plates.
Reflection Hologram with Diode Laser
The setup for making holograms with the HeNe laser
is on the holography table. The optical components
must be 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.
Unscrew the black collimating lens and spring from
the laser and store in the laser’s container. Clip the
laser with a clothespin and stick the other end of the
clothespin in a cup of sand to dampen vibrations.
Position the cup with the laser about one foot above
and one foot horizontally from the subject. Connect
the laser to the power supply, and observe the beam.
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
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
Holography
being the reference beam and the light from the
object being the object beam.
The camera in the setup provides a convenient
shutter. Use the bulb setting and the remote shutter
release cable when you are setting up your object.
When you are convinced that the subject will be
properly illuminated, close the camera shutter and
cock the camera. Change the shutter setting from
bulb to an appropriate time (try various times, ¼ to ½
second may work well). 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.
Open the shutter of the camera and expose the film.
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 half 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. Develop
exposed film and plates in the darkroom.
Holography