Download HOLOGRAM(REFLECTION)

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.
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
11
Holography
1
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 is associated
with 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
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.
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.
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
light-colored. Try using coins or buttons. 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.
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.
Holography
5.
Dark Room
6.
Chemical Preparation:
1.
2.
3.
4.
5.
Set out four glass trays.
In the first glass tray, mix 75 ml of Stock
Solution A with 75 ml of Stock Solution B.
This will be the developer or “working
solution” and should be discarded after use.
Fill the second glass tray about 1/4 full with
distilled water, which can be discarded
when finished.
Fill the third glass tray about 1/2 full with
the Bleach Solution. This can be returned
to the bottle after use.
In the fourth glass tray, mix 10 ml of Post
Treatment Solution with 100 ml of water.
This mixture can be discarded when
finished.
7.
8.
Hand in all holograms made by the group, in an
envelope, whether they turned out or not.
Extra credit:
Make a good transmission hologram. See instructor
for details.
Place the enameled tray in the deep sink so that tap
water can continuously be running into it. This
allows the holograms to be washed in running
water.
Development Process:
1.
2.
3.
4.
Wash in running water for 3 minutes.
Inspect the hologram in the wash and sponge
off any visible powder-like precipitates
before the final treatment under bright lights.
Under bright lights, soak the hologram in the
Post Treatment Solution until the hologram
turns from pink to a light brown color.
Wash the hologram in water for 3 minutes.
Use a rubber squeegee to squeegee the
hologram. Dry the hologram with the hair
dryer set on warm. One can also hang the
hologram to dry. A reflection hologram
must be dry before it can be viewed.
Turn out all lights in the dark room except
for the green lamp.
Develop the film or glass plate in the
working solution developer for 2 minutes
with constant agitation by rocking the tray.
Rinse by sloshing the hologram in distilled
water for 10 seconds. Wash in running tap
water for 3 minutes.
Bleach in the Bleach Solution with the
hologram held vertically or with the
emulsion side down until it is transparent,
normally less than 2 minutes. Leave in the
bleach for an additional minute. After
bleaching the room light can be turned fully
on.
Holography