Creation of Simple Holograms
with a Single Beam Setup
Karl Fey
Vestal High School
Laser Teaching Center
University at Stony Brook
August 2000
What Is a Hologram?
A hologram is like a photograph - they both create a record of an
image by exposing a photosensitive surface to light.
A hologram is not like a photograph - the hologram records all the
information about light reflecting off of an object, including phase
and amplitude. A photograph records only the average intensity of
light from an area. This allows the hologram to display the object in
three dimensions.
Theory
The hologram is a record of the interference pattern created when
two beams of laser light interfere on the holographic surface. One
beam, called the reference beam, strikes the holographic plate
directly from the laser, or after bouncing off several mirrors. The
other beam, called the object beam, reaches the holographic plate
after bouncing off of an object being holographed. These two beams
are initially coherent and in phase with one another, but after the
object beam bounces off of an object, it will be out of phase with the
reference beam. The two beams will interfere at the plate and create
areas of high amplitude and low amplitude, light and dark bands.
These are recorded by the holographic surface, and preserved through
the developing process. The bands of light and dark act as an
extremely sophisticated diffraction grating, so that when light passes
through the plate or film, it interferes to form the exact image of
the object that was recorded. Viewing the hologram at different
angles will give a different view of the object, thus giving it its
three dimensional appearance. The 2 eyes on a human are separated
several centimeters, giving different views which the brain interprets
as being in three dimensions. The exact wavefront produced by an
object is duplicated by the hologram.
Types of Holograms
- Reflection hologram This hologram is formed when
the reference beam and the object beam are incident on opposite sides
of the holographic surface. They interfere and record an image. To
reconstruct the image, a point source of white light illuminates the
hologram from the proper angle, and the viewer looks at it from the
same side as the light source. Reflection holograms require the
simplest setup, and are visible without laser light.
- Transmission hologram This type is created when the
reference beam and the object beam are incident on the same side of the
holographic surface. They are viewed by shining a spread out laser light
through the emulsion side of the hologram at the same angle the hologram
was recorded at, with the viewer looking on from the opposite side.
Transmission holograms must be viewed with laser light, and they appear
the same color as the laser used to view and create them.
Properties of Holograms
- Sensitivity to vibration
Holograms record specific information about light at the level of
individual light waves. Thus, the holographic plates are sensitive
enough to record a single wavelength of light, 650 nm for red laser
light. Any vibration greater than a few hundred namometers is
detectable and could ruin the hologram. One of the major obstacles in
creating holograms without much expensive equipment is creating
stability.
- Divisibility A hologram can be divided into small
pieces that still retain the entire image of the object. Holograms are
like a window in this sense; if there is a person that can be seen out of a
window, and the window is then partially covered up, the entire man can
still be seen by shifting the viewer's position. A hologram does this
also, unlike a photograph. The object's wavefront is preserved throughout
the hologram, and can be viewed from a tiny piece of the whole.
Why Make Holograms?
I made these holograms in order to test the feasibility of a simple, low
cost hologram production setup. In the past, powerful lasers and exotic
equipment were necessary for the creation of even basic holograms. In the
creation of these holograms, I used only a stable optical table and a $150
holography kit.
How Did You Make the Holograms?
Supplies
- Integraf The supplies were ordered from
Integraf, a company in Illinois. They have everything necessary for
making simple holograms.
- Holographic plates or film These have a very
fine emulsion side on them that can record the hologram. Plates or
film are usually sensitive only to certain wavelengths of light, but
there are types of these that can record the entire spectrum of
visible light. I used 2.5 inch square plates that looked like small
pieces of glass. They were sensitive to laser light in the 625-650 nm
spectrum.
- Developing chemicals
These are all the necessary ingredients for making two solutions that when
combined form a working solution, plus a bleaching solution.
- Laser Integraf supplied a diode laser. This is
nice, because its natural tendency is for the beam to diverge, unlike
a HeNe laser. I removed the collimating lens from this little guy to
make it into an effective illuminating machine. It diverges roughly
10 degrees along the vertical, and 60 degrees along the horizontal,
giving a rectangular illumination shape.
- Beam spreader A narrow beam laser can also be
used, and its light reflected off of this first surface concave mirror
to give even illumination. I haven't tried this yet, but it should
produce an even circle of laser light that may more useful for
creating holograms.
- Optical table
Stability is absolutely necessary for holograms. Optical tables
give this sort of stability, enough for fringes to appear on a
Michelson interferometer, or to create a hologram.
Exposure
- Darkness As little white light as possible
should be allowed to reach the holographic plate. The room should be
as dark as possible, with visibility approaching zero. A green safe
light is acceptable to provide lighting, because most holographic
plates will not be sensitive to that wavelength of light.
- Stability As little vibration as possible is
optimal. If possible, prevent any air movement from air conditioners,
turn off any music and prevent noise, and avoid touching or moving
anything. Cars passing by may cause trouble, or people in another
room. Tolerances are still unclear, however. Testing the setup by
using a Michelson interferometer may be one way of knowing if it is
stable; if fringes are visible, then holograms are definitely
possible.
- Exposure time A holographic plate should be
exposed to a 5 mW laser for roughly 4 seconds in a basic reflection
hologram setup, according to the integraf manual. my findings
disagree with this; the time is too short.
Development
- Supplies
3 containers for the chemicals is enough. 2 trays for developing
the plates are needed. For trays I used inexpensive small round glass
dishes intended for serving food. Glass may have an
advantage over plastic trays.
- Conditions darkness, again. the developing
room should be like a dark room for developing photographs. instead
of a red safe light though, it should be green.
- Procedure Combine solutions A and B to form the
working solution. Develop the plate in working solution for 2
minutes, then rinse under running water for 3 minutes. Use the bleach
solution for 2 minutes, then rinse again. Allow the hologram to dry,
and then it is complete.
What Holograms Have You Made?
In the time available I made first one white light reflection hologram
and then two transmission holograms.
1. White Light Reflection Hologram
- Setup see the figure ...
- Exposure This was made in near darkness,
without the green safe light. The laser warmed up for 40 minutes
before the exposure, to prevent polarization fluctuations. I removed
a holographic plate from the box in total darkness, to protect all the
plates. The emulsion side was placed facing the object; the emulsion
side is determined by wetting one finger and touching a corner. If it
feels sticky, it is the emulsion side. The exposure time was 5
seconds, after which I transported the plate wrapped in several black
bags and a box to the developing room.
- Developing solutions see the table ...
| Solution A
| Solution B
| Bleach
|
1 l distilled water
20 g Catechol
10 g ascorbic acid
10 g sodium sulfite
75 g urea
|
1 l distilled water
60 g sodium carbonate
|
1 l distilled water
5 g potassium dichromate
80 g sodium bisulfate
|
- Development procedure I combined 40 mL of solutions A and B
together in one tray, creating the working solution. This is usable
for eight hours. 80 mL was enough to submerge the plate, all that is
necessary. I immersed the plate in this solution for two minutes,
rinsed it off under running water for 3 minutes, immersed it in bleach
until it became transparent (roughly 1-2 minutes), rinsed for 3
minutes, and let dry. The entire process was done in the dark.
- Results The hologram was best viewed with a
point source of white light in an otherwise darkened room. The image
was partial, displaying the front portion of a Thorlabs photodetector,
and was difficult to view. This was likely due to under exposure: 5
seconds was not long enough. The shiny surfaces on the detector were
the only ones visible because they reflected the laser light best.
Also visible on the hologram were bands of light and dark. These
indicate an interference pattern created by a slight vibration of the
setup. However, the fact that anything at all can be observed makes
this experiment a success.
2. First Transmission Hologram
- Setup This was again a single beam setup. This
exposure was performed on an optical breadboard (provides stability
like an optical table) placed on top of the optical table. This
provides a lot of stability; Michelson fringes did not shift.
- Exposure A green safe light was used this time.
It had no effect on the final image, so these lights are safe to use.
Because of the different setup, I used an exposure time of close to 20
seconds.
- Development Same as for the first hologram.
- Results This hologram is viewed by shining an
expanded laser beam at 45 degrees through the plate, then looking at
the plate from the opposite side. The proper angle must be found by
tilting the plate and shifting your head. A better image of a flower
is formed, yet it still appears under exposed.
3. Second Transmission Hologram
- Setup ... same as first transmission hologram,
except that a pile of screws was used as the object. They had a
detailed surface, and reflected a good deal of light.
- Exposure
This lasted for 40 seconds.
- Developing Same as before. However, 2 patches
of a dark film appeared on the plate this time. I don't understand
these.
- Results The best hologram so far, but not great.
Still somewhat under exposed, the objects are a bit difficult to see.
Only some surfaces are visible; this will always be a problem as long
as a single beam setup is used. A beam splitter may be necessary for
higher quality holograms. Also, the diode laser may not provide
enough intensity with the collimating lens removed. The beam diverges
over a significant area. Yet the vertical dimensions of the beam are
significantly smaller. If the beam has a Gaussian intensity profile,
as it likely does, then holograms of any larger objects must either
have the laser far away, which costs intensity and makes for higher
exposure times, or else use a narrow beam that is spread evenly by a
beam spreader. This hologram does show that exposure times of over 40
seconds are viable, because the image was the clearest yet.
What Are Holograms Used For?
- Interferometry Holograms are sensitive to the
smallest vibrations. This property can be used as a measuring tool.
Two exposures can be made of the same object, one after the object
undergoes some stress, and the interference patterns created by the
deformation can be observed, measured, and used to calculate the
amount of change.
- Optical elements Holograms can be made that have
the same properties as diffraction gratings. In effect, holograms are
nothing more than sophisticated diffraction gratings. Holographic
diffraction gratings are cheaper and easier to produce than other
kinds.
- Security Holograms are difficult to duplicate.
Some credit cards have holograms on them to protect against
counterfeiting.
- Art Holography offers a new artistic medium. Many
artists use holograms to capture their subjects. Holograms also can
be used to preserve art. A holographic copy can be made that is
nearly as good as the original, without damage the original at
all.
- Data storage Multiple holograms can be made on the
same holographic recording surface without interfering with each
other. Ideally, a great deal would be stored in a very small area,
allowing access to individual images by using laser light incident at
slightly different degrees.
How Can These Images Be Improved?
- Exposure time Proper exposure times are still
unknown. These can be found either through trial and error, or by
using a light meter to find the intensity of the reference beam
compared to the object beam.
- Setups A multiple beam setup will probably be
necessary for higher quality transmission holograms. High quality
reflection holograms might be made with only a single beam setup,
because the plate or film can be placed directly on top of the object
and the light comes from behind it.
How Good Does It Get?
Some high quality demonstration holograms made by Integraf were also purchased.
Hopefully holograms of this quality can eventually be made in the laser teaching center.
What Other Sites Should I Check Out?
Contact Me!
I'll be happy to provide additional information about my experiments
with holography or my experiences in the Laser Teaching Center. My
email address is: Karl_Fey@hotmail.com
