Francis Pham
3.22.2006
PH464
Holography
(Photo taken from http://hyperphysics.phy-astr.gsu.edu/)
Abstract
Holography is an advance technique of projecting image in 3-dimensions. Holography was invented in 1948 by a Hungarian physicist Dennis Gabor (1900–1979), for which he received the Nobel Prize in physics in 1971.
Unlike common photography technique which recorded the object’s image onto a film. Holography technique does not. Instead, it records the grating onto a film which was formed by the constructive light interference so when re-constructed, the lights refracted off the film grating and bend in the same angles as the original light and that the image formed will be very similar to the original one.
Early holograms are made using Mercury arc lamp because it’s the only source of coherent light available. Early holography was limited to film transparencies using a mercury arc lamp as the light source and the image contained distortions and twin image. Later development of laser provides true coherent light dramatically improved holography technique.
Holography
There is two common type of holograms; transmission and reflection. In Transmission hologram the reference and object beam hits the photographic plate at the same side. Reflection hologram is different, the reference and object waves traverse the emulsion from opposite side. What happen in reflection hologram is the reference beam goes through film plate and hit the object and bounce back forming interference with the other light. The resolution of film emulsion must be very high in reflection holography; consequently, it’s viewable under white light but the exposure time is longer and it will not tolerate vibration as well when recording. The transmission hologram on the other hand is often monochromatic, but color is possible by using three different lasers of different wavelengths which combined will give illusion of color when projected. Transmission hologram are viewable only by laser and must be put at the exact angle as when it was recorded or else it will not project clearly or not at all.
(Photo taken from http://www.ph.utexas.edu/~jfelk)
Transmission holography
(photo taken from http://www.litiholo.com)
Laser light are being split into two beams using a beam splitter; the straight ray will be the object beam and the side ray is the reference beam.
Object beam will be spread out using a concave lens and then reflect with a mirror to change the angle toward the recording medium to illuminate it. The mirror cannot be regular glass mirror or else it will experience twin-image effect due to the reflection property of glass therefore the mirror’s reflection layer must be surface coated.
The reference beam also goes through the same process; being spread out by a lens and then to angled with mirror reflection. Instead of shining it to the recording object, it is shine toward the film. What happen is that the laser lights reflected back from the recording object combined with the laser light from the reference beam hitting the film together formed a constructive light interference, it then get recorded onto film as an interference patterns.
The film used for holography must have a very high resolution or else it will not be viewable. With very high resolution, tiny movement will print false grating or double up the grating lines and therefore it could spell catastrophe. Everyone who dealt with holography will know that vibrations are the enemy when recording hologram.
To view the transmission hologram only the reference beam is used, lights hit the interference pattern on film and reflect off at specific angles, the effect very much like how grating bend light into rainbows, different light angle combined form an image what is known as hologram.
Reflection Holography
(Photo taken from http://www.phys.ufl.edu/courses/phy4803L/holography/)
Unlike the transmission hologram, reflection hologram does not need laser to view and the recording method is easier less the complication of beam splitter and the use of a mirror for ray changing angle. One method of recording the reflection hologram, the laser light get spread out and shine straight toward the film and the object is located right behind the film plate. What happen is, some of the light from the laser hit the recording object and bounces back combined with the remaining light formed an interference patterns, and those patterns are printed on the film making line gratings. On the above picture, the laser light get spread out by the concave lens and then refracted from the mirror. The purpose is to divide up and re-route the distant of laser to film so it would fit on to the table. Same result could be achieved by aligning everything in a straight line, this way you wont need the mirror, but this would require a longer table.
Experiment
For the experiment I use the 1mW diode laser module obtained from a laser pen and solders two wires to it; one to the negative terminal and the other to the positive terminal which is the chassis. What I’m doing was trying to use external battery for it because the smaller button battery supplied with the laser doesn’t last very long. Initially I started with four cells trying to overload the laser a bit so it could output more wattage, but then the laser light kept getting dimmer and dimmer; perhaps the higher voltage overheated the laser. I had to scale down from four to three batteries to operate the laser diode and it seems to do fine with three double-A batteries.
The laser diode laser was mounted to a wood stand using superglue with the beam ray adjust ~2 inches parallel to the table and then spread out with a lens. The laser beam was spread out with a diameter of 3” when it hits the film plate which would give an area of pi*(3/2)^2 = ~7in^2. Since the laser is 1mW so 1mW/7in^2 = ~143 Micro Watt/in^2. It’s a very weak laser.
For the first time, I use the conventional transmission holography method involving beam splitter and reflective mirror but since the laser is too weak and will not produce light intense enough to activate the film because so much lights is lost through the beam splitter. My solution was to instead use another transmission holography method found on the www.litiholo.com website.
(Image taken from litiholo.com)
In the above photo, the laser light get spread out by a lens and came toward the recording object and the film plate. Portions of the light hit the film plate and the remaining light hit the object. Lights, reflected off the object combined with lights hitting the film plate formed a light interference pattern, which then get recorded on the film. So after recording, simply remove the object and look on the other side through the plate, if done correctly you will see a 3-d image of the original object like it was there.
This method so far is the only method that works for me since my laser module is too weak. Using the above setup I manage to produce my first hologram image after fifteen exposures and lost fifteen holographic plates (a lot of money, a little over $3 for each plate). Other previous hologram exposure failed to work regardless of how I change the positions as well as the exposure times anywhere from five minutes to twenty minutes. Later I think I’ve found the problem; I guess it was because of the recording object is not bright enough to reflect light. Of all fifteen plates, the only hologram that works is the one that I use an Energizer battery as the recording object, perhaps due to its highly reflective surface. I believed what happen is that only the light reflected back from the silver color battery is bright enough to activate the film’s chemical since the film I use is a little slow; it needs red light (620nm-680nm) with intensity of at least 60mJ/cm^2 to activate.
This holographic method is able to tolerate vibration a lot better than the conventional way because of its simplicity and shorter light travel distant between the laser diode to film. Furthermore, the conventional way involves mirrors and beam splitter, so vibrations is further amplified by each device. I think the vibration that the film experience is equal to the product of the vibration of the lens and the amplification of the vibration angle of the mirror. By eliminating the mirror, this hologram could tolerate a lot more vibration thus easily be produce over an ordinary kitchen table (mine was taken on the PSU library floor inside a study room).
Conclusion
The class demonstration of my hologram only produces half of the recording object image due to the weak laser and short exposure time. Had I invest in a better laser and use a different recording object with high contrast; one with simpler geometry and bright color such as a dice. I think the hologram will turn out to be a lot better. Also what I could do is increase the exposure time from eight minutes to perhaps twelve or fifteen minutes.
Reference
http://en.wikipedia.org/wiki/Hologram
http://www.litiholo.com
http://www.phys.ufl.edu/courses/phy4803L/holography
http://hyperphysics.phy-astr.gsu.edu
http://www.ph.utexas.edu/~jfelkl/Holography.pdf