Student Participation at The Center for Applied Optics
- Laboratory Work
- Information About the Laser
- Laboratory Procedure
- Other Websites
Welcome to our offering of holography to you as a student. You may have the opportunity to make film, expose the hologram with a laser and develop the film in a process much easier than the silver halide based films. Holography is indeed an art though, the art of perfecting the film, controlling the uncontrollable during developing; in general, seeing the image manifest itself into the expression you intended is not as easy as it might appear. Things like relative humidity, temperature, and ratio of water to alcohol in the developer, even choice of object for recording all play an important roll in the successful realization of your hologram.
The holograms you are to make require many aspects of optical research. The study of holography covers interference, optical imaging, optical engineering, chemistry, physics, and mechanical engineering.
There is the laser, a single frequency light source producing extremely bright light beams. The laser is "coherent" which allows us to record three dimensional images. The laser is still one of the most profound elements of research in the world today. It is used in virtually every facet of experimentation, in one way or another. Check out the National ignition facility NIF website, this laser is the most powerful laser ever built, and for what? The answer lies in the need to further understand matter and its interaction with the universe, or at least its immediate surroundings.
Then there is Chemistry, the film is mixed in the lab using Ammonium Dichromate, an optically responsive material, and mixing it with water and gelatin. This mixture is spin coated onto thin glass plates giving us a thin layer of photosensitive film. Film research in the holographic community continues to advance the sensitivity and color replay of holographic films enabling a wide variety of applications in industry from auto displays to computer memory.
Mechanical engineering covers things like the optical table, which helps to keep the floor vibrations that are present in "every" room down to a minimum. Vibrations that find their way into scientific hologram recording sessions may result in a failed attempt.
Display holograms using the laser, vibration isolated tables, and lab made film, are what we will be working with in the laboratory.
This particular lesson will be devoted to studying Denisyuk's holographic recording method. It is indeed the most convenient method to use for artistic holographic displays and it is possible to record holograms of the highest quality, those which can be displayed in room light or in the sunlight. This method got its name from the famous Russian scientist,Yuri N. Denisyuk http://www.holography.ru/histeng.htm) who invented the procedure of reflection hologram recording on transparent photographic plates in the beginning of the seventies.Before that, holograms were recorded according to Leith-Upatnieks method which required the use of a laser too but it was also necessary to use a laser for observing. Denisyuk proposed to expose the plate and the object with the same laser beam to see holograms in the usual white light. This successful demonstration of holograms that could be seen in white light (the sun, or a halogen light) spawned a whole new generation of holographic displays with amazing depth and clarity. Today, full sized holograms of people and or large objects are routinely made with the most amazing results. We are, unfortunately limited to small objects in the laboratory, objects such as coins or small metal relief buttons or the like. We have a smattering of good display items so you do not have to find anything to work with; we have that stuff in the lab.
During this class offering we will have already made the film and have things prepared for you so that you can enjoy the fine art of holography by trying various objects in your recordings; you will expose, develop, seal and take home your work. You will be given four two inch by two inch glass film-plates to work with. On the other hand, we also offer “from scratch” holography; you make the film, expose, develop, seal, and take home your work. You will be given two eight inch by eight inch bare glass plates to work with.
The hologram you make will be environmentally sealed and will be yours to keep. Do not fret over poor holograms for your first attempt, as I have found that for some folks, you will only get a few good ones out of the whole batch. Other times I have found that virtually all the exposures produce great holograms. Sometimes it just depends on the alignment of the planets, but most times it depends on the bath temperature and agitation during developing, and of these, mostly agitation.
So,we assume for now:
- We have already mixed, coated, and cut-to-size the film plates for this exercise.
- The laser will be turned on for you, provided it works, and the exposure level for the displays will be preset for you.
- You will have assistance when you need it and a helper will be in the lab with you to guide your effort.
- We will allow all students in the laboratory at the same time, it could become crowded, but we will deal with that then.
- Any waiting students will be able to investigate other types of optics aspects such as diffractive optics, lenses, telescopes and acousto-optic modulation of laser light.
- Most importantly, have fun and remember that patience is a virtue; we may have to meet a second time to complete our study of the hologram.
Now then, A few important words about the laser:
This must be carefully understood:
The laser is extremely bright! Any direct contact with the pencil beam (the small diameter beam) with skin clothes or your eyes may result in damage to you, especially the eyes. You will be using glass plates for the film and the glass will reflect the laser beam so be careful and stay clear of the direct beam while you wait your turn at exposing the film. As you will see, the laser beam has been directed from the research optical table to the display optical table and is incident on a turning mirror, which directs the beam downward to the object that we are recording. The room is "laser safe" and all stray reflections have been accounted for, however you should not look at the beam bouncing off of any mirrors in the same way you would not look at the sun or any direct reflection of the sun. Laser safe goggles will be available for your protection if you are concerned.
Most of all don't be too concerned.
The image we will be making is a small astronaut, coin or NASA logo, which can be found on the display optics table. The objects can each be placed in the laser beam so adjustments to the beam will not be needed. The holder for the film plate will also be in place just above the object. After putting on protective latex gloves, you will be given a piece of film. Do not touch the active part of the film. Then you will need to place your film in the holder and install an aperture cover to keep from exposing the edges of the film plate. The film should be placed in the holder with the film side down (facing the object). To check which side the film is on, use the scribe and scratch near the edge of the film plate, the rough feeling side is the one with the film. Once the film and aperture is in place depress the exposure button, there will be a ten second delay then the shutter will open and expose the film.
For those of you interested, the exposure is 50 miliJoules. One question is: can you find the intensity or "irradiance" (Watts/cm2) if the laser beam on the film if the exposure time is fifteen seconds? We know that one Watt is one Joule/second. Did you get 3.33 miliwatts/cm2? If not, do not fret, it is not critical to be able to find the intensity. This is just a way for us to calculate the exposure time when we measure the irradiance at a detector placed where the film goes. We could expose the film anywhere from 50 to 400 miliJoules and still get good results. This means that the film is not that sensitive to light, as you will see.
To develop the film we will be using 2-proponal (alcohol) and water. The water has been pre-mixed with the 2-proponal in each of the three developing baths. You will, with the protective gloves on, submerse your exposed film plate into the first bath and agitate; the key to success is agitation, for one minute. This bath wets the film and allows the unexposed regions of the film to absorb water and swell. Next, you will plunge (and agitate) the film plate into the second bath for one minute, which will begin the drying process within the film; slowly extracting the moisture from the unexposed part of the film.
For those of you who may be interested, the chromium three-plus ions from the dichromate will begin to migrate within the water-swollen gelatin and make cross-links with the amino acids in the gelatin. These atomic sized cross-links will change the index of refraction (responsible for bending light) wherever they occur leading to the three dimensional image you see in the finished hologram.
Finally dunk the film into the third bath with agitation for one minute. This third developing bath will dry the hologram and remove any remaining water. The dry film result is what is called "index modulated". The film has (on a micro-scale) thousands of regions of index modulation or changes in the index of refraction on the scale of the wavelength of light. This modulation isthe hologram. Once the film comes out of the third and final bath, the film side of the plate is blown dry with a heat gun for ten seconds or so. This step is necessary to rapidly dry the alcohol off of the film so that the film does not begin to absorb any water or become patchy in its reconstruction.
The completed film must next be sealed from the environment so that it will not absorb water in the future. This process involves the use of index matching glue, which has to be exposed to UV light for ten minutes. The glue residue will be quite sticky and hard to remove from the glass plates without chemicals. For this reason we will have everything ready for you to clean them, with our help. You may take home the plate that you exposed and developed and sealed here in the lab
We will now summarize your procedures:
- Remove film plate from the box.
- Check which side the plate has the thin film.
- Place film in the film holder.
- Install the aperture.
- Put on laser goggles if you are concerned.
- Depress the exposure button for the shutter.
- Remove film from holder after the shutter closes.
- Place film in developer bath #1 for one minute with agitation.
- Place film in developer bath #2 for one minute with agitation.
- Place film in developer bath #3 for one minute with agitation.
- Place film side of plate in front of a heat gun for ten seconds or until dry.
- Use light source to see how well your hologram turned out.
- Seal and receive the completed hologram.
- Who is Yuri N. Denisyuk and what was his contribution to holography?
- After you visit the web site, name at least one other significant contributor to holography; what did he do?
- Why is the wetting and drying process for developing the film important?
- Why do you have to environmentally seal the hologram?
- What working wavelengths do you find in an Argon laser? Name two.
Please take the time to visit these websites and review the references. The references are publications from students of holography or their professors; the web sites are full of answers to your questions. Also, this information will help you to understand holography more deeply so that you can ask the more difficult questions to your assistants.
- Leith E. N., Upatnieks J. Journ. Opt. Soc. Amer., v. 54, 1295, 1964.
- Denisyuk Y. N. Optika i Spektroskopiya, v. 15, 522, 1963.
- Fyodorov B. F., Cibulkin L. M. Golographiya. Moscow, Radio I Svyaz, 1989
This is a good one!