In Search for Holofilm

Me, Chung HH, Chu CY

This was an experiment devised by ourselves, implemented by ourselves and evaluated by ourselves. I was in La Salle College Form 6. Some classmates and I were going to participate in the Joint School Science Exhibition. We desperately wanted to do something new and innovative. At that time, we were thinking about revolutions in movie production.

In the past, movies were solely taken by on-site video cameras. At present, many special effects could be done with computers indoors. How about in the future? We predict that a three dimensional movie made up of holograms ensues. We called it Holofilm. Even today, there is currently no known holofilm in the world, and we decided to build our first holofilm in 1996.

Just like many static pictures making up a film, we believe a holofilm should consist of many holograms displayed in succession rapidly. In order to produce holofilm, we first need to develop skills to create holograms. As we were still in secondary school, we did not have the necessary knowledge, equipment, fundings, and time to do such a big experiment.

Preparation

We started by visiting the Kowloon Central Library afterschool. There were not many books on hologram. Occasionally, we can find a few relevent pages from some thick tomes. Briefly speaking, two monochromatic coherent light sources are required. One of which goes to the film directly. The other one shines on the object, whose reflected light interfere with the first ray. The film records the interference pattern. After development, the same light source shining on the film could reproduce the interference pattern, creating an image of a three dimensional object.

The requirements of the experiment is incredibly high. Only laser beams can do the job. One of the best selling points of laser beams is that it has negligible divergence over a long distance. However, we need to diverge substantially the light coming out otherwise we cannot picture even the smallest object at hand, because we would have left only a tiny spot of light reaching the film. What makes things worse, the whole setup cannot tolerate even the slightest vibration. According to the books, a dog one metre away re-positioning its tail would fail the experiment.

At that time, the new airport was still under construction. Our school was directly under the flight path. During peak time, an aeroplane landed every two minutes. Whenever it flied by, the whole building shaked. In order to minimize its effect, we booked an underground store room. Well, the room had another advantage. It did not have any window. After closing the doors and switching off the light, you cannot see anything, including your own hand and a piece of white paper sticked on the wall.

We found a jumping bed and a table in the room. We inverted the table and put it upside-down on the jumping bed. The damping effect was excellent. Following instructions from the books, we bought a bag of sand from the Kowloon City and carried it uphill back to the school. The sand were spread evenly on the table, providing extra dampings.

Then, we borrowed a laser beam from the Physics Laboratory. It was the most expensive equipment we used. Should we bought it ourselves, we would have spent HK$4000. Although holograms are best made with green laser beam, it was exceedingly expensive. We used a neon laser beam giving out red light instead.

Our Setup

In order to have two monochromatic coherent light sources, we need a light spliter. Our school did not have such high-end facility. So our Chemistry teacher borrowed one from a local university for us. It was a thick cylindrical glass one inch in diameter coated with some chemicals. About half of the incident light passes through the glass while another half is reflected. The light spliter costs about HK$1000.

Guess what we used to diverge the light? Even the lens with the smallest focal length in our Physics Laboratory did not suffice. We even tried telescope eyepiece and microscope objectives! At last, we bundled three concave lenses to achieve the desired degree of divergence. In addition to all these optical instruments, we added a plane mirror to direct the reflected light from the light spliter to the object we are picturing.

As each hologram requires an exposure time of two to five minutes, we cannot use existing cameras. So we made our own film holder out of cardboards. The whole setup was mounted an another large piece of cardboard, which was then placed on top of the sand.

The Experiment

Our hologram experiment was conducted three times. We took as many holograms as possible in each run, to maximise the chance of getting at least one successful hologram. In our first trial, we used normal colour films. Later, we were told that we should use black and white films instead of colour films to record the interference pattern.

We asked the teacher advisor of the Photography Society about which film to choose. He suggested us that TMax-100 was a very popular black and white film. After our second trial, we learnt that TMax-100 used the technology of colour film to simulate black and white film.

Through my acquaintance in Hong Kong Astronomical Society, we sought the advice of Dr. C. H. Yeung of the Department of Applied Chemistry of the Hong Kong Polytechnic University. He referred me to Dr. Y. W. Wong of the Department of Applied Physics of the same university. Dr. Wong told me to use Technical Pan 2415.

Unluckily, this film could not be found throughout Mongkok. My classmate went to some professional photography shops in Central and Sheungwan. The salesperson told him that Technical Pan 2415 was out of stock, and suggested him to buy a similar thing Technical Pan 36/135. Without any other alternative, we used this film in our last trial.

Films are the least sensitive to red lights. That is also why the developers lit a small dim red lamp when developing the films. Since our laser beam is red, a red lamp during developing may destroy the interference pattern recorded. To maximise the chance of success, we decided to develop the film ourselves. It costed us nearly HK$400 to buy the equipment and chemicals for developing the film.

It was also the first time I saw the orange-brown bromine water coming out of the development tank. Later when I learnt the silver bromide chemistry of photographic films, it served as good memory aids to me.

Well, no doubt, we failed again. Later, Dr. Yeung told me that a normal colour film has a resolution of about 100 dots per cm. Technical Pan 2415 has about 1000 dots per cm, which is barely enough for making holograms. But Technical Pan 36/135 is only about half that value. Other possible causes of failure include the fact that plane mirror from the laboratory of a secondary school may not be plane enough, etc!

Concluding Remarks

Although the dream for creating the first holofilm in the world was broken, I treasure the process rather than the result. We learnt how to think of an innovative idea, how to design, implement and carry out the appropriate experiment, how to solve unforeseeable problems. We learnt the physics of interference, the chemistry of photographies ahead of most of our peers. Above all, We also became good friends that help each other.

May I take this opportunity to thank all my partners and advisors for their support. Chu Chung Yin carried out the whole project with me. Tan Khain Sein and Chung Hon Hei helped us to do many parts of the work and gave us invaluable advice. Cheung Wai Hei helped us to keep the timings for all the exposures.

Dr. Yeung

We would like to thank Dr. C. H. Yeung of the Department of Applied Chemistry of Hong Kong Polytechnic University, Dr. W. Y. Wong of the Department of Applied Physics of the same university for their invaluable advice. We would also like to thank our teacher advisors in this program Miss Becky Lam, Mr. S. T. Leung. and the technician of the Physics Laboratory Mr. Simon Leung. Without their help the whole thing could not even proceed.

We did not continue with our experiment because by then it was already one week after the Chinese New Year Holiday. We were getting busier and busier. Easter Holiday still had a long way to come. So we returned to our normal studies again. We decided not to demonstrate holofilm but some other things in the Joint School Science Exhibition. Nevertheless, we all sincerely wish holofilm to come true one day.

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