Photography in Three Dimensions
By “Awake!” correspondent in the British Isles
VISITORS to London’s Royal Academy of Arts in March 1977 saw demonstrated a new and fascinating concept of photography, a 20th-century photographic miracle called “holography.” On display at the exhibition was a floating three-dimensional image of a telephone, suspended in thin air, and so realistic that you would be excused for attempting to make a call with it.
The exhibition, designed to impress the public with the science and fun of laser beams, was fittingly named “Light Fantastic.” As the visitors came to realise, with holography, instead of having the picture on a flat card, an image is projected in full three-dimensional form in space. You can actually look at it from various angles to see different parts of it.
You may have heard the word “holography” or “hologram” in connection with the recent uses of laser beams for entertainment purposes. In laser beam lightshows, swirling, darting, twisting multi-coloured laser beams are choreographed to music, and sometimes the 3-D imagery of holography is used for special effects.
How It Works
The word “holography” is used to describe the process, because the prefix “holo” means “entire” or “complete.” Holography takes the picture in a much more complete manner than is achieved with an ordinary camera.
We can understand the basic principle behind holography by comparing it with sound recording and reproduction. Consider, for example, a symphony orchestra playing a piece of classical music. The musical notes and tones generated by various instruments result in a complex pattern of sound emerging from the orchestra. This pattern can, of course, be recorded, the record “storing” the sound in a coded form (actually by variations in its grooves). When the record is played, a pattern of sound is produced that duplicates the original notes that came from the orchestra. The identical sound waves have been regenerated.
In a similar manner holography records light waves for later reconstruction. Let us see how this is possible.
First of all, what is involved in seeing another person, a scene or an object? As we cannot see in the dark, light is necessary from the sun or from some other source. In fact, every tiny part of an object we are looking at reflects the light, but in varying amounts and in varying colours. A complex pattern of light is thus produced, emerging from the object like the sound emerging from the orchestra. We see the object when this pattern reaches our eyes and is interpreted by the brain.
Let us suppose that the pattern of light waves emerging from a friend sitting opposite to you is interrupted and recorded, or “stored,” similar to the gramophone record’s “storing” the sound. Your friend gets up and leaves. On “playback” of this “light-record” the identical pattern of light could be regenerated and thus, to the eye and brain, the person would seem to reappear. Furthermore, since the regenerated light duplicates the original (as in the case of sound reproduction), the image that is seen is in full 3-D form, exactly like the person.
This is the key difference between photography and holography. Photography involves making a flat image of a scene or of a person, like an artist’s painting, but holography reconstructs the original pattern of light waves themselves.
Making the Hologram
The record on which the light waves are “stored” is known as the “hologram.” It is essentially similar to the film for an ordinary camera but is of better quality and generally in the form of a photographic plate made of glass.
Figure 1 shows how the recording is done. An expanded beam of light from a laser is first divided into two parts by a special mirror. One part (called the “reference beam”) travels directly to the photographic plate, while the other part illuminates the object to be holographed. The complex pattern of light reflected from the object then also travels to the photographic plate. Light is thus arriving on the plate from two directions, producing a very detailed recording of the pattern on the plate.
Figure 2 shows how the playback process is done to give the 3-D image. The plate is first developed (as in ordinary photography) and the object removed. A single beam of light is now directed onto the plate. The light passes through the plate, but in so doing it is modified by the pattern embedded in the plate. The result is that the emerging light exactly duplicates the original light that came from the object, and so the object seems to reappear. To the viewer, the photographic plate is like a window through which the object is seen in full depth. By looking through the “window” in different directions, the object is seen from different angles. The image manifests such vivid realism that the viewer may be tempted to reach out and touch it, but, of course, nothing is there!
Interesting Properties
Holograms and the images they produce have many curious and fascinating properties. The hologram plate is equivalent, in holography, to the negatives obtained from an ordinary film. However, it is quite different in certain respects. For example, if you have some black-and-white negatives available, hold them up to the light and you will notice that they contain the picture (actually, in reversed form—the dark areas are light and the light areas dark). Hold the hologram plate up to the light and you will find that it bears absolutely no resemblance to any picture. Only under a microscope can the pertinent information be seen, but, even then, just as a highly irregular, unintelligible pattern of lines, blobs and whorls.
If part of an ordinary negative is damaged or cut away, then, obviously, that portion of the picture will be ruined or missing in prints made from the negative. Smash the glass hologram plate, however, and you will be surprised. The whole image can be reconstructed from any of the pieces! The quality will be impaired somewhat, depending on the size of the piece. Nevertheless, the image will always be complete!
The 3-D realism of the image produced from holograms is evident in several ways. If you change your viewing position through the “window” (the glass hologram plate), the perspective of the picture changes just as it would if you were looking at the original scene. If something in the foreground of the picture obstructs an object behind it, then by moving your head to the side you can look past it to see the hidden object. You will also find that the focus of your eyes will change when you look at near and far points in the scene and if you are nearsighted then your spectacles will help!
An interesting effect occurs if, say, a diamond ring is holographed. In the holographic image the diamond reflects glints of light from its facets and these appear and disappear as the viewer moves his head—exactly like the real diamond!
In short, the reconstruction has all the visual properties of the real thing.
Some Developments
Although the basic principles of holography have been known for over 30 years (holography was invented by Dennis Gabor in 1948), it was not until the invention of lasers in the 1960’s that the full capabilities of holography could be demonstrated. A laser is a source of pure, regular or “coherent” light and, in general, this type of light is necessary for recording holograms of 3-D objects. However, the use of lasers has disadvantages when practical applications of holography are considered. They are expensive and in some cases hazardous. Could their use be minimised in any way?
A major advance in this regard was made by the Russian investigator Yu. N. Denisyuk. He had the remarkable idea of combining holography with a form of colour photography invented by the French physicist Gabriel Lippmann in 1891. With Denisyuk’s idea, while lasers are still needed to record the hologram (Figure 1), in the reconstruction or playback process (Figure 2) the laser can be replaced by an ordinary light bulb. Further, by using three lasers during recording, corresponding to the three primary colours (red, green and blue), the hologram yields a full-colour image.
There is one rather special way, known as the “multiplex technique,” in which the use of lasers can be avoided altogether. The method involves making the hologram from a large number of ordinary photographs. For example, a person is seated on a slowly rotating platform, and an ordinary cinecamera takes hundreds of pictures, recording his appearance from all directions. The pictures are then synthesised into a single hologram from which a 3-D image can be reconstructed. The technique has made it possible to record some degree of motion in the hologram; a person can be seen moving his hand or giving a smile. It’s rather like the early days of moving pictures, but this time it’s in true 3-D!
Practical Applications
Making and viewing holograms is fascinating, but what practical applications does holography have?
One might immediately think of 3-D movies and television, where holography would provide the ultimate in reality. While it may be possible in principle to produce such a system, for the moment it is a long way off. The problem is due to the vast information content of the hologram plate. A 200-mm (8-inch)-square hologram plate has a potential information content over 300,000 times as great as a single static television picture. Present television systems come nowhere near the ability to handle such a vast amount of information.
At present, holography is finding application as a display and advertising medium. A company responsible for many of the billboards on the London Transport Underground has expressed its interest in using holograms for advertisement purposes. And the sales representative of the future may well carry holograms as samples of bulky or heavy products.
In museums, treasures can be replaced by holographic replicas. This technique has been pioneered in the U.S.S.R., and the Hermitage Museum, Leningrad, is now making a library of holograms for loan to other museums. Production of 3-D portraits will no doubt be an important application in the near future.
Holography has also found some important applications in industry and research. For example, in the production of high-precision motorcar cylinders, a hologram can be made of a perfect specimen. The holographic image is then exactly superimposed over the real production-line cylinders; any flaws and defects immediately show up as a characteristic fringe pattern. Errors in shape of less than a micron can be detected. (A micron is one millionth of a metre [0.00004 of an inch]!)
In research, events that happen too fast for the eye to catch can be holographed by using pulsed lasers. Pulsed lasers, like a super flashgun fitted on the holographic camera, emit pulses of light that last for only an instant. A ruby laser, for example, can produce a flash lasting for only 0.00000003 of a second! The light flash effectively captures an event that happens in less than a millionth of a second, or freezes the motion of an extremely fast-moving object. The event is recreated in the holographic image. Vibrations in objects, such as machinery or musical instruments, can be studied, and the method offers possibilities for analysing rapid chemical reactions.
Holography is still a rather expensive and cumbersome operation when compared with ordinary photography. It is also somewhat limited, at present, as regards the size of hologram that can be made. So rather than replacing photography, holography has emerged as an advanced form of photography for use in certain special areas. It represents another use of natural laws—actually, laws of the Creator—for the benefit and enjoyment of mankind. As further improvements are made in the process and in reducing the cost, there is no doubt that new ideas will be conceived for using holography to touch our lives much more than at present.
[Diagrams on page 13]
(For fully formatted text, see publication)
Figure 1
Pure light from laser source
Beam-splitting mirror
Beam
A pattern of light waves is transmitted to the plate from two directions
HOLOGRAM
Glass plate
3-D Object
Transparent photographic emulsion
[Diagram]
Figure 2
From laser
HOLOGRAM
3-D Image
EYE: sees 3-D Image identical to 3-D Object