WHEN Princess Leia sends a message, she does it in style. Not content with voice or video communication, she dispatches a moving holographic recording-on a floppy disc, no less. Many filmgoers might assume that this compelling image is nothing but a Hollywood flight of fancy. And back in 1977, when the film was first released, they would have been right. But today, as a revamped version of Star Wars hits the world鈥檚 screens, the technology is becoming a reality-moving holographic videos have already been made and can even be viewed on electronic displays.
That said, holographic videos aren鈥檛 exactly commonplace. Only one or two laboratories in the world are able to produce moving holograms electronically and even then, the pictures are far from Star Wars quality. The technology behind holographic videos is still so difficult that researchers have to work with small, low-resolution holograms even when using state-of-the-art computers and electronic displays. But as computers become faster, display technology improves, and communications networks grow in capacity, holographic video will become more practical.
Sit very still
But first, the basics of holography. A hologram is a recording of an interference pattern. Conventional holograms are generated when two beams of coherent light fall on a photographic plate. The beams are created by splitting a single laser in two. One beam hits the recording plate directly while the other reflects off the subject onto the plate ( see Diagram). When the two beams meet, their wavefronts interfere, creating an interference pattern of light and dark fringes-the hologram.
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Decoding the image is a simple matter of illuminating the hologram with the original laser beam. The viewer sees the interference pattern created by the reflected light, which appears as a three-dimensional image of the original subject. As the viewer moves round and sees different parts of the interference pattern, he or she sees the subject from different angles.
The art of holography is much more difficult than conventional photography. The interference fringes created in the process are extremely fine-fractions of a micrometre. To record these, holographers use a light-sensitive film that is hundreds of times finer and many times more expensive than that used by photographers. Holographic emulsions are relatively insensitive to light and require long exposure times.
During exposure, the image is easily destroyed if either the subject or any of the optical components move by more than a quarter of a wavelength of the laser light. That is why most holograms are of inanimate objects: they can be isolated from vibration. But it is possible to freeze the motion of living or fast-moving subjects using very short, high-power laser pulses.
So why are holographic videos so difficult to make? Why not make a series of holograms and project them one after the other, just like a conventional film? Creating a series of holograms in this way is relatively simple but displaying them is difficult. To get the full 3D effect, the viewer must see the interference pattern directly. Projecting the image onto a screen prevents this. One option would be to view the holographic plates themselves one after another. Japanese and French researchers are pursuing this approach with rolls of film wide enough to allow a single viewer at a time.
The plates can be magnified, but even then the final image cannot be larger than the lenses or mirrors used in the process. Using a real lens the size of a cinema screen would be impractical and even smaller systems would be prohibitively expensive because of the optical elements needed to reduce aberrations. However, holograms of mirrors and lenses, which behave like the real thing, may reduce the cost of large optics to acceptable levels.
Despite the problems, researchers have managed to create holographic movies. The first was made in the Soviet Union at about the same time that Star Wars was being filmed. In 1976 Victor Komar, a researcher at the Cinema and Photo Research Institute in Moscow produced a series of holograms at the rate of eight a second which were projected onto a metre-square holographic lens. A few years later at NASA鈥檚 Lewis Research Center in Cleveland, Ohio, Arthur Decker achieved a recording rate of 20 frames per second, mainly because of improvements in the rate at which powerful laser pulses could be produced.
Solid entertainment
Researchers at the French-German Research Institute of Saint-Louis, France, have used such techniques to show how objects such as brakes and loudspeakers move in three dimensions. In 1986, the group even produced an art film. However, these approaches are limited by the size of the images they create.
But there is another method with the potential to produce large images-modern electronic displays. With electronic imaging, the recording and the display of holograms are done by different devices and information is transferred between them. And this changes everything.
The challenges facing scientists developing electroholography follows from the huge amount of information that holograms contain. To get an idea of the scale of the problem, imagine a holographic video display comparable in size and resolution to a notebook computer screen. In theory, each pixel seen in the 3D image must be representable on the 2D screen. A black and white screen might have an array of 640 by 480 pixels in a rectangle roughly 25 centimetres diagonally across. That鈥檚 a total of 307 200 pixels.
Now imagine that the image has a depth of about 10 centimetres at the same resolution of 30 lines per centimetre. This means that the information content is multiplied by 300. Multiply again by three to produce a colour image and the final count is 276 million pixels. An electronic camera would need a similar number of pixels to record such an image. But the maximum available today is just 4 million.
In fact, electroholography boils down to three separate problems: how to create or record this huge amount of data, how to transfer it to the display and how to display it. 快猫短视频s are finally beginning to make progress.
Nobuyuki Hashimoto is a research engineer at the Citizen Watch Company鈥檚 technical laboratory in Tokorozawa, Japan. Although electroholography might sound an unusual pursuit for a watch company, Hashimoto has persevered because he believes developing holographic video is an excellent way to test the performance of his company鈥檚 liquid-crystal displays. He and colleague Shigeru Morokawa have attacked the technology鈥檚 problems head on. In March, at a communications research laboratory workshop in Tokyo they described their results.
To capture the image of a holographic fringe pattern electronically, the pair used a charge-coupled device (CCD)-a light-sensitive chip similar to those found in conventional video cameras. These chips have only a limited number of pixels-in this case 400 000. So to cut down the information content of the hologram, the researchers placed a small aperture between the subject and the camera to ensure that only light coming squarely off the subject could interfere with the reference beam.
The result was a moving, 2D holographic image with no 鈥渓ookaround鈥�. While such an image is of little use for humans hoping for 3D displays, Hashimoto and Morokawa believe it may be a useful way of handling and analysing information in the optical computers of the future.
Some researchers believe that this approach is doomed. Holograms simply contain too much information to be recorded with today鈥檚 CCD cameras, they say. Stephen Benton is one who holds such views. A professor at the Massachusetts Institute of Technology鈥檚 Media Laboratory, he is best known for developing holograms that can be viewed in white light. The tiny rainbow holograms on credit cards are only possible because of this work.
His approach to holography, and that of the Spatial Imaging Group which he heads, is also to reduce the amount of information holograms contain but in a way that viewers do not notice. One trick is to reduce the resolution, especially in the depth axis where people notice it least. It is also possible to cut out vertical parallax so that viewers can look around objects as they move from side to side but not as they move up and down. Rainbow holograms are a good example of this.
Other qualities can also be manipulated. Hold your finger in front of your face while looking at this sentence and focus on it. This article in the background should appear blurred. Focusing back again now makes your finger appear blurred. This ability is called accommodation and the brain uses it, in addition to stereo vision, as a way of judging depth. But accommodation is not essential. For example, Victorian stereograms produce a 3D effect without it.
Even the fact that objects nearer to the viewer hide objects further away, a property called occlusion, can be manipulated to reduce information content. The 3D displays that dispense with this property show ghostly semitransparent images superimposed on one another.
Information overload
Exercising this kind of control over a hologram can only be done by a computer. In fact, Benton does away with the optical recording entirely and generates his holograms on computer. Researchers at his lab can now make moving holograms of any object that can be represented by a 3D data set, including people, outdoor scenes and even fictional objects.
Computationally, this is a huge task and very different from the kind of manipulations normally done on 3D computer graphics. To make a hologram in this way, the computer creates a virtual version of the diagram. Instead of projecting real laser light, it simply calculates the path and phase of the light that would reflect off each point on a virtual object. It then calculates how this beam would interfere with the reference beam. The result is a virtual hologram that can be manipulated in the same way as any other data set.FIG-mg20914901.jpg
When Benton started this work in the 1980s, researchers at the Media Lab used a Thinking Machines supercomputer with 16 000 parallel processors to do the job. Even then, each frame in their films took several hours to compute.
So the researchers have spent the past 8 years paring down the information content of their holograms and improving the efficiency of the computer codes used to produce them. Today, Silicon Graphics workstations perform the calculations, even for large images, in just half an hour. These holograms have almost all the depth information contained in a real scene- accommodation, occlusion and horizontal parallax-but no vertical parallax. Benton says that by cutting out accommodation, the calculation time drops to just 5 seconds. And as computers become faster, the calculation time will become even quicker. These holographic images take up a mere 36 megabytes compared with the 276 megabytes of the early holograms.
There are trade-offs, however. Without accommodation, the hologram may appear flat to people with only one eye or with differing eye strengths. Also, the perception of depth may be inaccurate, even for fully sighted people. So the images might not be suitable for precise medical applications.
Although Benton has managed to reduce the information content of holograms to an impressive level, his electroholograms are still huge compared with 2D video images. This has important implications for the rate at which the images can be displayed.
Ideally, moving, flicker free images should be displayed at around 60 frames per second. This requires a data link with the display capable of handling 2 gigabits per second. (By comparison, most office networks can handle less than 10 megabytes per second.) Although this data rate is possible, what is difficult is building a frame buffer-a device that assembles all the data in each image before sending them to the display.
To get around this, Benton uses a digital video processor known as Cheops which was developed at the Media Lab. In addition to assembling the data for each image, Cheops is able to perform fast, reasonably sophisticated data processing operations. This allows it to handle images in compressed form, dramatically reducing the amount of data that needs to be sent. Despite this, Benton鈥檚 holographic videos can only be displayed at a rate of five frames per second. But future computers will do better.
So, let鈥檚 assume that R2-D2 can compute the Leia hologram in real time using technology that is only a little bit better than that available today. The disc that Leia uses would have to store a huge amount of information, but even this may be plausible with technology now on the horizon. What is not possible is the way in which R2-D2 displays the hologram floating in midair, at least not with any foreseeable technology.
Other kinds of display are possible, however. The approach taken by researchers at the Citizen Watch Company is to shine a laser beam through a liquid-crystal display containing 100 000 pixels. To act like a hologram, each pixel in the display must diffract the light so that it interferes with light from neighbouring crystals. The refractive index of liquid crystals, a measure of the extent to which light passing into the crystals from another medium is bent, can be varied by applying a voltage across them.
This means that the scientists can introduce a phase difference in the light passing through them. And by controlling this phase difference, they can create interference fringes that mimic those of a hologram.
The problem is that liquid-crystal pixels are huge, almost a hundred times bigger than fringe patterns should be. To get round this problem, Hashimoto and Morokawa turned to the bizarre properties of light. When light passes through an aperture similar in size to its wavelength, it diffracts. But the strange laws of optics also allow diffraction to occur at much larger apertures if the image of the light passing through the aperture is demagnified. So by demagnifying their liquid-crystal display by a factor of 100, the Japanese researchers have managed to create a holographic image.
At the Media Lab, researchers are using a different approach. One way to alter the refractive index of certain types of crystal is to pass a travelling wave through them-a technique known as acousto-optic modulation. This wave can be created by squeezing the edges of the crystals very quickly using piezoelectric actuators, which change shape according to the voltage applied to them.
The wave train can be thought of as a series of fringes. By varying the rate of squeezing, the size of the fringes and spacing between them can be altered. With this type of modulation, the waves can be made to encode the information required to create a hologram.
The trick that Benton has perfected is a way of displaying these waves by passing a laser beam through the crystal. The problem is that the waves move at a rate of more than 600 metres per second. So the image has to be stabilised before it can be viewed. Benton does this with a set of spinning mirrors that exactly cancel out the motion of the waves through the crystal (see Diagram). The resulting image need only be demagnified by a factor of 10.
Benton鈥檚 results are impressive. He has already built a black-and-red 3D display measuring 15 centimetres wide by 8 centimetres high by 8 centimetres deep and a full-colour display that is the size of of a 3-centimetre cube.
Flash of genius
Better displays are in the pipeline. Liquid crystal displays with pixels small enough to diffract light are in development. And miniature display technologies, based on micromechanical structures, could also be roped into help electroholography techniques.
鈥淲hat we鈥檙e waiting for is some young genius to come along and take us to a no-moving-parts display technology,鈥� says Benton. He draws a parallel with the position John Logie Baird found himself in after developing a complex bulky, partly mechanical television set. Within a few years, Philo T. Farnsworth had developed an electronic version. Benton is confident that a similar revolution could transform the prospects for holographic video.
The most promising applications for holographic video are in the medical and industrial fields, where accurate 3D renditions could help surgeons to visualise organs in the body or designers to see their work in 3D without having to build prototypes. Benton believes that the first such applications may only be a few years away. Although, in the best tradition of Hollywood fairy tales, princesses may have it sooner.
