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Holography - Introduction, Concepts and Technique, Types of Hologram, Applications, Changing Status

holograms image light object

SEAN JOHNSTON
University of Glasgow Crichton Campus

Introduction

The word hologram is derived from Greek roots meaning whole picture and is frequently used today in credit card emblems and as an imagined technology in Star Wars and Star Trek. It has a history extending back to the mid-twentieth century. Holography is a subject that grew around holograms and had a difficult birth. It created a new kind of technical imaging professional, the holographer. Conceived by Dennis Gabor in 1947 as a means of improving electron microscopy, his approach used wavefront reconstruction. It began to quickly evolve into holography in the 1950s when physicists and electrical engineers working mainly in classified laboratories combined findings in information theory and coherent optics. Yuri Denisyuk, an engineer and graduate student at the Vavilov State Optical Institute in Leningrad (now St Petersburg), developed the so-called wave photography in 1958. At the University of Michigan’s Willow Run Laboratories, in 1953, Emmett Leith worked on synthetic aperture radar (which has theoretical similarities to wavefront reconstruction). In 1960, with colleague Juris Upatnieks, he worked on the development of so-called lens less photography. From late 1962 their insights from electrical engineering and communication theory were combined with newly available lasers to yield a powerful imaging technique; by the end of 1963 their research had yielded astonishingly realistic three-dimensional imagery.

Despite the popularization of lens less photography, the analogy between the hologram and the photograph is nevertheless a loosely defined one. The Leith-Upatnieks hologram is a kind of transparency, but the image is observed by looking through the hologram as though through a window. It is a two-step process and it creates the featureless surface often described as storing the image information necessary to later reconstitute a visual representation of the original scene. Thus a photographic contact copy of a hologram does not yield a negative image, but another positive. And unlike a photograph, the hologram can recreate a view of the entire image from any part; the pieces of a broken hologram still work. The dynamic range (variation from bright to dim in the image) is much larger in holograms than in photographs. But the technique is restrictive: Only small laboratory scenes (normally a few cubic meters at best) can be recorded. The first generation of holograms was made using the laser as a light source. The laser was not just for initial recording but also required for subsequent viewing of the holograms. Despite these unfamiliar attributes, photography was to be a convenient guide to understanding the new medium of holography and to forecasting its future development.

From 1966 Willow Run and other laboratories extended what was known as holography to create stunning displays and exquisitely sensitive optical measurement techniques. Within a decade about 100 books, 200 theses, and several thousand scientific papers had been published on the subject. Commercial and military funding skyrocketed, and predictions for applications multiplied.

Concepts and Technique

Gabor, Denisyuk, Leith, and Upatnieks developed distinct optical geometries for holography, but all of them relied on a two-step process. Initially two beams from a single coherent source of light (usually a laser) are combined to yield interference fringes. One of those beams (known as the reference beam) strikes the photosensitive surface directly. The other (the object beam) is reflected onto the surface, or otherwise modulated, by the object to be imaged. The combination of the two beams yields a fine pattern of interference fringes. This is a series of dark lines at positions where the two beams differ in phase by a half-wavelength of light and bright lines where the difference amounts to a whole wavelength. The average scale of this pattern (known in communication theory as the carrier frequency) is related to the angle between the reference and object beams. In Leith-Upatnieks holograms, the angle is typically between 10 and 100 degrees; in Denisyuk holograms, the angle is about 180 degrees and the reference beam strikes the photosensitive plate from the side opposite the object beam. Because the optical setup must remain motionless to a fraction of a fringe during exposure, the Denisyuk technique demands even more scrupulous vibration isolation than does the Leith-Upatnieks method.

The details of the interference pattern bear no obvious relationship to the object itself, but depend on the light scattered from every point of the object, its relative distance, and the intensity of reflection. In general, every point recorded on the holographic plate is a combination of light from every part of the reflecting object. The pattern amounts to an encoded recording of the wavefront of light scattered from the object.

The microscopic pattern—consisting of hundreds to thousands of irregular lines per millimeter—can be recorded on conventional high-resolution silver halide photographic film and processed by standard techniques. Such recordings, known as amplitude holograms, can also be made on photoresist materials. Alternatively, the fringes can be recorded as variations in refractive index or thickness in materials such as bleached silver halide emulsions, dichromated gelatin, synthetic photopolymers, or thermoplastics. Embossed holograms, available since the 1980s, record height variations of the fringe pattern by various proprietary processes. These may begin with a silver-halide hologram or photoresist from which a master, usually in metal, is produced. Copies can then be made by either casting or (more commonly) embossing in PVC and then applying a reflective coating. This yields an inexpensive product, stamped by little-modified conventional printing presses to yield a copy of the fringe pattern in the form of fine surface corrugations. All of these so-called phase holograms produce the same effect in the second step of imaging, but yield brighter images than amplitude holograms.

The second (reconstruction) step is less technically demanding than the first (recording) step. In the hologram, the illuminated copy of the original reference beam is used. The complex fringe pattern of the hologram causes diffraction of the beam and regenerates as an image that is, in all visual respects, a copy of the wavefront that originally reached the recording plane.

Types of Hologram

The description above is incomplete because, in practice, holograms are usually created in ways that allow them to be viewed in white (non-coherent) light. These variants allow the reconstruction step to be considerably eased, although still more demanding than viewing a conventional photograph.

The most elegant white-light holograms are those made using Denisyuk’s geometry. Such holograms record an unusually minute fringe pattern through the depth of the emulsion. Like Gabriel Lippmann’s color photography process, which won the 1908 Nobel Prize for Physics, these planes selectively reflect a narrow range of wavelengths. Denisyuk’s reflection holograms reconstruct a high-quality yellow or green image, even when illuminated by white light, but require thick, high-resolution emulsions and are less amenable than other types to mass production.

The first white-light holograms created in the Western world, however, were known as image-plane holograms. These holograms were created by focusing an image using a lens or the real image reconstructed from a master hologram, onto or near, the hologram plane. For points close to the plane, the image appears sharp and colorless (by contrast, Leith-Upatnieks holograms, which record objects behind the plate without lenses, act as complex diffraction gratings and produce an image smeared into a spectrum of colors when viewed in white light).

A third popular type is the white-light transmission hologram, or rainbow hologram, developed by Stephen Benton of Polaroid Corporation in Cambridge, Massachusetts. This interposes a narrow slit between the object and the hologram plate. By doing so, it allows the hologram to reconstruct an image in white light, but sacrifices vertical parallax in the process, so that moving from side to side still allows the viewer to look around the object, but vertical movement merely shows the same sharp image in different spectral colors.

The image-plane and rainbow techniques are frequently combined in display holograms, and can be further combined with other techniques, notably integral holograms (also known as holographic stereograms). An integral hologram is synthesized as separate side-by-side vertical strip holograms from individual two-dimensional images such as movie frames. Each eye views a different strip image, yielding a binocular stereoscopic view. In the earliest commercial version developed by Lloyd Cross in San Francisco and known as multiplex holograms, animated rainbow images could be viewed by walking around a cylindrical hologram illuminated by an unfrosted light bulb at its base. Later versions were usually recorded on flat surfaces, and sometimes aluminized to allow them to be mounted on a wall and illuminated by a halogen lamp.

Applications

The applications of holography have changed in the decades since its invention. During the 1960s, companies such as Conductron Corporation in Ann Arbor, Michigan, attempted to market Leith-Upatnieks and Denisyuk holograms for product displays and magazine inserts with little success, and developed pulsed lasers to record live subjects for advertising displays. The most successful application at that time was holographic non-destructive testing (HNDT) based on holographic interferometry. Through the 1970s, HNDT became popular with automotive and aerospace manufacturers as an extremely sensitive technique for mechanical engineering design.

Since 1969, a handful of artists have taken up holography with a few major exhibitions (notably New York in 1975, Stockholm in 1976, London in 1977–1978, Tokyo in 1978, and Rome in 1979), which touted the potential of the medium for fine art and commercial display. Such applications were also supported by regular conferences organized by the International Society of Optical Engineering, by wider ranging activities of the New York Museum of Holography (1976–1992), and by accredited teaching programs such as the Holography Unit of the Royal College of Art in London (1985–1994).

Despite the sprouting of a cottage industry and artisan holographers, display holograms failed to attract a large market. Specialist applications developed during the 1970s, though, for military and corporate sponsors. Holographic optical elements (HOEs), which allow the combination of reflective, lens-like, and diffractive optical properties in a single compact device, became increasingly sophisticated, finding their way into products such as camera viewfinders and bar-code scanners. Head Up Displays (HUDs) used in fighter aircraft also incorporated HOEs to combine an image of the instrument panel with the view out the cockpit window.

The applications took a new and more profitable turn with the development of embossed holograms as a low-cost medium. From 1983, embossed holograms graced magazine covers (notably National Geographic covers of 1984, 1986, and 1988) and reflective stickers for toys, but the flexible backing and their viewing under uncontrolled lighting made the image quality unreliable; they fell from favor by the end of the decade.

On the other hand, a sustained market for embossed holograms appeared in 1983 with the introduction of holographic security patches on MasterCard credit cards, an application first promoted by American Bank Note. These proved popular, spreading to most credit cards by the end of the decade, and then to security holograms on product packaging, currency, and identity documents. A similarly growing market was holographic packaging, in which imagery was sacrificed for attention-getting optical effects. For both applications, reliance on optically recorded holograms gradually fell, with more complex patterns achievable by computer-generated holograms. At present, security and packaging holography applications dominate the market.

Changing Status

In 1971, Gabor was awarded the Nobel Prize in Physics for his seminal work in what had become a burgeoning new field. Since then, holography has been taken up by a broad group of technical communities. The original cluster of scientists and engineers, who were originally supported by military and industrial contracts, were later joined in the early 1970s by artists and artisans. They recast the methods and goals of holography, transforming it from an expensive and delicate art into a publicly accessible medium. But while investigating and applying this new invention and aesthetic medium, these new communities of workers found that their early expectations were not entirely realized: Holography failed to make sustained progress in many of its forecast directions. Although it had been popularized as an extension and analog of photography, the subject did not evolve in the same way. Schools and galleries of the 1970s and cottage industries of the 1980s began to close, research groups retrenched, and the number of artists active in the medium diminished.

Today, research and development in holography has been channeled to a small number of important applications such as anti-counterfeiting measures, mechanical testing, and graphics for packaging, but it continues to attract new communities of enthusiasts. While publishing and conference activities have diminished by half compared to their peak during the 1980s, patents in holography continue to rise. Two areas, both reliant on increased computer power and its integration with modern optics, are of particular interest. The first of these is the long anticipated potential of holography as a convenient display technology for portraiture, based on improvements in automated digital hologram printers, an application being pursued by Japanese firms. The second is the generation of real-time holographic displays from computer-generated data, a tour de force of calculation that is becoming increasingly feasible. Most recent applications of holography, however, are in relatively arcane branches of modern optics, where it has been gradually integrated.

Holsey, Albon L.(1883–1950) - Organization executive, writer, Begins Work at Tuskegee Institute, Chronology, Writes Numerous Publications [next] [back] Holographic Interferometry

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Holography - Introduction, Concepts and Technique, Types of Hologram, Applications, Changing Status