Holography

Type of physical science: Classical physics

Field of study: Optics

Holography is an imaging technique which can record the interference pattern produced by light scattering off an object and can reconstruct a three-dimensional image of the object from that interference pattern.

Overview

A hologram is a record, usually in the form of a photographic image, of the interference pattern produced when two beams of coherent light are permitted to interact with each other. In practice, when one of the light beams is allowed to scatter off an object before the interference pattern is produced, the hologram can be used to produce an image of that object. This holographic image is three-dimensional; that is, by changing the viewing position, it is possible to see around the sides of the object.

Although holographic images are generally associated with laser light, the theory of holography was developed and demonstrated before the laser was invented. In principle, the theory of holography could have been understood as early as 1816, when Auguste Fresnel developed a mathematical explanation for wave diffraction and interference. The birth of holography, however, can be traced to research in 1947 by Dennis Gabor (1900-1979), a British electrical engineer, on how to improve electron microscopy.

The problem confronting Gabor was that images produced by electron microscopes were not sharp because the electromagnetic lenses that were used to focus the electron beam were poor. He sought a technique in which images could be obtained without the need for lenses.

Since a beam of electrons with the required properties was not available at the time, Gabor conducted his experiments using visible light from a filtered mercury arc lamp. He described the holographic imaging technique in his 1949 paper "Microscopy by Reconstructed Wavefronts."

He coined the word "holography," from the Greek words holos (meaning whole) and grapho (meaning to write), to describe the images that he produced. Gabor was awarded the 1971 Nobel Prize in Physics for this research.

For more than a decade, holography remained only a scientific curiosity because light sources with the coherence required to produce good holographic images were not available. The development of the laser, a device that can produce intense beams of coherent light, in the early 1960's gave impetus to new ideas on holography. In the United States, Emmett Leith and Juris Upatnieks pioneered the development of holographic techniques using laser illumination. In 1963, they produced the first "laser transmission hologram" (a hologram that must be viewed in coherent light, usually from a laser) of a three-dimensional object--in this case, a model railroad engine. Laser transmission holograms of the type developed by Leith and Upatnieks are generally used in scientific and industrial applications of holography.

Simultaneously and independently Yu. N. Denisyuk, of the Soviet Union, pioneered the reflection hologram technique, allowing holograms to be viewed in ordinary white light. This technique has found popularity in art. When coupled with a method for duplicating holograms in a manner similar to producing compact discs (CDs), it became possible to manufacture reflection holograms in mass quantities for distribution to consumers. Although the early holograms were in one color because of the monochromatic nature of the light beams, Denisyuk, in the early 1980's, demonstrated a technique for producing natural color holograms.

The idea of holography rests on the physical principle of interference between waves, which can be understood by analogy to ocean waves. When one is standing at a fixed point, the surface of the ocean will be observed to rise, then fall, then rise again in a cyclic or periodic manner. Overall, the crest (high point) of the wave will be seen to move in a direction called the propagation direction. All waves have two properties: an amplitude, which is a measure of the maximum height, and a phase, which is a measure of where the wave is in the cycle of rising and falling motion at a specific location and time.

When two waves meet each other, they "interfere," or combine with each other to form a single wave. When their phases are such that both waves are at or near their crests, their displacements are added. This sum gives a very high crest of the combined wave, a condition called "constructive interference." When the phases oppose each other, however, so that one wave is very high while the other is near a low point, the interference is "destructive"; that is, the displacements are subtracted.

In normal photography, only the intensity of the light scattered from the object of interest is recorded on the photographic film. The process of holography allows information on both the intensity and the phase of the scattered light to be recorded on the film, which is accomplished using the interference principle. A coherent beam of light is split into two beams.

One beam, called the reference beam, is allowed to travel directly to the photographic film. The second beam, called the object beam, is reflected from the object of interest and then allowed to travel to the photographic film. The interference pattern produced by the interaction of the reference beam and the object beam at the surface of the film contains information on the amplitude and phase of the scattered light. This information is recorded by the film as a hologram.

In order to produce a holographic image, it is important that neither the photographic film nor the object move during the exposure. Since the holographic image is actually a record of the interference between two light beams, even a motion as small as one-tenth of the wavelength of the light, or 6 x 10^-6 centimeters, will affect the visibility of the image.

If viewed directly, the resulting transmission hologram does not seem to contain an image of the object at all. When viewed in normal light, the holographic image appears as a series of gray smudges on the photographic plate. When viewed in the coherent light of a laser, however, a three-dimensional image of the object appears. The hologram has a number of unusual properties. Perhaps most unusual is that, if a hologram is cut into a number of pieces, each piece contains the interference pattern. Thus, when viewed, the entire object still appears.

Only the resolution of the image is reduced as smaller and smaller fragments of the original hologram are viewed.

Applications

The invention of the laser ushered in the era of practical holography. Beginning in the early 1960's, patents were issued for a variety of holographic devices, ranging from holographic stress testing of aircraft wings to holographic, three-dimensional television systems.

Holography makes it possible to store large amounts of information in very small spaces--ten to one hundred times the amount of data per unit area than on microfilm or microfiche--and to read these data back at a very high rate. A major advantage of holographic data storage is that a particular datum, such as the printed letter a, would appear as a single symbol on microfilm and could be obliterated by a dust particle or a scratch. Using holography, the whole image contains the symbol, and a single small scratch or dust particle does not cause a noticeable degradation in the data.

Holography has been applied to many practical problems in engineering and manufacturing. The discovery that two or more holographic images could be compared very precisely by simply superimposing them allowed the study of variations in objects over time.

This technique was used to evaluate the shape of a microwave antenna which was 2.75 meters in diameter, ensuring that its contour was accurate to thousandths of a centimeter.

By obtaining a series of holograms, properly spaced in time, the response of an object to shock or stress can be studied without attaching sensors to the object itself. Since such sensors frequently alter the response of the object to the shock or stress, the holographic technique offers a considerable advantage. The National Aeronautics and Space Administration (NASA) and its contractors have employed sequential holographic images to the study of titanium welds, the deformation of honeycomb structures, and the formation of microcracks.

NASA has also pioneered the development of holographic techniques that can visualize fluid flow, or map the temperature and density changes that occur in wind tunnels and rocket nozzles. The use of holography, as well as other optical techniques, allows the replacement of conventional temperature and pressure probes, which altered the original flow patterns.

The Environmental Protection Agency, working with private contractors, has developed a holographic system to measure the effectiveness of "scrubbers," the devices designed to remove pollutants from smokestack emissions.

In the medical field, holography has been employed as a method to store three-dimensional data in a small space. Holograms can be made from computerized axial tomography (CAT scan) images, allowing doctors to view three-dimensional images of tumors or other growths directly rather than being presented with sequential "slices," or two-dimensional images. Dental records, particularly dental casts, can be stored and the resulting images measured later using traditional tools such as calipers. Sequential holograms have also been employed to map the stress patterns on normal hipbones and their artificial replacements, allowing the artificial joints to be optimized to mimic the load transfer of the natural joints.

Biological applications of holography have generally focused on microscopy. If an ordinary camera is used to produce a photographic image of an object seen in a microscope, then only a thin slice of the object appears in focus. If the object is three-dimensional, such as a fluid drop containing moving micro-organisms, then the micro-organisms can move from slice to slice as multiple pictures are taken. A single holographic image of the fluid drop, however, will freeze the motion at a single instant in time. The hologram can then be placed in the microscope and illuminated with the laser reference beam, and a series of traditional photographs can be taken showing the positions of all the micro-organisms at one instant of time.

The almost magic nature of the holographic image has also appealed to the advertising industry. At the opening of an automotive building, the image of an automobile was shown through four holographic windows in a large display case. Occasionally, the interior lights in the display case were turned on to demonstrate to the viewers that the case was actually empty.

Holography has served as a tool in art. Initially, the three-dimensional imaging capability of the hologram was important. It allowed easy distribution of three-dimensional images of art objects, such as sculptures, around the world. Holography has also emerged as a creative tool, however, with the resulting hologram being the artwork. Some artists, for example, move the object slightly during the image formation, which results in a three-dimensional black spot where the object was located.

Mass-produced holograms have also made an appearance as consumer products. The production of eleven million reflection holograms of an eagle, serving as an insert on the cover of the March, 1984, issue of NATIONAL GEOGRAPHIC, was the first mass distribution of a reflection hologram. Reflection holograms have also been used as book illustrations, particularly for art books showing three-dimensional objects such as statues, and on credit cards as counterfeit-protection devices.

Context

Although the principle of holography was recognized in 1947, it was not until the laser was invented that holography became practical. Beginning in the early 1960's, the laser-generated holograms moved from the realm of a scientific curiosity to a tool for scientific research, engineering analysis, and quality control in manufacturing.

Holographic data storage holds considerable promise because of the large amount of information that can be contained in a single holographic image. In theory, it should be possible to store and retrieve vast quantities of information using light-sensitive crystals such as lithium niobate. When coupled with modern, high-speed computers, such holographic data storage devices may allow a revolution in information-storage technology.

Holograms can be made using all forms of waves, such as X rays, microwaves, and acoustic waves, not only visible light. X-ray holograms offer the promise of three-dimensional views of human cells, which would assist biologists in studying how the cells work. Microwave holography offers promise in mapping terrain contours from an aircraft, thus allowing maps to be produced more rapidly and accurately. Acoustic holography is being explored for nondestructive defect detection in manufacturing, as well as in medical diagnosis.

Holography also offers promise in the area of pattern recognition. Experiments have been conducted to convert law-enforcement files of fingerprints to a holographic format, allowing rapid comparison and information retrieval. The military has explored the possibility of comparing battlefield holographic images of tanks or other armored vehicles with images stored in memory, thus increasing the reliability of the correct identification of enemy vehicles. As artists continue their experimentation with the holographic process, new and unusual types of images are sure to emerge.

The success of holographic images as a counterfeit-protection device on credit cards caused many national governments to consider holographic images as a counterfeit-protection device on currency. The first experimental holographic currency was released by Australia in 1990. Holographic images have also appeared on postage stamps.

Patents have been issued for processes to display holographic three-dimensional images using television or film projectors. Although the amount of information contained in a single hologram seems to preclude the transmission of holographic moving images by conventional television broadcast stations, the larger information capacity of videodiscs may make holographic television a possibility. Holographic projection devices, which would permit three-dimensional, color images in film theaters, have also been the subject of experimentation and patents.

Since the early 1960's, applications of holography in scientific research, engineering, manufacturing, and art have developed. Breakthroughs in the quality and cost of holographic-image reproduction make holographs a likely consumer product of the future.

Principal terms

AMPLITUDE: the maximum displacement of a point on a wave from its average value

COHERENT LIGHT: light of a single frequency which is vibrating in phase

FREQUENCY: the number of wave crests that pass a fixed point in a fixed unit of time

INTERFERENCE: the process of combining two waves in which, at every instant in time and point in space, the displacements of the two waves are added

LASER: a device which produces coherent light by a stimulated emission of radiation

MONOCHROMATIC: radiation, such as light, of a single frequency

PHASE: the relationship between the position of a wave's crest and a fixed reference point

WAVELENGTH: the physical distance between two adjacent crests of a wave

Bibliography

Caulfield, H. John. "The Wonder of Holography." NATIONAL GEOGRAPHIC 165 (March, 1984): 364-377. A well-written and well-illustrated account of the history of holography and its applications in science, technology, and the arts. Includes good illustrations of how a holographic image is formed and how it is viewed.

Dudley, David D. HOLOGRAPHY: A SURVEY. Washington, D.C.: National Aeronautics and Space Administration, 1973. An early summary of the practical applications of holography, citing research and engineering uses developed at NASA centers and in industry. Contains well-illustrated, nonmathematical descriptions suitable for general readers.

Hariharan, P. OPTICAL HOLOGRAPHY. New York: Cambridge University Press, 1984. An extremely good, but mathematically detailed, reference on the theory of holography. Contains an extensive bibliography.

Kaspar, J. E., and S. A. Feller. THE HOLOGRAM BOOK. Englewood Cliffs, N.J.: Prentice-Hall, 1985. A well-written discussion of the principles of holographic-image formation using simple, well-illustrated, geometrical arguments.

Saxby, Graham. PRACTICAL HOLOGRAPHY. Englewood Cliffs, N.J.: Prentice-Hall, 1988. A 488-page description of holography, including a thorough discussion of its applications in science, industry, and the arts. Well illustrated and contains an extensive bibliography.

Unterseher, Fred, Jeannene Hansen, and Bob Schlesinger. HOLOGRAPHY HANDBOOK: MAKING HOLOGRAMS THE EASY WAY. Berkeley, Calif.: Ross Books; 1982. A 407-page practical guide to holography which emphasizes the how-to aspects. Contains a good historical discussion and a very well-illustrated description of the theory of hologram-image formation.

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