Gabor Develops the Concept of Holography

Date 1947

Dennis Gabor created a lensless system of three-dimensional photography, one of the most important developments in twentieth century optics.

Locale Rugby, England

Key Figures

• Dennis Gabor (1900-1979), Hungarian-born inventor and physicist and 1971 Nobel laureate in physics
• Emmett Leith (1927-2005) and
• Juris Upatnieks (b. 1936), radar researchers at the University of Michigan who produced the first laser holograms

Summary of Event

Since 1900, the recording of images using the technique of photography has been commonplace. The optical lens had been in use for several centuries and the formation of images using lenses was well understood. The development of photography in the early twentieth century increased greatly the importance of the lens to the scientific community. Combining the optical lens and the process of photographic emulsion made possible the recording of events and information in a way unknown before the twentieth century: photographing star clusters, recording emission spectra of heated elements, storing data in the form of small recorded images (for example, microfilm), photographing microscopic specimens, and many others. Because of its vast importance to scientists, the science of photography has developed steadily.

An understanding of the photographic process and of the holographic process requires some clarification of the wave behavior of light. Light is an electromagnetic wave that, like a water wave, has an amplitude and a phase. The amplitude corresponds to the wave height, while the phase indicates which part of the wave is passing a given point at a given time. A cork floating in a pond bobs up and down as waves pass under it. The position of the cork at any time depends on both amplitude and phase. Waves from more than one source arriving at the cork combine in ways that depend on their relative phases. If the waves arrive in phase, they add and produce a large amplitude; if out of phase, they subtract to produce a small amplitude. The total amplitude, or intensity, depends on the phases of the combining waves.

In ordinary, nondigital photography, light radiating from a point of the object being photographed is focused by a lens on a photographic film to create an image of the point. Images of each point are created similarly. A record of the intensity of the image points is stored on the film without any phase information about the combining wave fronts. The clarity of the image depends in large part on the quality of the focusing lens.

Dennis Gabor, the inventor of holography, had been intrigued since his teenage years by the way the photographic image of an object was stored by a photographic plate, but he had been unable to devote any consistent research effort to the question until the 1940’s. At that time, Gabor was involved in the development of the electron microscope. By 1947, the electron microscope had improved the resolving power of the ordinary light microscope by a factor of 100, but it was still unable to resolve objects the size of atoms and small molecules. This inability resulted from imperfections in the objective of the microscope. If the aperture of the objective was decreased to reduce the spherical aberration of the lens, then the diffraction of the electrons produced a blurred image. If the aperture was opened to reduce diffraction, then the spherical aberration of the lens blurred the image. The theoretical work of O. Scherzer indicated that the limit of the resolving power of the objective was twice that needed to “see” atoms. The practical limit at that time was about twelve times. The problem faced by Gabor seemed insurmountable.

Gabor was pondering the problem of how to improve the electron microscope while sitting beside a tennis court on Easter morning in 1947 when the solution came to him. He would attempt to take a poor electron picture and then correct it optically. The process would require coherent electron beams, that is, electron waves having definite phase. This two-stage method was inspired by Lawrence Bragg. Bragg had formed the image of a crystal lattice by means of diffraction from the photographic X-ray diffraction pattern of the original lattice. This double diffraction process is the basis of the holographic process. Bragg’s method was limited because of his inability to record phase information of the X-ray photograph. So, only crystals, for which the phase relationship of the reflected waves could be predicted, could be studied.

Gabor devised a way of capturing the phase information. By adding coherent background to the wave reflected from the object, an interference pattern was produced on the photographic plate. When the phases of the two waves are identical, a maximum intensity will be recorded; where they are out of phase, a minimum intensity is found. Therefore, what is recorded in a hologram is not an image of the object but rather the interference pattern of the two coherent waves. It looks like a collection of swirls and blank spots. The hologram (or photograph) then would be illuminated by the reference beam and part of the transmitted light would be a replica of the original object wave. By viewing this object wave, one sees an exact replica of the original object.

Gabor’s original intention was to improve the resolving power of the electron microscope. He intended to form the hologram with electrons and then to illuminate it with visible light. The wavelength of visible light is on the order of 100,000 times the wavelengths of electrons. The magnification achieved should be on the order of 100,000.

To demonstrate the feasibility of the concept, Gabor attempted to make a hologram using visible light. The major impediment at the time in making holograms using any form of radiation was a lack of coherent sources. For example, the coherence length of the mercury lamp used by Gabor and his assistant Ivor Williams was so short that they were able to make holograms of only about 1 centimeter in diameter. The early results were rather poor in terms of image quality and also had a double image. For this reason, there was little interest in holography and the subject lay almost untouched for more than ten years.

Interest in the field returned after the development of the laser in 1962. Emmett Leith and Juris Upatnieks, who were doing radar research at the University of Michigan, published the first laser holographs in 1963. Leith and Upatnieks used an off-axis reference beam rather than the in-line arrangement that Gabor was forced to use. Gabor actually had noted from the start that this off-axis method would allow the separation in space of the twin image. The laser was an intense light source with a much longer coherence length. Its monochromatic nature allowed the off-axis reference scheme to be used to its full potential and improved the resolution of the images greatly. Also, there was no longer any restriction on the size of the object to be photographed.

The availability of the laser allowed Leith and Upatnieks to propose another improvement in the holographic technique. Before 1964, holograms were made of only thin, transparent objects. A small region of the hologram bore a one-to-one correspondence to a region of the object. Only a small portion of the image could be viewed at one time without the aid of additional optical components. Illuminating the transparency diffusely allowed the whole image to be seen at one time. This development also permitted the recording of holograms of diffusely reflecting three-dimensional objects. Gabor had seen from the beginning that this should allow the formation of three-dimensional images.

After the early 1960’s, the field of holography developed very quickly. Because it is basically different from conventional photography, the two techniques often complement each other. Gabor saw his idea blossom into a very important technique in optical science.

Significance

Because of the lack of intense, coherent sources of light, the early holograms produced images of rather poor quality. For this reason and because of the technical difficulties in making holograms, the new technique of holography aroused little interest for the first fifteen years after its development. The development of the laser and the publication of the first laser holograms in 1963, however, caused a blossoming of the new technique in many fields. Techniques were developed quickly that allowed holograms to be viewed with white light and also holograms that could reconstruct multicolored images.

In biology, the use of holography makes it possible to overcome basic limits of the microscope. Holographic methods also have been used to map terrains with radar waves and in Earth surveillance for forestry, agriculture, and meteorology. The advertising profession as well has not been slow to see the possibilities of holography. The three-dimensional image of a hand holding a diamond necklace over the entrance to a jewelry store attracts customers. At the opening of a new automobile plant, a display case contains alternately a coach and the body of a new car. From time to time, the case is illuminated with diffuse light to show that it is really empty.

Advances in computer memory and digital technology has made holography a multibillion-dollar industry. It has found applications in advertising, in the making of art, in security devices on credit cards, as well as in scientific fields. An alternate form of holography, also suggested by Gabor, uses sound waves. Acoustical imaging is useful wherever the medium around the object is opaque to light rays, for example, in medical diagnosis.

Bibliography

Gabor, Dennis. “Holography, 1948-1971.” Science 177 (July 28, 1972): 299-313. A revised version of Gabor’s Nobel lecture. It traces the development of holography from its beginnings in a nonmathematical way. Good for readers with a very general knowledge of physics.

Johnston, Sean F. Holographic Visions: A History of New Science. New York: Oxford University Press, 2006. The story of how holography spread as a science to the arts, science fiction, business, counterculture, and popular culture. Illustrations, bibliography, index.

Kasper, Joseph, and Steven Feller. The Complete Book of Holograms. New York: John Wiley & Sons, 1987. Geared for use in an introductory course at the high school or college level. Kasper and Feller explain holography using many diagrams but no mathematics. There are discussions of applications of the technique and descriptions of how to make holograms.

Leith, Emmett, and Juris Upatnieks. “Photography by Laser.” Scientific American 212 (June, 1965). A fairly nontechnical description of the first production of holograms using laser light. Includes numerous photographs and diagrams.

Pennington, Keith S. “Advances in Holography.” Scientific American 218 (February, 1968). Discusses the advances made in the holographic technique as well as some applications of holography in the mid-1960’s.

Williamson, Samuel J., and Herman Z. Cummins. “Holography.” In Light and Color in Nature and Art. New York: John Wiley & Sons, 1983. This college-level introductory text is geared for those with little background in mathematics or science. Describes various types of holograms and illustrates the principles of holography in a basic manner.