Holography

The invention: A lensless system of three-dimensional photography that was one of the most important developments in twentieth century optical science. The people behind the invention: Dennis Gabor (1900-1979), a Hungarian-born inventor and physicist who was awarded the 1971 Nobel Prize in Physics Emmett Leith (1927- ), a radar researcher who, with Juris Upatnieks, produced the first laser holograms Juris Upatnieks (1936- ), a radar researcher who, with Emmett Leith, produced the first laser holograms Easter Inspiration The development of photography in the early 1900’s made possible the recording of events and information in ways unknown before the twentieth century: the photographing of star clusters, the recording of the emission spectra of heated elements, the storing of data in the form of small recorded images (for example, microfilm), and the photographing of microscopic specimens, among other things. Because of its vast importance to the scientist, the science of photography has developed steadily. An understanding of the photographic and holographic processes requires some knowledge 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: The phase determines on which part of the wave the cork is floating at any given time, and the amplitude determines how high or low the cork can be moved. Waves from more than one source arriving at the cork combine in ways that depend on their relative phases. If the waves meet in the same phase, they add and produce a large amplitude; if they arrive out of phase, they subtract and produce a small amplitude. The total amplitude, or intensity, depends on the phases of the combining waves. Dennis Gabor, the inventor of holography, was intrigued by the way in which the photographic image of an object was stored by a photographic plate but was 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. On Easter morning in 1947, as Gabor was pondering the problem of how to improve the electron microscope, 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 with a definite phase. This two-stage method was inspired by the work of Lawrence Bragg. Bragg had formed the image of a crystal lattice by diffracting 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 the phase information of the X-ray photograph. Therefore, he could study only those crystals for which the phase relationship of the reflected waves could be predicted. Waiting for the Laser Gabor devised a way of capturing the phase information after he realized that adding coherent background to the wave reflected from an object would make it possible to produce an interference pattern on the photographic plate. When the phases of the two waves are identical, a maximum intensity will be recorded; when they are out of phase, a minimum intensity is recorded. Therefore, what is recorded in a hologram is not an image of the object but rather the interference pattern of the two coherent waves. This pattern looks like a collection of swirls and blank spots. The hologram (or photograph) is then illuminated by the reference beam, and part of the transmitted light is a replica of the original object wave. When viewing this object wave, one sees an exact replica of the original object. The major impediment at the time in making holograms using any form of radiation was a lack of coherent sources. For example, the coherence of the mercury lamp used by Gabor and his assistant IvorWilliams was so short that they were able to make holograms of only about a 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 was rekindled after the laser (light amplification by stimulated emission of radiation) was developed in 1962. Emmett Leith and Juris Upatnieks, who were conducting radar research at the University of Michigan, published the first laser holographs in 1963. The laser was an intense light source with a very long coherence length. Its monochromatic nature 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 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 made it possible to record holograms of diffusely reflected three-dimensional objects. Gabor had seen from the beginning that this should make it possible to create three-dimensional images. After the early 1960’s, the field of holography developed very quickly. Because holography is different from conventional photography, the two techniques often complement each other. Gabor saw his idea blossom into a very important technique in optical science. Impact The development of the laser and the publication of the first laser holograms in 1963 caused a blossoming of the new technique in many fields. Soon, techniques were developed that allowed holograms to be viewed with white light. It also became possible for holograms to reconstruct multicolored images. Holographic methods have been used to map terrain with radar waves and to conduct surveillance in the fields of forestry, agriculture, and meteorology.By the 1990’s, holography had become a multimillion-dollar industry, finding applications in advertising, as an art form, and 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 whenever the medium around the object to be viewed is opaque to light rays—for example, in medical diagnosis. Holography has affected many areas of science, technology, and culture.