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Holograms appear all over the place.  They can be seen in the television and movie industry or even in modern art galleries.  Yet, what exactly is a hologram?  Who invented it? How does it work?

Holography dates from 1947, when British/Hungarian scientist Dennis Gabor accidentally developed the theory of holography.  Gabor intended to increase the power of electron microscopes by using a light beam instead of an electron beam.  Instead of achieving his goal, Gabor created the first hologram.  Gabor then coined the term hologram from the Greek words holos, meaning "whole," and gramma, meaning "message." Further development in the field was prevented during the next decade because light sources available at the time were not truly coherent, or having a constant wavelength.
            This barrier was overcome in 1960 with the invention of the laser.  This pure, intense light was ideal for making holograms. A laser has one characteristic that makes it different from all other light sources.  A laser is coherent.  In 1962, Emmett Leith and Juris Upatnieks, who worked in side-reading radar, realized that holography could be used as a 3-D visual medium. In 1962, they read Gabor's paper and "simply out of curiosity" decided to duplicate Gabor's technique.  Leith and Upatnieks used a laser and an "off-axis" technique, which was borrowed from their work in the development of side-reading radar. The result became the first laser transmission hologram of 3-D objects (a toy train and bird). These transmission holograms produced images with clarity and realistic depth.  However, they required laser light to view the holographic image.  Leith’s and Upatnieks’ pioneering work led to standardization of the equipment used to make holograms. Today, thousands of laboratories and studios possess the necessary equipment: a continuous wave laser, optical devices (lens, mirrors and beam splitters) for directing laser light, a film holder, and an isolation table on which exposures are made.  Stability is absolutely essential in a hologram.  Movement, even as small as a quarter wave-length of light during exposures of a few minutes or even seconds, can completely spoil a hologram. The basic off-axis technique that Leith and Upatnieks developed is still the foundation creating the holographic method.
            Also in 1962, Dr. Uri N. Denisyuk of the U.S.S.R. combined holography with the work of Laureate Gabriel Lippmann, the 1908 Nobel-prize winner.  Lippmann’s main work consisted of natural color photography.  Denisyuk's approach produced a white-light reflection hologram.  This, for the first time, could be viewed in light from an ordinary incandescent light bulb.
            In 1960, the pulsed-ruby laser was developed by Dr. T.H. Maimam of the Hughes Aircraft Corporation. This laser system (unlike the continuous wave laser normally used in holography) emits a very powerful burst of light that lasts only a few nanoseconds. It effectively freezes movement and makes it possible to produce holograms of high-speed events, such as a bullet in flight or of living subjects. The first hologram of a person was made in 1967.  This paved the way for a specialized application of holography: pulsed holographic portraiture.
            In 1965, Robert Powell and Karl Stetson published the first paper on holographic interferometry. With this technique, small distortions between two holographic exposures of the same object -- one at rest and the other under stress -- are displayed as contours on the image. Holographic interferograms are useful in non-destructive testing of materials, fluid flow analysis, and quality control.
            Shankoff and Pennington developed the use of a dichromated gelatin as a holographic recording medium in 1967. This made it possible to record a hologram on any clear, non-porous surface.  From 1975 - 1984, Rich Rallison (International Dichomate Corp., Draper, UT) pioneered the use of dichromate holograms that were used as jewelry pendants and other premium items. This type of holography has been best used for high performance diffractive optics.
            The 1967 World Book Encyclopedia Science Yearbook contained what is arguably the first mass-distributed hologram.  This was a 4"x3" transmission view of chess pieces on a chess board. An article describing the production of the hologram and basic information about the history of holography accompanied it. A .05 watt Helium-Neon laser was used on a nine-ton granite table in a 30-second exposure to make the original.  All copies were then produced from this original.
            Another major advance in display holography occurred in 1968 when Dr. Stephen A. Benton invented white-light transmission holography.  He did this while researching holographic television at Polaroid Research Laboratories. This type of hologram can be viewed in ordinary white light creating a "rainbow" image from the seven colors which make up white light. The depth and brilliance of the image and its rainbow spectrum soon attracted artists who adapted this technique to their work.  These artists soon brought holography into further public awareness.
            Benton's invention is particularly significant because it made mass production of holograms possible while using an embossing technique. With this technique, developed by Michael Foster in 1974, holographic information is transferred from light sensitive glass plates to nickel embossing shims. The holographic images are "printed" by stamping the interference pattern onto plastic. The resulting hologram can be duplicated millions of times for a few cents apiece. Consequently, embossed holograms are now being used by the publishing, advertising, banking, and security industries.
            The first holographic art exhibition was held at the Cranbrook Academy of Art in Michigan in 1968. The second holographic art exhibition took place at the Finch College gallery in New York in 1970.  This attracted much national media attention.
            During the same year, Lloyd Cross, a physicist, and Canadian sculptor Gerry Pethick developed a sand-table system for making holograms that did not require expensive laboratory optics and an isolation table for stability during exposures. Optical components were stabilized by using PVC plumbing pipes inserted into the sand. This revolutionized the medium by making it accessible to artists.  Cross and his associates started the San Francisco School of Holography in 1971, the first such place for artists and scientists to learn the new medium.
            In 1972, Lloyd Cross developed the integral hologram by combining white-light transmission holography with conventional cinematography to produce moving 3-dimensional images. Sequential frames of 2-D motion-picture footage of a rotating subject are recorded on holographic film. When viewed, the composite images are synthesized by the human brain as a 3-D image.
            Later, Cross founded the Multiplex Corporation that produced hundreds of images using his holographic stereogram technique. That same year, Benton modified his white light transmission technique to make black and white (achromatic) images.  Three years later, the International Center of Photography in New York City featured Holography '75: The First Decade, produced by Jody Burns and Posy Jackson. While limited exhibition and productive work by scattered individuals proceeded slowly in the Western countries (mainly the United States, Germany, and Sweden), the Soviet Union rapidly pushed ahead research and production. It gave priority status to artists and scientists to work in elaborate state-financed laboratories. New developments were made in holographic movies and film emulsions.
            Holograms are diverse as most of our modern technology. Holograms take many different forms.  A myriad of miniscule differences separate them. As with most scientific and mathematical advances in our recent times, the common man cannot differentiate between the many different holograms without the interpretation of a holographic specialist.  However, here is how a simple hologram works:  The basic theory behind all holograms is that holograms are made by recording light interference on to a light sensitive area.  Then a light source, a laser since it has a constant wavelength, is split into two beams.  One of the beams is aimed at the object itself.  In the case in the above figure, the object is an apple.  This beam is called the reference beam.  The second of the two laser beams is directed towards the holographic plate.  These two beams then overlap and interfere with each other.  This creates the interference region.  This is shown in Figure 1.

Figure 1. How a Hologram Works                                                                               

         Waves are unique.  They each have special characteristics.  A basic wave is shown in Figure 2.  When two waves interfere with each other, one of two things can happen.  In one instance, there can be constructive interference.  This occurs when a trough meets a trough, or a crest meets a crest.  If this happens, they will add to each other and make the entire wave larger.  In the second case, destructive interference can occur.  Destructive interference happens when a trough meets a crest or a crest meets a trough.  If this occurs, each wave will cancel each other out.

Figure 2:  Characteristics of a Wave



            A person might wonder what this has to do with holograms.  The pattern of the wave interfering with each other is the actual recording being done in a hologram.  In a hologram, the light sources’ direction is recorded through the wave interference.  Therefore, the image is recorded as it is really there.  That is why the object becomes a 3-D image.  As stated before, stability is needed in order for a hologram to work.  The light source cannot move.  The object cannot move.  The surroundings have to remain the same.  The image will not appear if there is movement in even a fraction of a wavelength.  One way to counteract this movement is to buy a pulse ruby laser which makes the exposure very quick.  However, these lasers are very expensive.  A different, more affordable way is to construct an isolation table.  The purpose of this is to isolate all the objects from any source of movement.