Holography

History
Denis Gabor made the first hologram while working on electron microscopes in England in 1947. He used a mercury arc lamp and produced a hologram of a small transparency containing the names of famous scientists. His main problem was the lack of a coherent light source. The invention of the laser in the 1960s gave holography the coherent light source needed to give the size, brightness and depth of image which have intrigued and delighted viewers ever since. At first lasers were only available in universities and research laboratories, but now they are much cheaper, making them available to individuals. Further development introduced pulsed lasers which give a very powerful flash of only a few nanoseconds, enabling holograms to be made of living subjects.
In 1962 Emmett Leith and Juris Upatnieks of the University of Michigan recognized from their work in side-reading radar 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 using the laser and an "off-axis" technique borrowed from their work in the development of side-reading radar. The result was the first laser transmission hologram of 3-D objects (a toy train and bird). These transmission holograms produced images with clarity and realistic depth but required laser light to view the holographic image. Their 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 because movement 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 staple of holographic methodology.
Yuri Denisyuk, a Russian, devised an elegant method of using a single beam to act as both reference and object beam. Placing the film between the laser and the object allows the beam to pass through the film and be reflected off the object, so that the reference beam impinges on the film from one side, and the object beam from the other. Transmission holograms illuminated by white light give a "rainbow smear" effect. In 1968 Stephen Benton discovered a way of eliminating this effect. He made a transfer hologram masking off the master hologram through a narrow horizontal slit; when the second hologram is flipped, the image of the slit appears in front of the hologram close to the viewpoint, and only light of one colour is seen. This gives rise to the "rainbow" effect; as the viewpoint is moved vertically, the colour of the hologram changes. The invention of an embossing technique meant that holograms could be mass-produced for use as security devices and to enhance magazine covers. I
n 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.

Types of Holography

Transmission Holograms: Viewable with laser light. They are made with both beams approaching the film from the SAME side.
Reflection (White Light) Holograms: Viewable with white light from a suitable source such as spotlight, flashlight, the sun, etc. They are made with the two beams approaching the holographic film from OPPOSITE sides.
Multiple channel holograms: Two or more images are visible from different angles. There are different types of multiple channel holograms: Simple ones with 2, 3, or a few images each viewed from a different angle.
Multiplex: A large number of "flat" pictures of a subject viewed from different angles are combined into a single, 3-dimensional image of the object. A COMPOSED hologram.
Rainbow holograms: The same image appears in a different color when viewed from different angles.
Real Image Holograms (H-2's): These are usually reflection holograms made from a transmission original (H-1). The image dramatically projects IN FRONT OF THE PLATE toward the viewer. Most holograms in holography museums are of this type. The procedure for making them is quite elaborate and demands precise control of angles.

How It Works

The way that holograms work is described here. Holograms work by recording the patterns of light and interference patterns on holographic film or plates. You see light is a wave (and a particle but that part isn’t relevant here.) As with any wave, light has crests and troughs. The crest is the top of the wave and the trough, the bottom. When two waves meet, one of a few things happen: The crests and troughs coincide. When this happens the amplitude (”height”) of the wave doubles. This is called constructive interference (sounds like an oxymoron, doesn’t it?)
The two waves are completely out of sync. This causes the crests and troughs to overlap. If the two waves had the same amplitude, they will cancel each other out completely. This is destructive interference.
The last possibility is when the waves aren’t in perfect sync, or perfectly inverse. When this happens you get a unique pattern of interference. How does that make holograms?” When you make a hologram, you are actually recording those patterns of interference. The principle is that if you take two waves and put them together, you get a unique third wave. By subtracting one of the first two waves from the third you can re-create the other original wave. So by shining light on the hologram, you get one of the patterns of light waves back. Because the light you shine on the hologram is constant (not in a pattern) it looks the most like the reference beam (the one that shines directly on the film). This causes the other light waves to be re-constructed, giving you the object beam. And the object beam is a “copy” of the light reflected off the original object. Because the light picked up by each eye is a little different, and because the interference pattern is different for every light beam that hits the hologram, it comes out differently. Because the eyes pick up slightly different versions of the image, the brain is fooled into seeing a 3D image. So in reality this is basically an optical illusion.

Current and Future Uses

Some current applications that use holographic technology are:
· Holographic interferometry is used by researchers and industry designers to test and design many things, from tires and engines to prosthetic limbs and artificial bones and joints.
· Supermarket and department store scanners use a holographic lens system that directs laser light onto the bar codes of the merchandise.
· Holographic optical elements (HOE’s) are used for navigation by airplane pilots. A holographic image of the cockpit instruments appears to float in front of the windshield. This allows the pilot to keep his eyes on the runway or the sky while reading the instruments. This feature is available on some models of automobiles.
· Medical doctors can use three-dimensional holographic CAT scans to make measurements without invasive surgery. This technique is also used in medical education.
· Holograms are used in advertisements and consumer packaging of products to attract potential buyers.
· Holograms have been used on covers of magazine publications. One of the most memorable Sports Illustrated covers was the December 23, 1992 issue featuring Michael Jordan. Holograms have also been used on sports trading cards.
· The use of holograms on credit cards and debit cards provide added security to minimize counterfeiting.
· Holography has been used to make archival recordings of valuable and/or fragile museum artifacts.
· Sony Electronics uses holographic technology in their digital cameras. A holographic crystal is used to allow the camera to detect the edge of the subject and differentiate between it and the background. As a result, the camera is able to focus accurately in dark conditions.
· Holography has been use by artists to create pulsed holographic portraits as well as other works of art.

Future applications of holography include:

· Future colour liquid crystal displays (LCD’s) will be brighter and whiter as a result of holographic technology. Scientists at Polaroid Corp. have developed a holographic reflector that will reflect ambient light to produce a whiter background.
· Many researchers believe that holographic televisions will become available within 10 years at a cost of approximately $5000. Holographic motion picture technology has been previously attempted and was successful in the 1970s. The future of holographic motion pictures may become a reality within the next few years.
· Holographic memory is a new optical storage method that can store 1 terabyte (= 1000 GB) of data in a crystal approximately the size of a sugar cube. In comparison, current methods of storage include CD’s that hold 650 to 700 MB, DVD’s that store 4.7 GB, and computer hard drives that hold up to 120 GB.
· Optical computers will be capable of delivering trillions of bits of information faster than the latest computers.