Holography is a three-dimensional imaging technique. It uses laser light to record the patterns of light waves reflected from an object onto the emulsion of light sensitive film (or glass plates). When that film is developed, and re-exposed to laser light (or normal incandescent light like most holograms today), it re-creates -- in space -- all the points of light that originally came from the object. The resulting image, either behind or in front of the holographic film, has all the dimensions of the original object and looks so real that you are tempted to reach out and touch it -- only to find nothing there but focused light.

Unlike photography or painting, holography can render a subject with complete dimensional fidelity. A hologram creates everything your eyes see -- depth, size, shape, texture, and relative position – from many points of view. In fact, the term "hologram" was coined from the Greek words holos, meaning "whole," and gramma, meaning "message."

More than one 3-dimensional image can be recorded on the same piece of film. For example, the two-channel hologram, "Brain/Skull" (produced by The Polaroid Corporation in 1982), displays two different images as it is viewed from left to right.

There are two basic types of holograms -- reflection and transmission. They can be distinguished by the way in which they are illuminated.

Reflection holograms are lit from the front, reflecting the light to you as you view it, like a painting or photograph hung on a wall. Different film emulsions produce images with different characteristics.

1. Silver Halide is the emulsion of choice for most artists and holographers who use silver halide glass plates to achieve the highest quality images. Holographers also use silver halide film, which is cheaper, less fragile and easier to handle but does not produce the depth, resolution and projection possible with glass plates. However, film is used successfully for longer production runs.

2. Dichromated Gelatin (DCG) is a chemical-gelatin mix that is coated onto a piece of glass. Dichromates produce very bright images in a golden-yellow color. The images have the least range of depth, but they are viewable in normal room light without special spotlights. This makes Dichromates suitable as small gift products for the consumer gift market or the premium & incentive industry. Dichromates have largely been replaced with cheaper mass produced photo polymer holograms. The medium still works best for high performance diffractive optics and probably will for a long time to come. Bright, clean DCG masters are still made for copying into photo polymers.

From 1975 - 1984, Rich Rallison (International Dichromate Corp., Draper, UT) pioneered the production of glass sandwich dichromate holograms that were used as jewelry pendants, key chains, paper weights, and other premium items. He produced these various sized dichromates (up to 3") with Jason Sapan, Holographic Studios, NYC in 1978-79.

3. Photo polymer is the newest of the recording materials. Developed by Polaroid and Dupont, photo polymers have a plastic backing and are suitable for long production runs. The image depth of photo polymers is slightly less than that of silver halide; however, the images are brighter, with a wider angle of view.

Transmission holograms are lit from the rear (like a photographic transparency) and bend light as it passes through the hologram to your eyes to form the image.

1. Laser transmission holograms are made with lasers, like all holograms, but also must be lit with lasers to be viewed. Therefore, the images appear in the color of the laser used in illuminating them for viewing, usually red (helium neon laser). Other types of holograms use a laser transmission hologram as the master, from which copies are made. This is the earliest type of hologram developed by Leith and Upatniks in 1962.

2. White light transmission holograms are illuminated with incandescent light (white light) and produce images that contain the rainbow spectrum of colors. The colors change as the viewer moves up and down and are often called "rainbow" holograms. Holographers have developed considerable control over the colors displayed in this type hologram to produce images in a specific color or in near full, natural color.

This is the first full color hologram, a 4 x 5" full color one step white light transmission hologram by Dr. Stephen A. Benton, Herbert Mingace, Jr. and William R. Walter, The Polaroid Corporation, in 1979.(Photo by S.A. Benton, Collection of MIT Museum, Cambridge, MA)

Other techniques have been developed to record living subjects, to show movement and to extend the mass-production capabilities of the medium.

Pulsed holography uses a quick, intense burst of laser light to record the subject in few nanoseconds – too quick for movement to be a factor. This is similar to strobe or flash photography and has been used extensively for portraitures.

Integral holography, developed by Lloyd Cross, combines holography with cinematography to record a stereogram as a white-light transmission hologram. Several frames of 2-D motion picture footage are converted frame-by-frame to narrow, slit holograms stacked side-by-side on a piece of holographic film. Early integrals were mounted in a circular or semi-circular format and produced an image that appeared in the center with several seconds of movement. Any subject that can be recorded or reproduced as movie footage, video or computer graphics can be made into a holographic image.

This is a series of photographs taken of "Kiss II" (1974), an integral hologram produced by Lloyd Cross, inventor of the process. The hologram -- which was made from approximately 360 frames of motion picture footage -- was typically mounted in a semi-circular, wall-mounted display and illuminated by a single light bulb below. The floating, 3-dimensional image of Pam Brazier blows a kiss and winks as the viewer walks by. (Photo by Daniel Quat, 1977)

Embossed holograms are transmission holograms with a mirror backing that are applied to magazines, promotional items and credit cards. Embossing is the most frequently used method of mass-production in holography. The 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 and then backing the images with a light reflecting foil. The resulting hologram can be duplicated millions of times for a few cents apiece.

How a Hologram Is made
The diagram below illustrates a typical holographic layout on a vibration-free table top in a completely darkened room. The exposure must be made in an environment void of any movement or vibration. Even movement to the degree of half a wave length of light will prevent the recording of the image on the film.

A beam of laser light is optically separated into two beams. One, the reference beam, is directed toward a piece of holographic film and expanded (its diameter increased) so that the light covers the film evenly and completely. The second (object) beam is directed at the subject of the composition and similarly expanded to illuminate it.

When the object beam reflects off the subject, it carries with it information about the location, size, shape and texture of the subject. Some of this reflected object beam then meets the reference beam at the holographic film, producing an interference pattern which is recorded in the light sensitive emulsion.

Holographers use basically the same kinds of film and developers as photographers. The standard silver-halide film (or glass plate) is similar to its photographic counterpart; but it is slower and requires longer exposure times, has a finer resolution, and is more sensitive to red light. Processing involves the same developer, hypo and fixer used in photography. Bleaches are used to brighten the image, and more exotic chemicals are used occasionally to shrink or swell the emulsion to control the image color.

After the film is developed, the hologram is illuminated at the same angle as the "reference" beam during the original exposure to reveal the 3-D image.

A hologram must be illuminated to produce the image. Although laser light is used to make holograms, holograms are usually illuminated with normal incandescent spotlights.

To see a holographic image, the viewer must look at the film. "Projected" images appear in the space between the film and the viewer. There is no current technology that gives the ability to project an aerial or focused image into open space without interposing some type of screen such as smoke, mist, film or a concave mirror. Images cannot be projected from the film to a distant point. A Hologram can produce a real or virtual image in space, but only when it is viewed through or reflected from the physical film.

The projected image of Princess Leia from the chrome-dome of R2D2 in the 1977 movie Star Wars was a Cinematographic special effect – not holography. Perhaps holography will make this a reality in the future.

Attention students and educators: You can learn to make holograms and teach holography by visitng Integraf L.L.C., a one-stop resource for holographic film & plates, holography kits, holograms, books and instructional materials.

Integraf has also produced a superb. free article, "Simple Holography, The Easiest Way to Make Holograms", that offers instructions for first-time holographers, such as students, teachers, and hobbyists with a smaller budget.
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Glossary of Terms
Amplitude - The height of a wave crest or depth of a wave trough, measured from the wave's mid-point.

Beamsplitter - A device used to divide the light from the laser into two separate beams - the reference and object beams. It consists of a partially transparent mirror which reflects part of the laser beam and transmits the rest.

Coherent light - Light which is of the same frequency and is vibrating in phase. The laser produces coherent light.

Film - Whether photographic or holographic, film consists of light sensitive chemicals (the emulsion) spread on a surface. A film's resolution measures its ability to distinguish between details. Because holographic films must be able to record very detailed information, they have a resolving power of 50 or more times that of photographic film. They require either exposure to a high intensity pulsed or a long exposure to a continuous wave laser. Holographic film is developed in a manner similar to photographic film, by bathing it in a series of chemical agents.

Hologram - A medium for bending and focusing light. It uses the interference of monochromatic object and reference waves to record and reproduce multiple 2-D images for a 2-D view. Unlike photography, which records an image as seen from a single view point, a hologram is a record of an image as seen from very many viewpoints.

Holographic stereogram - Made by converting conventional movie frames into holograms, displayed in a cylinder or as a flat picture. The viewer sees two separate images at a time, for an illusion of depth and motion.

Interference pattern - When two waves overlap, their amplitudes add at every point. This results in an interference pattern which records the relative phase relationships between the two waves, storing each individual wave's characteristics. This is how a hologram works.

Laser - An acronym for "Light Amplification by Stimulated Emission of Radiation." A laser is a device that produces a concentrated beam of coherent light. Some, called continuous wave lasers, produce a continuous beam of light. Others, called pulsed lasers, emit more light in brief pulses which are able to freeze motion.

Lenses - Lenses are devices which redirect light. In photography, lenses are used to focus an image for the film. Holographers use lenses to widen the lasers beam to illuminate all of the subject which is to be holographed.

Object beam - The light from the laser beam that illuminates the object, and is reflected to the holographic film.

Phase - The position of a wave in space, measured at a particular point in time.

Reference beam - The portion of the laser beam that goes directly to the holographic film. The interference pattern which results from the object beam meeting the reference beam at the holographic film is recorded on the film.

Reflection hologram - One that forms an image by reflected light. Reflection holograms are lit from the front, reflecting the light to the viewer.

Transmission hologram - One that forms an image from the light passing through the holographic emulsion. Transmission holograms are lit from the rear, bending the light as it passes through the hologram to your eyes.

White light - Light which contains most of the wavelengths in the visible spectrum, such as light from the sun or a spotlight. White light is incoherent, while laser light is coherent. A white light transmission hologram (or rainbow hologram) is one which can be displayed using ordinary white light. Early holograms required viewing with coherent laser light.

The Guide to Practical Holography, by Chris Outwater and Van Hamersveld is an excellent book for the beginner. It's illustrated and free on line.

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