In 1947 the British physicist Dennis Gabor began experiments to overcome problems of image formation that were due to aberrations. Gabor recognized that for an object illuminated with coherent light, the pattern generated by interference between the wave scattered off the object and a coherent reference wave contains all of the visual information about the object. This interference pattern can be recorded photographically. Then, if the reference wave alone is used to illuminate the developed photographic film, it is scattered by the film to generate a new wave, identical to the original wave reflected from the object. The result is the formation of an image without the use of a lens.
Gabor coined the word hologram for this photographic recording of the interference pattern produced by the combination of reference and object beams. This word is derived from the Greek words holos, meaning whole, and gramme, meaning what is written or drawn. The making and study of holograms is called holography. Gabor successfully demonstrated the effect in 1948, more than a decade before the invention of the laser. In the ten years immediately following his discovery, holography received little attention and its development was hampered because of the lack of a suitable source of coherent light.
After the invention of the laser, Emmett N. Leith used coherent laser light to create images in the manner first demonstrated by Gabor. With Leith's introduction of the laser and use of a reference beam at an angle to the object beam, holography became practical.
The images created from holograms contain more information than images produced with ordinary photographic techniques. Holographic images are three-dimensional and have both depth and parallax. As you move your head when viewing a holographic image, it seems as if the original objects are present within the image. Even matched stereo pairs of photographs do not contain the visual information of a hologram.
The production of a hologram requires two coherent beams of light. These two beams are generated from a single laser beam by a half-silvered mirror or other beam-splitting device. One beam, called the reference beam, shines directly on the photographic film. Mirrors direct the other beam to the object whose image is to be recorded. Light scattered from the object to the photographic film is called the object beam. No lenses are used to image the object onto the film; instead, the object and reference beam both illuminate the film. Since these beams are coherent, they interfere and the photographic film records that interference pattern.
You can see a holographic image of the object by illuminating the developed film with a laser beam similar to the reference beam. In general, two images are formed. If the reference beam used in making the hologram was a beam of parallel light and the reconstructing beam is also parallel, then one of these two images is a virtual image. You can observe the virtual image by looking through the hologram. The three-dimensional image appears behind the film. The second image is real and can be formed on a screen. If you are farther away from the film than the image is, your eyes can focus the rays diverging from the real image.
So far we have been describing holograms that produce sharp images only when viewed with light that is nearly monochromatic. Reflection holograms can be viewed in white light. In a reflection hologram the object and reference beams (which do need to be monochromatic, coherent light ) are brought together from opposite sides of the photographic film. The resulting interference pattern has structure perpendicular to the plane of the film. If the thickness of the emulsion is greater than about 15 mm, then the interference pattern recorded in the film is truly three-dimensional, with twenty or more layers within the emulsion. When the developed film is illuminated by a white light beam in the same direction as the original reference beam, the hologram diffracts some of the light backward. The resulting wave recreates the original object wave. For each particular angle of incidence, only a narrow range of wavelengths is reflected into the viewer's eye. The hologram automatically selects the proper wavelength for each angle of incidence. Thus we can view reflection holograms with white light, provided it comes from a point source.
Another type of white-light hologram is the rainbow hologram, invented by Steven Benton of the Polaroid Corporation in 1969. The rainbow hologram may be viewed with transmitted light from an incandescent lamp. The different wavelengths are dispersed so that each forms only a small portion of the total image, resulting in a rainbowlike change of color across the image. This effect is achieved in a two-step process. First make a transmission hologram of the object in the usual way. Then make a second hologram, using the real image from the first hologram as the object. A narrow horizontal slit is placed next to the first hologram during this process. This eliminates the vertical parallax in the second hologram, but the second hologram still records the image of the entire object.
When the second hologram is illuminated with a white light source, each wavelength forms an image of the slit. Each of these images occurs at a different vertical angle, as expected from the laws of diffraction. When looking for the image of the object, an observer actually looks through one of the colored images of the slit. The original object appears with horizontal parallax. As the observer moves up and down, the image remains constant, showing no vertical parallax‹but it changes color depending on the vertical position.
In recent years, holograms have been made using a special photographic emulsion called photoresist. Instead of variations in opacity, as in normal photographic emulsion, the developed photoresist has variations in thickness in response to the intensity of the light incident upon it. By using electrolysis, we can deposit a thin layer of nickel on the developed photoresist, making a three-dimensional mold. Duplicates of the interference pattern, made from the mold, can be stamped into plastic and coated with a thin layer of aluminum. The aluminum functions like a mirror, reflecting incident white light back through the interference layers to produce the holographic image. The result is called an embossed hologram. Because they can be reproduced inexpensively, embossed holograms are finding their way onto magazine covers, buttons, and novelties. Since 1984, credit card manufacturers have been incorporating embossed holograms into their cards in an effort to thwart counterfeiting.