Optics: Pure Light for Practical Pictures

From repairing damaged retinas in the human eye to burning precision holes in industrial diamonds, the list of uses for laser light has grown steadily since the fierce, pure beams were first projected less than ten years ago. A recent application may yet prove to be one of the most practical of all. With lasers (for “light amplification by stimulated emission of radiation”) to help them take their pictures, Professor George Stroke and his associates at the University of Michigan are perfecting the techniques of holography—three-dimensional photography without the use of a lens.

Holography produces no familiar photographic negative or print. But when light is directed upon a holographic negative—or hologram—its smudgy and apparently meaningless patterns of concentric circles and parallel lines become a window through which a viewer sees the scene that was photographed. By moving his head from side to side, he can look through that window at different angles and change the perspective of the three-dimensional view; he can look around an object in the foreground to see what is behind it, just as if he were examining the actual scene.

Disciplined Waves. The basic principles of holography were worked out 20 years ago by British Physicist Dennis Gabor, but they could not be put to use effectively without the peculiar light that lasers now provide. Unlike “white” light from the sun or an electric light bulb, which radiates in all directions and consists of a whole spectrum of colors, light waves from a laser are highly disciplined or “coherent.” They are of only one color—which means that they are all of the same frequency. And they all emerge from the laser in step—in phase with each other and traveling along precisely parallel lines.

To produce a hologram, light from a laser is split into two beams, one of which is directed by a mirror onto a sheet of photographic film. The other beam is used to illuminate the subject. When the laser light hits the subject, it is scattered by the irregular surface and reflected back toward the film. As a result, many of the reflected light waves are jumbled and out of phase both with each other and with the light from the undisturbed beam reflected by the mirror. When the light waves from subject and mirror are reunited at the surface of the film, they interfere with each other in strange patterns of bright and dark areas that are recorded on the film. “In the hologram,” says Electrical Engineer Stroke, “the light waves are stored in a manner similar to the way a musical tone is stored in a piano string. It is there, but it is not released until the string is plucked.”

Holography Handicaps. To pluck a hologram and release its light waves, a laser beam is passed through it. As the laser beam hits the hologram’s interference pattern, it is diffracted into light waves that duplicate those that were reflected from the subject. The viewer sees the subject of the picture in three dimensions, apparently suspended behind the hologram at the same distance it was from the sheet of film.

Though holography is the subject of intense research in commercial and university laboratories across the country, its practical use has been limited by two handicaps: holograms have displayed their pictures in only one color, the color of the original laser beam, and viewing the picture has also required laser light, which is not only expensive and difficult to handle but can cause serious eye injury as well. Holographer Stroke has apparently eliminated both these difficulties. At a meeting of the Optical Society of America in Washington this week, he reported that he and Student Antoine Labeyrie have produced holograms that can be seen with ordinary light and show their images in true colors. “The amazing thing,” says Stroke, “is that no one had gone to work on eliminating these problems. It was widely assumed that it could not be done.”

Multicolor Image. Stroke & Co. went to work on the problem last year, and in December decided to apply the principles of a photographic process that won Physicist Gabriel Lippmann a Nobel Prize in 1908. By changing the position of the holography mirror, Stroke directed the undisturbed laser light to the back, rather than the front, of a sheet of film. As the beam passed through the film, it met the scattered light reflected from the subject coming through in the opposite direction. The new arrangement had the effect of producing layers of interference patterns in the emulsion of the film. When a beam of ordinary white light was directed at the developed film, these layers filtered out all of its components except the color of the laser beam used to illuminate the subject. Thus only waves of this frequency were reflected back to the viewer as a single-color, three-dimensional image.

Last month, with the aid of scientists at the Bell Telephone Laboratories, Stroke began using overlapping red and blue laser beams to illuminate his holography subjects. The combined beams produced even more complex layers of interference patterns in the emulsion of the film and added a new facet to the hologram. In addition to carrying information about the intensity and phase of the light reflected from the subject, it now contained full color information—even though the hologram itself was made on black and white film. When ordinary white light was reflected from the new hologram, two colors—red and blue—reached the viewer’s eyes in varying combinations that produced a multicolor image.

Movies in 3-D. Though there are many technical problems still to be solved before holograms come into widespread use, the University of Michigan development should speed the transition of holography from a laboratory curiosity to a valuable industrial and scientific tool. Stroke also sees it as a living-room entertainment medium. His new holograms can be framed and hung on the wall, where standard illumination will transform them into windows revealing . three-dimensional scenes Though movies and television present more difficult technical problems, holography may eventually be used to present them in three dimensions and in full color. “In our field,” says Stroke, “this breakthrough is the equivalent of a successful Apollo shot. We asked for the moon and we got it.”