Holograms are a medium for storing three-dimensional information. They stand in the same relation to photography that sculpture stands in relation to oil painting. Holograms, though they are not nearly as common as photographs, have, nevertheless, become fairly commonplace. Small, extremely simple holograms can be found on many bank cards as a security measure.
The magazine National Geographic has released a few issues that feature holograms on the front cover—the December 1988 issue is particularly well known—and the U.S. Postal Service has issued a holographic stamp. But holography will not soon replace photography as a visual medium. Holograms do not capture colors as well as traditional photography; they are often harder to view, and as a consequence, they may seem inferior to photographs, but they are not. Holograms record information that one cannot capture with traditional photography.
To understand how holograms work, it helps to look a little more deeply into the wave nature of light, especially the phenomenon of interference. First, a note about terminology: The medium on which a hologram is recorded will be described as “film,” in part, because many applications of holography involve film. (The type of film used in holography is capable of capturing extremely fine detail; its performance far exceeds that of the digital cameras and the types of photographic films with which most of us are familiar.) Although not every hologram is stored on film, there is no convenient term that includes all holographic storage media, and enumerating each type of storage medium each time the discussion requires a reference to such media is awkward.
The word film in this context is, then, a generic term for any holographic storage media. How do holographers record information on the phase of the incident light? The film that is used for holography could just as easily be used to record a photograph, and photographs contain no information at all about the phase of the light waves. The difference between a photograph and a hologram lies in the image that appears on the film used to create a hologram. In contrast to a photo, which forms an image of the subject, the hologram forms an interference pattern. In effect, the subject of the hologram is an interference pattern. That interference pattern is later used to create an image of the original object, but if one examines the image on the film, one finds only a spaghetti-like collection of closely packed lines and curves, a record of the interference pattern that the holographer created.
The first step in creating a hologram is to obtain the equipment. There is surprisingly little equipment required. First, the holographer must obtain a laser that produces the purest monochromatic (single wave length) light and the most coherent beam available. In theory every laser produces monochromatic light—or in the case of a dye laser, for example, several pure wavelengths simultaneously from which one wavelength can be chosen—but in practice there are always some unwanted, low-amplitude wavelengths emitted along with “the” laser light. The situation is analogous to a sound recording. A CD produces much less hiss than an LP, but even with a CD there are the occasional noises resulting from scratches, stray voltages, and so on. The same sort of statement can be made about the coherency of laser light. Laser light is highly coherent, but over a long enough distance— the length of which depends on the quality of the laser—the beam eventually loses its coherency. The longer the light emitted by the laser retains its coherency, the better suited that laser is to the process of holography. The lesson is this: A laser is always a far better source of monochromatic, coherent light than an incandescent bulb, but there is always some difference between theory and practice. Some lasers produce better, or more monochromatic light, than others. Next, one needs a device called a beam splitter. This is a simple optical device into which one shines the laser.
The beam splitter divides the laser beam into two component beams that emerge in different directions. They have names: One is called the reference beam, and the other is called the image or object beam. It is the interaction of these two beams on the film that creates the interference pattern. The reference beam is directed by one or more mirrors—it depends on the geometry of the setup—toward the film. As it makes its way toward the film, the reference beam will also generally pass through one or more lenses. This is done so that the reference beam will expand and illuminate the entire film. Otherwise, the reference beam is unaltered. In particular, there is no concept of focusing the light on the film as is done in photography. The reference beam consists of an unadulterated set of planar wave fronts, which are, in effect, a record of the light that first emanated from the laser. The other beam that emerges from the beam splitter, the object beam, is used to illuminate the object that is to appear in the hologram. The object beam is often redirected one or more times with mirrors, and it, too, passes through a lens in order to broaden the beam sufficiently to illuminate the object. It is also helpful to imagine the object beam as a series of planar wave fronts—at least until the object beam impinges on the object itself.
Notice that in the diagram on image above, the object is an apple. If one imagines a planar wave front impinging on the apple, it is not difficult to see that because the surface of the apple is curved, different parts of the wave front will reach the apple at different times. As a consequence, they are reflected by the apple at different times and in different directions. The resulting reflection is a highly distorted version of the original planar wave front. That part of the wave front that impinged on the most forward part of the apple is reflected first. Those parts of the planar front that strike the parts of the apple that are farther back will be reflected later.
It is a simplification, but a helpful one, to imagine that the reflected front carries a three-dimensional image of the apple on it. A more accurate, but more complex, way of imagining what is happening is to picture each point of the apple reflecting light outward in an ever expanding front, much the way a circular water wave expands out across the surface of a quiet pond at the drop of a pebble. Some of this reflected light is cast across the film, which is also illuminated with the reference wave. Taken together, these two light sources, the reference beam and the object beam, form an interference pattern on the film, and, in fact, the goal of this entire process is to obtain a permanent record of this interference pattern.
Obtaining a usable interference pattern is not easy. The interference pattern is extremely delicate in the sense that if any of the holographic equipment is vibrated, the interference pattern is smeared, and the hologram is ruined. The amount of motion needed to ruin a hologram is measured in fractions of a wavelength of the reference beam, an unimaginably small amount of movement. For this reason, holographers must give a great deal of thought to the table used to support their equipment. Tables weighing several tons are common as are various clever techniques to dampen any remaining vibrations. Once the holographer completes the exposure, the film is developed. What remains bears the same relation to the light that shone on the film that a plaster cast has to a sculpture around which it was formed. It was, for example, once common practice to make casts of famous sculptures with the goal of making replicas. These casts are the mechanical analogues of holograms. There are casts of many Greek works as well as the statues of Michelangelo.
The method involved forming a plaster cast around the statue and then removing it. What was retained was the three-dimensional structure of the object. The cast was then pieced together and used to create duplicates. In the same way, the interference pattern recorded on the film is a sort of cast of the light that impinged upon it.
One way of recovering the desired image from the interference pattern is to place the developed film in front of the reference beam. As the light passes through the film, the interference pattern dampens the amplitude of the light and redirects the passing wave front to create the same wave front that was reflected off the original object. In this sense, the film can be pictured as an extremely complex lens that alters the planar wave front as it passes through to produce a new wave front with many of the properties of the object beam after it reflected off the object.
The resulting hologram is a three-dimensional object. One can, for example, look at it from the left and see the left side of the subject, or if one looks at the hologram from the right, the right side becomes visible, but the left side of the subject is no longer visible. When the holographic image is obtained by shining the reference wave back through the film, one obtains what is called a transmission hologram. Another type of hologram, called a reflection hologram, does not require a coherent light source to shine through it from the back. The light necessary to make these holographic images appear comes from the observer’s side of the hologram.
These images are created in such a way that when incident light composed of a variety of wavelengths is shone upon the hologram, various wavelengths are absorbed as they pass into the hologram. What emerges, or is reflected back toward the observer, is that wavelength of light chosen to make the hologram appear. These holograms are more common because they can be viewed without the hardware needed to see a transmission hologram. They are also somewhat more problematic since they depend crucially on ambient lighting and the willingness of the observer to tilt and squint at the hologram until it appears. There is a corresponding loss of control by the hologram creator over the quality of the final image.