A HOLOGRAM IS A KIND OF photograph, but unlike a normal photograph it creates an illusion of depth and also allows the viewer to see the imaged object from various points of view. Indeed, the image can be so realistic that it appears to be the object itself. There are various ways to make a hologram, in all of which film is exposed for several seconds to laser light that has been scattered from the object. Many of the methods are notoriously sensitive to vibrations during the long exposure, so that holographers must often go to great lengths to steady their apparatus.

Recently Roland M. Bagby of the University of Tennessee and Laurie Wright of the Eastman Dental Hospital in London sent me a report on how to make holograms without any fuss. Their rig was designed by Wright and the late Brian Keane of the Royal Sussex County Hospital in England and subsequently modified by Bagby. It is based on a technique for making holograms that was invented in 1962 by Yuri N. Denisyuk of the Soviet Union. The apparatus is so small and sturdy and the technique is so insensitive to vibration that Bagby can load the rig into his car, drive to a local school and then set it up on an ordinary table and begin making holograms within minutes. As a bonus, these holograms can be viewed in white light from an incandescent bulb as well as in light from a laser.

In order to appreciate the Denisyuk technique you need to know the basic principles behind a hologram. Its illusion of depth and faithful representation of perspective stem from the fact that it is a record of an interference pattern created by two beams of light during the exposure. One beam, called the object beam, was scattered from the object, whereas the other beam the reference beam-was not.

The beams must originate from the same source (these days it is a laser) so that there is a fixed phase difference between them when they reach the film. Phase refers to the state of a light wave as it passes a chosen point. The wave is in a certain phase when a "crest" passes and in an opposite phase when a "trough" passes. If two waves of the same wavelength pass the point, their phase difference is a measure of how closely their states match: the waves are completely in phase if they are in the same state and completely out of phase if they are in opposite states.

When the waves are completely in phase, they are said to interfere constructively, and the point is brightly illuminated owing to the alignment of crests with crests and troughs with troughs. When the waves are completely out of phase, they interfere destructively, and the point is dark because of the complete misalignment. If the waves are long and continuous, their phase difference stays the same as they continue to pass through the point, and so does the level of illumination there. The direction in which the beams travel does not matter: they can be moving in the same direction, in opposite directions or at an angle to each other.

In a hologram an interference pattern is produced in the emulsion of a piece of film. The beams begin completely in phase because they come from the same source, but since the object beam undergoes scattering, they arrive at the emulsion with a variety of phase differences. At some points in the emulsion bright illumination activates the grains of silver and at other points darkness leaves the grains unchanged. When the film is developed, the altered grains are opaque whereas the unaltered grains are transparent. The film, now a hologram, is filled with tiny dark lines and transparent ones, a record of the original interference pattern. In certain processes, including the Denisyuk technique, the film is bleached to brighten the hologram. All the lines are then transparent, but they differ in their index of refraction and hence still provide a record of the pattern.

If the hologram is illuminated by a beam identical with the reference beam, the light scatters from the arrangement of lines and reconstructs the object beam. When you view the hologram from the appropriate angle and intercept some of the scattered light, you perceive an image of the object. As you shift your view somewhat you intercept a different part of the scattered light and gain a different perspective on the object.

The earliest holograms were made without benefit of laser, and so their images were dim and murky. Today holographic images are brighter and sharper because of lasers, better film and better ways of exposing the film. Commonly a laser beam is split by a half-silvered mirror into two beams, which are then successively reflected by other mirrors until they reach the film. Along the way one of them is scattered from an object and becomes the object beam. The other one, the reference beam, is sent into the film on the same side as the object beam but along a different path.

When holograms are made in this way, the rig must be carefully isolated from vibrations in order to maintain a consistent interference pattern on the film during the long exposure. If something along the path of either the object or the reference beam is jostled, the phase difference of the waves reaching each point on the film shifts and the recording of the interference pattern is washed out.

The technique usually has another disadvantage: the holograms must be viewed in the same type of light that exposed the film. The requirement excludes viewing a hologram in white light from a bulb, because white light consists of many wavelengths. The scattering the waves undergo in the hologram depends on wavelength; when there are many wavelengths, you intercept a number of scattered patterns yielding such a jumble of images that nothing is recognizable.

One way around the problem is to expose the film so that the finished hologram selects out only one wavelength to create an image. In this technique the object and reference beams are sent into the film from opposite sides; the film has a thick emulsion, so that many layers of constructive and destructive interference, separated by half a wavelength, span the emulsion. Information about the object is still recorded in the lateral variation of the interference pattern, but now the wavelength of the light is recorded in the spacing of the layers.

After the film is developed it is illuminated with a beam of white light along the path previously taken by the reference beam, and you view it from the light-source side. Although many wavelengths enter the hologram, the only light that is scattered in your direction is light whose wavelength matches that of the original reference beam. The selective scattering results from the half-wavelength separation between the embedded layers: light of the "proper" wavelength is strongly backscattered by the arrangement, whereas light with any other wavelength is not.

The scattering can be thought of as a form of reflection, and so this kind of hologram is called a reflection hologram. When you intercept some of the scattered light, you perceive an image on the far side of the hologram. It is a "virtual" image constructed by your visual system, which mentally extrapolates the rays your eyes receive back to their apparent origin. If you placed a card at the apparent position of the image and looked at the card directly rather than through the hologram, you would not see the image.

A Denisyuk hologram is a reflection hologram, but the laser light is not split into two beams by a half-silvered mirror. Instead it is spread by one or two lenses and then sent directly through a tilted, transparent film to reach the object, which sits immediately behind the film. Some of the light, acting as the object beam, scatters back to the film and interferes with the oncoming light, which acts as the reference beam. When the film is developed, you can view the hologram by shining white light onto it along the same tilted path taken by the initial laser beam. Note, incidentally, the advantage of the tilted orientation of the film during the exposure. If the laser beam had been perpendicular to the film, you would have to hold the white-light source directly in front of your face in order to view the hologram instead of off to one side.

The Denisyuk technique is particularly convenient because the rig requires little isolation from vibration. The object and the film are next to each other; if one wiggles, the other wiggles almost in unison, and so the interference pattern within the film is largely unaffected. If you position the . object farther away from the film, the advantage is lost and the rig needs to be isolated from vibrations.

You can make a hologram with the Denisyuk technique by building the rig designed by Wright and Keane. It is shown above and the parts it calls for are listed in Figure 2. The parts marked with an asterisk can be bought from the Mode Corporation, P.O. Box 1697, San Leandro, Calif. 94577. (For an extra 50 cents per piece the tubing will be cut to specified sizes; otherwise order it in 10-foot lengths.) The optical devices can be bought from the Edmund Scientific Co., 101 East Gloucester Pike, Barrington, N.J. 08007. The precise size and design of the parts are not critical, and Bagby and Wright suggest that readers may enjoy improvising.

Construct the main frame of the rig from the tubing and joints and the inserts that connect them. Use a rubber or plastic mallet to make the connections, but do not ram the pieces hard. Lay the frame on the plywood and test it for stability, if it wobbles, adjust the joints until it is stable, and then fasten it to the plywood with the shelf supports and screws.

A U-shaped mount that will support the film is made with three shorter pieces of tubing. (Hold the mount inside one end of the main frame to be sure there is a clearance on each side of about an eighth of an inch.) Outside the bottom section of the mount attach an identical length of tubing with bolts. Run the bolts through holes drilled in both pieces; either thread the holes in one of the sections to hold the bolts or secure the bolts with nuts. The extra section of tubing forms a narrow shelf to support whatever is to be photographed.

Find and mark the balance points of the mount and then prop it upright in the end of the main frame about three-quarters of an inch above the bottom tube of the frame. Mark the heights of the mount's balance points on the vertical tubes of the frame, remove the mount and then drill holes a quarter of an inch in diameter through the frame at the marks. Also drill threaded holes through the balance points on the mount. Return the mount to the end of the main frame, run a bolt through each hole in the frame, add washers to separate the mount from the frame and then turn the bolts into the threaded holes in the mount. There should be enough washers so that the mount can be easily rotated about the bolts but is kept in place, once positioned, by friction from the washers. Press the cladding channel (a rubberized track that holds the pane in place in some windows) onto the inside of the mount and slide a piece of plate glass into the channel. Later the film will rest against the glass. (You may be able to simplify the entire rig. Wright has made one of wood.)

The optical bench should be between one foot and two feet in length, which may necessitate your cutting a standard bench. Two pin holders are mounted on the bench to hold the pins that are screwed into the lens holders. You can either buy commercial pins and lens holders or hold the lenses with household broom clips, screw the clips into wood dowels and then insert the dowels into the pin holders. (A homemade optical bench might be substituted to reduce the cost of the rig.) The lenses are plano-concave or double concave, with short focal lengths of from-15 to -30 millimeters. Before final assembly the plywood, the bench and everything on it except the lenses should be sprayed with flat black paint to eliminate stray light during the exposure.

Choose a sturdy table to hold the rig. To help isolate the rig from vibrations, stand the legs of the table in coffee cans partially filled with some compliant material such as vermiculite. (If vibrations later prove to be a problem, you may have to mount the table on inflated inner tubes.) Put the bench and optical equipment into the rig, cut a white sheet of paper to the size of the film (about four by five inches) and lay the paper on the glass in the mount, which is tilted with its top part toward the lenses. Then adjust the lenses so that they are aligned with the center of the paper. Turn on the laser and adjust its height and the height and horizontal position of the lenses until the beam spreads uniformly over the paper. (Never look into a laser beam, and take great care not to allow any bright reflection of it to reach your eyes.) When the optical alignment is satisfactory, secure the bench to the plywood and mark the locations of the pin holders.

Bagby and Wright say any helium-neon laser will do; those that emit polarized light and have an output power of at least five milliwatts work best. (The weaker the laser is, the longer the exposures must be, and long exposures can make vibrations a problem after all.) The emulsion should be thicker than six microns, transparent to light on both sides (ask for film with a no-antihalation, or "NAH," backing) and sensitive to the red laser light: say Agfa film type 8E75HD NAH, Kodak spectroscopic film type 649-F or a Kodak high-resolution plate.

To process the film you will need a fine-grain, high-contrast developer such as Kodak type D-l9 and also a bleach mixture. The developer can be reused if it is stored in a cool place in a brown or opaque plastic bottle. The bleach is needed to brighten a hologram; without it, reflection holograms are disappointingly dark. Bagby and Wright sent recipes for two alternative bleach mixtures. To make one of them, add 25 grams each of potassium bromide and potassium ferricyanide to about 900 milliliters of water (distilled water is best). Stir until the powders completely dissolve, add enough water to bring the volume to 1,000 milliliters and then cautiously add 10 milliliters of concentrated sulfuric acid. (Do all of this in a sink whose appearance is unimportant, and run tap water into it so that any spilled acid is diluted before it gets to the pipes. And whenever you are handling a bleach mixture, be sure to wear safety goggles and laboratory gloves.)

The second bleach mixture yields even brighter holograms, but it also may shift the color of the image. (The shift is no problem when the hologram is viewed in white light, but if it is viewed in the original laser light, the shift may dim or eliminate the image.) The mixture is prepared by adding 30 grams each of potassium bromide and ferric sulfate to 900 milliliters of water, stirring and then adding enough water to bring the mixture to 1,000 milliliters.

You will also need some absolute (100 percent) methanol, a green safety light and a hair dryer, preferably one in which the heat and air speed can be controlled separately. The green safety light enables you to see while developing the film. The methanol serves to dry a hologram quickly, but you must be careful not to breathe it or bring it near any flame or spark, which could ignite it. The hair dryer is used in the final drying stage.

Now you are ready to make a Denisyuk hologram. With the room lights out and the laser on, recheck the beam alignment by placing a white sheet of paper on the plate glass in the mount, which should be tilted between 30 and 45 degrees from the vertical. On the paper lay a piece of plate glass somewhat larger than the film you will be using. Mark the positions of the left and right edges of the second glass with masking tape on the larger glass and then slip the paper out from the assembly.

Block the laser light with cardboard positioned in front of the laser and then slide the film between the two pieces of glass, maneuvering it so that it fits between the tapes. The emulsion side of the film should face the laser. Place the object to be photographed on the top layer of glass and wait for several minutes to allow any vibrations to damp out. Lift the cardboard slightly, wait again for about 30 seconds for any new vibrations to damp out and then lift it completely to expose the film.

The proper length of the exposure depends on the film, the strength of the laser beam, the size of the film and the reflectivity of the object, and so it requires experimentation. If the beam from a five-milliwatt laser is spread over film that measures four by five inches, and if the object has moderate reflectivity, the exposure might require about five seconds. To stop the exposure put the cardboard back in front of the laser. Then retrieve the film and put it in a lightproof compartment until it can be developed.

Develop the film in a room illuminated only by the green safety light. Wearing safety gloves, slip the film into the developer, making sure the emulsion side of the film faces up so that it will not be scratched on the bottom of the container. Swirl the film until it is quite dark; that can take from 30 seconds to two minutes, and getting it right will require some experimentation. Then bathe the film under running water for two minutes before putting it into one of the bleach mixtures. If the film does not soon become more transparent, it was overexposed, overdeveloped or processed with bleach that is too old.

If the film does clear, bathe it again in running water for two minutes. If the tap water is hard, rinse the film in distilled water to eliminate any deposits. Next, blot it with a soft paper towel enough to remove any clinging water but not enough to dry it fully. To complete the drying, submerge the film in methanol for about two minutes. (If the methanol bath gains too much water, the developed hologram will be murky.) After this submersion, work quickly: lift the film, allow the fluid to drain from it and lay it on a soft, dry paper towel with the emulsion side up. Gently apply another paper towel to the emulsion side and immediately complete the drying with the hair dryer, setting it for both heat and air. (Keep the dryer away from the methanol fumes in case it has any internal sparking.) You then have a hologram that can be viewed in white light, provided that the light source is small: a flashlight or a slide projector without its front barrel will serve, but a fluorescent tube will not.

If an object you want to photograph does not balance well on the narrow shelf of the U mount, you can mount it and the film horizontally on a piece of plate glass positioned to straddle the top of the rig. The glass should be a quarter of an inch thick and measure 12 by 14 inches. To reflect the laser beam up to the glass you will need a mirror, which should measure eight by 10 inches and have its reflecting layer on the front surface. Angle the top of the U mount away from the laser, place the mirror on it and adjust everything until the laser light spreads uniformly over a sheet of paper placed on the horizontal glass. Then follow the exposure procedure outlined above.

You may well wonder why all holograms are not made the Denisyuk way. The fact is that the illusion of depth is often weaker in a Denisyuk hologram than it is in a hologram made with a split-beam method. The weakness stems from the fact that a laser does not emit a single, continuous wave but rather a succession of continuous waves, none longer than about the length of the laser itself. The phase changes abruptly and randomly when one wave leaves off and another begins, and so if there is to be consistent interference between two beams of light when the film is exposed, the beams must come from the same wave. In the Denisyuk setup a wave essentially folds back on itself when it scatters from some point on an object. If the point is near the film, the returning part of the wave can interfere with the oncoming part of the same wave at the film, but if the point is too far away, the returning part of the wave meets an oncoming part of another wave; the phase difference between the two waves is unpredictable, and during the exposure there is no consistent interference at the film. Denisyuk holograms therefore record the nearby points of an object well enough but not the distant points.

Although the Denisyuk method is the simplest way to make holograms, it is still challenging; you may want to consult the references listed below for further advice. In addition, Bagby has volunteered to answer questions addressed to him at the Department of Zoology, University of Tennessee, Knoxville, Tenn. 37996-0810.

Bibliography

HOMEGROWN HOLOGRAPHY. George Dowbenko. American Photographic Book Publishing Co., Inc., 1978.

LASERS AND HOLOGRAPHY: AN INTRODUCTION TO COHERENT OPTICS. Winston E. Kock. Dover Publications, Inc., 1981.

HANDBOOK OF HOLOGRAFHY: MAKING HOLOGRAMS THE EASY WAY. Fred Unterseher, Jeannene Hansen and Bob Schlesinger. Ross Books; 1982.

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