It is now possible for amateurs to buy a hologram and to perform some interesting experiments with it, a subject to which we shall return. Sylvain M. Heumann of South San Francisco, Calif., is one of those who do their own holography. Discussing both the principles and the procedures he uses for making holograms at home, Heumann writes:

"In principle holograms should be easy to make. In practice they are not.

"The object to be recorded is placed on a solid platform and flooded with light from a laser. Light reflected by the object falls on a photographic plate that faces the object. The plate is simultaneously flooded by a second set of rays, called the reference beam, that is reflected by a mirror. The reference waves travel on a path that bypasses the object. After adequate exposure the plate is developed.

"No lens is used to form an image, and no image appears on the completed plate. Instead the emulsion records an abstract pattern of fine lines and whorls that may be roughly likened to a thumbprint. If rays of colored light are now directed through the hologram along the path of the reference beam, a new set of rays emerges from the back of the hologram. The new waves are in every respect identical with those that were reflected by the object. A viewer who sees them for the first time is likely to think he is being tricked, because the object looks so real.

"If the exposure were made with ordinary light, the photographic plate would merely blacken. Every part of the plate receives light from every point on the object and from the mirror. Laser light, however, is coherent: the waves are identical in length, and they proceed in step. At some points on the photographic plate the crests of waves reflected by the object coincide with the crests of waves in the reference beam. The two waves reinforce each other and expose the photographic emulsion at that point. At other places the crests of waves reflected by the object coincide with the valleys of waves in the reference beam. They cancel, so that the plate receives less exposure. Such interference effects vary at all points on the plate, depending on the shape and surface texture of the object.

"The pattern of fine lines in a hologram has the property of diffracting, or bending, light rays. The diffraction is greater with close spacing than with narrow spacing. Advantage is taken of this effect in the hologram to reconstruct the light waves that were reflected by the object. Rays that enter the hologram from the same direction as the reference beam are bent and scattered precisely, enough to match those that were reflected by the object. In effect they duplicate the object rays. The light used for reconstructing the object rays should be coherent, but remarkable realism can be achieved with ordinary colored light emitted from a pinhole source.

"The structure of the hologram involves dimensions that are determined by the wavelength of light and the angle made between the object beam and the reference beam. Normally the plate must record many thousands of lines per inch, a resolving power that greatly exceeds that of ordinary photographic plates. Indeed, the ultrafine structure of the pattern explains why the hologram can record more information than an ordinary photograph. It also helps to explain why holograms are difficult to make at home. During exposure the photographic plate must remain motionless with respect to the relative positions of the mirror and the object. Any relative movement between the three in excess of a few millionths of an inch causes the lines to blur; hence the quality of the reconstructed waves will be seriously degraded.

"For best results the photographic plate must be capable of recording about 60,000 lines per inch. Emulsions capable of this high resolution are comparatively insensitive. Those I use are rated at an ASA speed of only .003, in contrast with ordinary black and white film, which is rated from 400 up. The problems of making holograms, then, consist in devising rigid structures for supporting the apparatus, insulating the apparatus against vibration and maximizing the available light to minimize the exposure interval.

"The first requirement for making holograms is a laser. Mine was built at home. The apparatus described previously in this department will work splendidly if it is modified to develop somewhat more power and to generate light waves of a single frequency. The output power can be increased substantially by operating the laser on direct current, a requirement that is simple to meet A string of two or more silicon rectifiers, such as type CR210, can be connected in one lead of the neon-sign transformer as indicated by the accompanying illustration [right]. The resulting unidirectional current is smoothed by connecting a capacitor across the output of the rectifiers. The circuit must also include two resistors, one for limiting the current in the rectifiers and the second to compensate for the negative resistance of the laser tube. High-voltage rectifiers are expensive. The experimenter who has more time than money can substitute a synchronous rotary switch.

"The switch consists of an insulating shaft that carries two switch arms spaced 180 degrees apart. Each arm passes close to but does not touch an opposing pair of semicircular electrodes [left]. In the case of 60-cycle operation the switch arm rotates synchronously at 3,600 revolutions per minute. It is driven by a Barber-Coleman synchronous motor, type KYAJ622-328. The motor is available from the Edmund Scientific Co., 101 East Gloucester Pike, Barrington, N.J. 08007. The base of the switch and the supports for the electrodes can be made of Lucite or any comparable insulating material.

"Alternating current is connected to the switch arms through brushes made of brass shim stock that ride on brass slip rings. The slip rings make a snug fit with the shaft. Other essential mechanical details are evident in the illustration. The inner diameter of the semicircular electrodes must be at least two inches, and opposing electrodes must be spaced at least three-quarters of an inch apart. All the other dimensions can differ from those shown.

"When the synchronous switch is in operation, the blades must stand midway between the opposing semicircular electrodes at the beginning of each cycle. They must complete half of a revolution at the end of each alternation of current. In other words, the switch must operate in phase with the alternating current.

"To set the switch arms in phase, connect one output lead of the neon-sign transformer to the brush of one switch arm and connect the other output lead of the transformer to one of the semicircular electrodes. Apply power to the neon-sign transformer from a variable-voltage transformer, such as a Variac. Connect the motor to the power line. When the motor comes up to full speed and is running synchronously, gradually apply power to the neon-sign transformer and observe the gap between the switch arm and the electrode. When the voltage has been increased sufficiently, sparks will bridge the gap.

"Note the point on the semicircular gap where the sparks first appear Perhaps they will begin approximately halfway around the electrode. If so, shut off the power, stop the motor and rotate the switch arm 90 degrees on its shaft. Reenergize the apparatus and again observe the gap. Doubtless the sparks will now fill the entire arc of the semicircular electrode. Should the sparks originate at greater or lesser angles around the semicircular electrode, adjust the angular position of the switch arm on its shaft by an appropriate amount.

"After the switch arm has been positioned so that the sparks fill the complete arc of the electrode, fix it to the shaft with a dab of quick-drying cement. Then rotate the remaining switch arm 180 degrees from this position and similarly cement it to the shaft. Adjacent semicircular electrodes are interconnected. When an alternating-current source is connected to the rotating switch arms, unidirectional current can be drawn from leads connected to opposing semicircular electrodes.

"The switch functions as a full-wave rectifier and can replace the costly diodes. The switch requires no current-limiting resistor. A capacitor of about .25 microfarad should be connected across the output of the switch, however, and a resistor must be inserted in one lead between the capacitor and the laser to compensate for the negative resistance of the laser tube.

"Conduction between the switch arms and the semicircular electrodes is established through the spark. For this reason the switch unfortunately acts as a copious generator of electromagnetic noise at frequencies close to all television channels. In order to prevent the radiation of this noise, the switch, the neon sign transformer, the capacitor and the resistor must be installed in a grounded metal cabinet.

"In some cases it may also be necessary to insert choke coils and bypass capacitors in the alternating-current power line and the direct-current output leads. The choke coils and bypass capacitors should be potted in grounded metal containers and installed in the cabinet. The cost of the complete synchronous rectifier should not exceed $20. Warning: The high voltage is lethal. Handle it accordingly.

"To make certain that the laser will generate coherent light of a single frequency (that it will operate in the so-called TEM₀₀ mode), the resonator should consist of one mirror of spherical figure and one flat mirror. The flat mirror can be bought from Henry Prescott, 116 Main Street, Northfield, Mass. 02118. The laser described previously in this department was equipped with a pair of mirrors of spherical figure. Either one of these can be replaced with the flat mirror.

"To make the modification, align the two spherical mirrors so that the laser functions normally. Remove one mirror and replace it with the flat mirror, which can be aligned by inserting a microscope slide between it and the adjacent Brewster window at an angle of about 45 degrees, shining a small light on the slide and manipulating the adjustment screws while looking through, the spherical mirror and down the capillary tube. When the reflected light reaches maximum intensity, the flat mirror is in proper adjustment.

"The adjustment can also be made by the method described in this department in December, 1965. Occasionally a small additional adjustment is necessary. It is made by applying direct current to the tube and rocking the adjustment screws back and forth slightly until the beam appears. Direct the beam onto a white screen and observe the pattern. If it consists of an array of two or more spots, adjust the screws until the spots merge into a single disk of uniform intensity. Incidentally, the laser may not develop maximum intensity when adjusted for TEM₀₀, mode, but more intense multimode beams cannot be used for making holograms.

"The desired disk-shaped spot of light may contain a number of interference fringes and circles. Such spurious effects usually represent diffraction patterns that are caused by dust or by imperfections in the mirrors. The beam can be cleaned up by passing the light through a pinhole about .0005 inch in diameter. The pinhole must be located at the focus of the two lenses that will be used to spread the beam into a pair of broad cones. A good pinhole can be made by pressing a sharp needle into a sheet of aluminum foil backed by a piece of plate glass. The pierced foil can be mounted on a ring of cardboard for clamping into position in the optical train. Finally, the power of the laser can be further increased 10 to 25 percent by placing a series of reasonably strong horseshoe magnets every inch or so along the laser tube. The magnetic fields reduce the tendency of the laser to generate infrared waves and therefore concentrate the output at the desired wavelength of 6,328 angstrom units.

"In addition to the laser, the experimenter will require the following equipment: a heavy table that is insulated against vibration; four first-surface mirrors; two lenses for spreading the laser beam; two beam splitters, and a supply of high-resolution photographic plates together with chemicals for their development. All these materials, except the table and the chemicals, can be bought from the Edmund Scientific Co.

"My table consists of a granite surface plate mounted on dense polyfoam. It weighs 100 pounds. The polyfoam rests on the cement floor of my basement. Another amateur who goes in for holograms uses a stack of concrete blocks of the type sold by dealers in gardening supplies. Each block is two feet square and two inches thick. Six blocks are cemented together with roofing tar and placed on a foot-thick stack of old newspapers. The assembled table weighs 500 pounds. The heavier the table the better. It cannot be insulated too well.

"To check the stability of the table you will require a small interferometer consisting of a beam splitter (Edmund catalogue No. 578) and two first-surface mirrors (Edmund catalogue No. 40,040). These components can be secured to one corner of the table by wax, blocks of wood or rigid fixtures such as machinist's vises [see Figure 7]. Direct the rays of the laser into the beam splitter and adjust the position of the components until the two beams superpose on a screen that can be permanently mounted on a distant wall.

"The superposed beams will make a small spot of light on the screen. Enlarge the spot by inserting a lens with a focal length of 10 to 50 millimeters in the beam at a point within a few inches of the apparatus. Interference fringes will appear in the enlarged spot. They must show no perceptible movement. If they do, add mass to your table and improve the insulation. During the hologram exposure the fringes must show no movement. Street traffic and other sources of vibration can present a problem. In some regions exposures can be made only during the early hours of the morning when traffic is at a minimum.

"Once the table has become stable you can assemble the optical train of the holograph apparatus [see Figure 6]. You will require a piece of thick glass for the beam splitter (Edmund catalogue No. 2,183), a large front-surface mirror (Edmund catalogue No. 40,043), a small first-surface mirror (Edmund catalogue No. 40,040) and two simple lenses of good optical quality, any convenient aperture and a focal length of about 17 millimeters. The lenses need not be achromatic. The mounting supports can be improvised according to the tastes and resources of the experimenter. Again, stability is the essential requirement.

"The subject to be photographed should consist of small objects that will stand still. Chessmen are a good example. The available light from a homemade laser limits the size of the scene to about one square foot if the exposure is to be kept within a five-minute interval. The photographic plate should be placed vertically, facing the subject at a distance of about 10 inches. First, however, place a piece of white cardboard in the position the plate will occupy. The cardboard should match the size of the plate.

"Darken the room, direct the rays of the laser into the beam splitter and adjust the lens of the appropriate beam to floodlight the object. (The laser does not have to be on the stable table.) Block off this beam and adjust the lens and mirrors so that the second beam, as reflected by the small and large mirrors, floods the cardboard screen. If the diagram [Figure 6] has been followed carefully, the distance from the beam splitter to the object to the cardboard screen will be approximately equal to the distance from the beam splitter to the small mirror, large mirror and cardboard. In no case should an inequality exceed half of the length of the laser. If scattered light from the laser tube is perceptible on the screen, enclose the laser in an opaque housing.

"The two beams that now fall on the screen must be adjusted for relative intensity. The beam from the mirror should be two to three times brighter than the rays reflected to the screen by the object. The brightness is difficult to estimate, but it is not too critical. If the reference beam seems too bright, try shifting the position of the lens so that it picks up the rays that are reflected by the second surface of the beam splitter. If the beam still seems too bright, move the lens closer to the splitter or insert a neutral-density filter in the beam at the point where it is reflected from the beam splitter. If a filter is so used, place it exactly at right angles to the axis of the beam; otherwise light will be reflected back and forth internally between the glass surfaces and will introduce unwanted interference effects.

"The angle made at the photographic plate between rays from the object and those of the reference beam should not exceed 30 degrees. The spacing of the lines in the hologram varies inversely with the size of this angle and becomes so narrow at angles approaching 90 degrees that problems arise. Now replace the cardboard screen with the photographic plate. The emulsion side should face the object. (The emulsion side of a plate can be determined by the fact that it will stick to your lip.) The best emulsion for holograms is the Eastman Kodak Company's 649F, which comes in the form of four-inch by five-inch glass plates, packed 36 to a box. The plates are fairly expensive. They can be obtained from the Edmund company in smaller quantities. Other emulsions of lower resolving power can be made to work by using a narrow angle between the reference beam and the object beam. This arrangement generates somewhat broader fringes, which are better for such emulsions, but the adjustment is difficult and I do not recommend it to the beginner.

"Just before making the exposure, direct the laser beam into the interferometer and examine the fringes for movement. If they appear solid, switch the rays to the hologram beam splitter and make the exposure. A dim safelight can be used if it is kept at least 15 feet from the plate. The exposure time is a matter of trial and error. If the object is colored and not more than three inches in diameter, a laser output of five milliwatts should make an exposure of optimum density in about three minutes. If the plates are stored in a refrigerator, allow at least 30 minutes for them to reach room temperature before use. The Eastman Kodak Company recommends that 649F plates be developed for five minutes at 67 degrees Fahrenheit in Eastman H. R. P. developer. Thereafter the plates are fixed, washed and dried. "The dried hologram can be inspected immediately for an image. Place the plate in the diverged beam of the laser at the angle of the reference beam. You should then see-the object. Rotate the plate from side to side to find the angle that yields maximum brightness. Alternatively you can inspect the hologram by placing a filter of almost any color in the slide holder of a 35-millimeter projector and fitting a pinhole mask over the front surface of the projection lens. You can even use a flashlight of the penlight type if it is fitted with a self-focused bulb. When inspected by flashlight, the hologram will be fuzzy and the image will appear in the colors of the rainbow.

"If no image can be found, the probability is high that something moved during the exposure. Examine the plate under a microscope at a magnification of about 600 diameters. The pattern should consist of fine, crisscrossed lines. If these lines are not seen, some part of the apparatus certainly moved during the exposure If the emulsion is much darker or lighter than a conventional photographic transparency, appropriately increase or decrease the exposure during the next try. If the object has poor contrast, increase or decrease the intensity of the reference beam.

"You do not have to make a hologram to have fun with these fascinating playthings. Relatively inexpensive holograms on film can now be bought, with a viewing filter, from the Edmund company.

A number of engrossing experiments can be done with them. Try photographing the reconstructed light. You will discover that either the focus must be altered for recording sharp images of foreground and background objects or the lens must be stopped down to increase the depth of the field. Try making photographs of I various areas of the hologram. Each portion, however small, will reproduce the entire scene-yet the information contained by each area differs from that of other areas. The principal difference involves the effects of parallax: the relative displacement of objects as seen from various points of view. In addition, each area makes a contribution to the resolution of the entire plate. Features of the scene appear in sharpest focus when the reconstructed rays from the full plate are intercepted by the eyes. It is this property of the hologram that accounts for the fact that blemished plates can yield good results. Clear photographs of objects can be made from holograms that are dust-pocked or scratched-imperfections that would ruin a conventional photographic negative.

"The hologram is a special form of the diffraction grating, which is a flat optical surface ruled with thousands of parallel, uniformly spaced lines and used for diffracting white light into its constituent colors. Diffraction gratings transmit part of the light as a straight beam but bend and disperse other portions into bundles of rainbow colors that lie on each side of the central beam. The bundles are known as diffraction 'orders.'

"The hologram also diffracts the light into such orders. If your eye is close to the hologram, you will find a certain angle at which an apparent mirror image of the scene appears. The depth of the field in the inverted image may appear greatly exaggerated, depending on the angle between the reference beam and the object beam at which the hologram was made.

"To gain a full appreciation of the astonishing amount of information that can be compressed into the two-dimensional pattern of the hologram, examine a plate under a microscope. At 40 diameters of magnification you will find curving lines making fine patchwork designs. At 200 diameters these fine details turn out to consist of still finer features. At 1,000 diameters the structures will be resolved into an orderly pattern of relatively straight, interwoven lines that resemble the seat of a caned chair. Good holograms contain more than 25,000 such lines per inch. It is in their number, shape and density that the optical information is encoded. These paragraphs have mentioned only a few of the many new optical experiments that have been made possible by the advent of the hologram.

Bibliography

FUNDAMENTALS OF OPTICS. Francis A. Jenkins and Harvey E. White. McGraw-Hill Book Company, Inc., 1950.

A NEW MICROSCOPIC PRINCIPLE. D. Gabor in Nature, Vol. 161 No. 4098, pages 777-778; May 15, 1948.

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