Introduction

 

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Holography is one of the most significant discoveries humankind has ever made. Its discovery has had such a profound effect on our lives, that the person who discovered the process in 1947, Dr. Dennis Gabor, received the Nobel Prize in Physics in 1972.

   

Holography is the only visual recording and playback process that can record our three-dimensional world on a two-dimensional recording medium and playback the original object or scene, to the unaided eyes, as a three dimensional image. The image demonstrates complete parallax and depth-of-field. The image floats in space either behind, in front of, or straddling the recording medium.

   

There are many types of holograms and Hologram techniques, but this site deals exclusively with display holograms. Display holograms are exactly what their name implies: they are holograms that are hung on a wall and display three dimensional objects and/or scenes, complete with parallax and depth-of-field.

   

In Part I, I will discuss how you build a versatile display holography recording system. In Part II, I will demonstrate how to use the system to make various types of simple holograms and ultimately a white-light reflection display Hologram. You do not have to be a physicist or have a knowledge of physics to accomplish these goals. All that is needed is the desire to build this system, a few mechanical skills, and the information at this site to guide you through the process. Although I have been producing holograms for 35 years, I still get a feeling of awe and inspiration of the beautiful Hologram imagery I produce on my system.

 

 

Before you start building this display holography system and producing holograms, you need to see a visual overview of what a basic system looks like when it is completely set up and ready to record a Hologram. Figure 1 shows a photographic view (left) as well as a top-view diagram (right) of a basic system. The system I will be describing is tried and true, but there are other systems that can be built (like a sandbox system). I have found this system, though, to be perfect for creating excellent 4" x 5" white-light reflection display holograms. That's the objective of all this: to produce a high quality white-light reflection display Hologram that can be hung on your wall and shown-off.

 

 

 

Figure 1: (Left) Photograph of a transmission holography setup.
(Right) Diagrammatic top view of the setup.
( L = laser, DM = directional mirror, BS = beamsplitter, O = object beam,
R = reference beam, DL = diverging lens, PH = plate holder,
PM = parabolic mirror, OS = object scene ).

This particular system has five basic optical components. They are the laser (L), beamsplitter (BS), directional mirrors (DM), diverging lenses (DL), and the parabolic mirror (PM). In addition to the optical components, there is the object or scene (OS), the photographic plate holder (PH), the table mounts and the optical holders. Although this system shown in Figure 1 uses the same optical table and components that you will build, you will not be using this particular optical arrangement since it does not lend itself well to creating high quality display holograms.

The function of the optical table is to provide a vibration-free environment on its surface and a substrate for the table mounts. There are many commercially available optical tables but they are very expensive. Figure 2 shows a less expensive table that I use.

 

Figure 2: Granite optical table showing cinder blocks (a),
plywood (b), inner tubes (c), and granite table (d).

 

 

 

 

The table will need to have four legs. Each leg is made from three cinder blocks, 8 inches on a side, stacked on top of each other. Their location under the table surface is critical to providing the maximum dampening effect on ground vibrations. You should first draw your table surface on the floor with chalk or masking tape to determine where the legs should be placed. A leg is essentially placed at each corner of the table, just inside the width of the table. The center of the leg should be a distance of 22% of the length of the table from the end of the table. For a 4 foot long table, this is 11 inches from the end of the table. This distance of 22% has been experimentally shown to be the optimal distance for maximum dampening.

   

On the top of each cinder block stack, place a circular piece of plywood 16 inches in diameter, and 3/4 of an inch thick. These provide support for the inner tubes.

   

On top of each piece of plywood, place two inflated inner tubes that have an approximate diameter of 15 inches when inflated. The inflation pressure or psi (pounds per square inch) cannot be measured with a tire gauge since the pressure is too low, but a good indication of proper inflation is being able to push your finger into the tube about an inch. If the tubes are over inflated, high-frequency vibrations will be passed to the table surface from the ground. If the tubes are underinflated, low-frequency vibrations will be passed. The proper inflation is not all that critical, so don't loss sleep over it. Valve extensions can be connected to each inner tube valve to allow periodic inflation or deflation, or you can do what I do: have someone lift the corner of the table while you remove the tube, adjust the air pressure with an air pump, and re-insert it. Make sure nothing falls off the table while doing this! Better yet, remove all components from the table before removing a tube.

   

The final step for the table is to place a granite or marble slab, polished on one side, on top of the inner tubes, polished side up. This slab acts as the table surface and should have dimensions of at least 3 feet by 4 feet, and be a minimum of 3/4 inch to 1 inch thick. A slab of marble or granite this size is very heavy (about 200 lbs.) and will require four people to lift it. A heavy table is necessary to help dampen ground vibrations. Sometimes a slab that is slightly chipped or a slightly smaller or larger remnant slab can be found and purchased more cheaply. Substitutions for the marble/granite slab are laboratory tables or a piece of glass (at least 1/2" thick and edge polished).

 

 

The room where the optical table is to be located should be on the ground floor of your home or in your basement. This allows the earth to dampen all or most ground vibrations caused by movement in adjacent rooms or nearby traffic. It is essential that no ground vibrations reach the table surface during the exposure of the Hologram. The optical table discussed previously will handle ground vibrations nicely. In Part II, I will discuss how to set up a Michelson interferometer to visualize and test for these potential vibrations.

   

The optical table must also be located in a room that can be totally darkened. The reason for this is that you will be placing a photographic plate in a plate holder (without light-protection) on the optical table and no light other than that from the laser, during the exposure, should impinge on the plate. Your film/plate processing area must also be in a room that can be totally darkened. Hopefully, your processing area can be in the same room as the optical table or adjacent to it.

   

The room in which the optical table is housed should be large enough to accommodate an optical table at least 3 feet by 4 feet with a minimum of 2 feet of walking space around the table on three sides. Also needed is shelf or cupboard space to house photographic plates, optics, optical mounts, and other related items.

   

If the room is air conditioned by a single air conditioning unit, the unit will need to be turned off 30 minutes prior to the exposure. If the room is air conditioned or heated by a central home unit, it will need to be turned off by a thermostat if there is an input vent in the room or the airflow must be physically blocked at the input vent. Any airflow over the optical table will cause the optical components to move during the exposure and destroy the Hologram recording. Movement of any component during the exposure of more than one-half the wavelength of the laser light will destroy the image.

   

The optical table room should be kept clean and no smoking should be allowed. Smoke-related pollutants will eventually build up on optical surfaces, and cause distortion and attenuation of the laser beam.

 

 

 

You must use a laser to make a Hologram (Figure 3a). A laser is a source of coherent light necessary to produce a high-quality Hologram. Fully coherent light sources, such as lasers, are both spatially coherent and temporally coherent. A laser emits light in a very narrow beam and is considered a point source (spatially coherent), as opposed to an extended source (spatially incoherent) such as an incandescent bulb or a fluorescent lamp. A laser also emits light of a single color or wavelength (temporally coherent) whereas the light bulb or fluorescent lamp (temporally incoherent) emits light of many wavelengths.

   

 

Figure 3: Five milliwatt Helium-Neon laser (a), mounting structure (b).

 

 

The laser I use is a helium-neon (He-Ne) gas laser (Figure 3a). They range in price from $100 to $13,000 depending on the power output of the light in milliwatts. There are 3 criteria you should consider when purchasing a He-Ne laser. The light power output should be at least 5 milliwatts (mw), otherwise exposure times will be long and potential table vibrations may destroy the image. If you plan to make holograms smaller than 4" x 5", you can consider a laser with a lower power output but I won't go below 3 mW. If your table and environment is very stable, you could have exposures as long as 10 minutes. The higher the output power of the laser, the shorter the exposure time. My He-Ne laser is 5 mw and my exposures range from 70 seconds to 10 minutes, depending on the reflectivity and size of the object or scene, and the optical arrangement I'm using. Agfa's 8E75 films and plates were very fast, but unfortunately, Agfa no longer produces them. The PFG-01 and BB-640 emulsions have sensitivities of 100 microwatts/sec and 150 microwatts/sec, respectively. This is 7-10 times slower than Agfa's emulsion depending on whether you're using a PFG-01 or BB-640 emulsion.

   

Three other criteria to consider when purchasing a laser are: its polarization characteristics, its TEM mode and its wavelength. The laser should be polarized linearly 500:1 as opposed to randomly polarized. The TEM mode (transverse electromagnetic mode) should be single mode as opposed to multimode. Most He-Ne lasers emit light at 6328 angstroms (red) although they are available with infrared, yellow and green wavelengths. You should purchase a He-Ne with a 6328 angstrom output because the recording plate sensitivity is set to this wavelength. Regardless of what laser company you purchase from, their brochures will list the specifications for power output, polarization, TEM mode and wavelength. He-Ne lasers are air cooled, operate on 110-120 VAC, and have a life expectancy of greater than 20,000 hours. Never let an undiverged He-Ne laser beam hit your eye. It can severely damage your retina and your sight. It can blind you.

 

 

 

 

The beamsplitter is a critical optical component of the system. It can make producing a display Hologram easy or difficult, depending on how much money you're able to spend. The beamsplitter has three important functions:

To split the beam from the laser into two beams.

To control the individual intensities of each beam.

To set an initial direction for the reflected beam from the beamsplitter.

 

 

Two beams in a Hologram setup not only offer versatility, but are necessary to produce a HIGH QUALITY display Hologram. One beam is called the reference beam (R), and the other beam is called the object beam (O). See Figure 1 (right). After the beamsplitter separates these two beams, they are recombined at the photographic plate (PH) to produce the Hologram.

   

 

Figure 4: Circular variable beamsplitter (a),
slide and prism beamsplitters (b),
and variable linear beamsplitter (c).

 

 

The best type of beamsplitter to buy is called a circular-gradient variable beamsplitter (Figure 4a). A circular-gradient variable beamsplitter has a wheel with a reflectivity gradient on one side of the wheel to control the amount of laser light reflected and transmitted (the beam should impinge on the gradient side of the beamsplitter). As the wheel is rotated from 0 to 360 degrees, the intensity of the reflected beam is decreased while the intensity of the transmitted beam is increased. You can choose either the reflected beam or the transmitted beam as the reference beam. The other beam then will become the object beam. Referring again to Figure 1, the transmitted beam (R) is the reference beam in this setup.

   

During the recording process (Part II), there exists an important intensity relationship between the reference and object beams at the photographic plate plane. This is called the beam intensity ratio. The intensity of the reference beam must always be equal to or greater than any point in the object intensity at the plate plane. This ability to adjust the intensity between the two beams is made easier with the variable beamsplitter as it keeps the reflected beam propagating in only one direction while the intensities are varied.

   

If you cannot afford a circular-gradient variable beamsplitter, there are substitutes. The second best beamsplitter you can use is a linear- gradient variable beamsplitter (Figure 4c). As a third choice ( that decreases your control), you can use a microscope slide or a prism (Figure 4b). The problem with these last two optics is that as the incident angle on the slide or prism is changed to adjust the beam intensities, the angle of reflection of the reflected beam will change. Also, using a prism will cause the transmitted beam to change direction. These problems and their solutions will be covered when we discuss making the holograms in Part II. The bottom line is that you want to be able to change the intensities of the reflected and transmitted beams without changing the reflected or transmitted beam's direction.

 

Figure 5: Directional mirror in optical mount.

 

 

The directional mirrors (Figure 5) are front-surface aluminized mirrors that are used to direct the reference and object beams to various locations on the optical table. Because of the small 2mm diameter of the laser beam (about 0.08 inches), I purchase small mirrors about 1 inch x 1.5 inches in size. You could go down to 10mm (.4") diameter mirrors or 10mm x 10mm squared mirrors for those mirrors that are placed before the diverging lens. You can save money this way. In some setups, you'll need larger mirrors that are in the beam beyond the diverging lens. Use the diagrams to determine what you need and I will note in a particular setup what size mirror you need.

 

 

 

Figure 6: Diverging lens in optical holder (a), microscope objectives (b), and spatial filter (c).

 

 

The function of the diverging lenses (Figure 6a) is to diverge (expand) the small diameter reference and object laser beams into wider diameter beams so that the photographic plate and object are uniformly illuminated. The diverging lenses should be simple double-concave lenses, or they can be more complex optical components such as microscope objectives used by themselves (Figure 6b) or in conjunction with an optical device called a spatial filter (Figure 6c). Simple double-concave lenses will work fine provided they are kept clean and scratch free. All the optical components (beamsplitter, directional mirrors, diverging lenses, and parabolic mirror) must be clean and free from scratches to produce a clean, high-quality Hologram (photographic) plate. The first three optics can be cleaned with cotton swabs and acetone. The parabolic mirror can be cleaned with window cleaner and cotton balls.

   

If you can afford a spatial filter or two, get them. Spatial filters have two functions. The first is to diverge the laser beam as simple diverging lenses and objectives do, and the second is to eliminate noise caused by dust and scratches on the optics and the internal noise created in the laser cavity. Although simple double-concave lenses and microscope objectives will do adequately as diverging lenses for producing high quality Hologram images, they must be kept clean and scratch-free. Using a spatial filter will provide the ultimate in totally clean Hologram images irrelevant of scratches and dust. It is comprised of three micrometers, a microscope objective, and a pinhole (Figure 6c). The X and Y micrometers move the pinhole to center on the objective's central light axis, and a Z micrometer moves the objective so that the narrowest point of its focal point is located at the center of the pinhole. When this is achieved, you have a very clean beam at the plate holder from the reference beam. Spatial filters are expensive. You need not consider these for your initial holograms. When you are setting up an optical arrangement, use a -0.32 inch (8 mm) focal length, simple, double-concave lens or an equivalent 20x microscope objective when a diverging lens (DL) is required. Depending on the optical arrangements, you may want to have handy a range of objectives from 5x to 40x (or a range of double-concave lenses) to allow versatility.

 

 

 

The Parabolic Mirror

 

Figure 7: Parabolic telescope mirror in its mount (a), mirror mount (b).

 

 

The function of the parabolic mirror is to collimate (make parallel) the diverging reference beam used in the transmission Hologram so that the Hologram image has a magnification of x1. This is very important because we will use this transmission Hologram image to create a reflection Hologram that can be viewed with white light (light bulbs) instead of the laser light. If you can't afford this mirror, I will discuss an alternative method in Part II: Producing Transmission and Reflection Holograms.

   

I use a telescope parabolic mirror that is coated with enhanced aluminum on its front surface (Figure 7a). A 6-inch diameter mirror is recommended with a focal length of 24 inches. This mirror will uniformly illuminate the 4" x 5" inch photographic plate.

 

 

 

 

Each optical and non-optical component of the Hologram recording setup will need at least one table mount. This adds up to a minimum of 16 mounts. The most complicated optical arrangement in Part II uses 20 mounts. Each mount is made of a 5- pound lead scuba diver's weight (the lead provides necessary mass stability) and a 13- inch long, 1/2- inch diameter aluminum pole (Figure 8). To construct the mount, first take the 5- pound weight and using a 9/16 inch bit, drill a hole in the center of the weight but not all the way through to the bottom. Make sure you drill as perpendicular to the weight as possible. (Caution: diving weights are made of lead, so be sure to clean up all loose lead and wash your hands afterwards.)

 

 

Figure 8: Table mount showing lead diving weight (a), 13-inch aluminum pole (b),
Plexiglas base (c), clamp (d), short rod (e), and optical holder (f).

 

 

 

 

Half-inch diameter aluminum poles are available commercially usually in twelve-foot lengths and the company will cut the poles to six-foot lengths for easy transportation. Each mount pole should be cut to 13- inch lengths and the edges filed smoothly on the ends. Mix up a small amount of 5-minute epoxy glue and insert about a teaspoon into the lead weight hole. Next, insert the pole into the hole. It will be a tight fit and you may find that a pocket of air develops in the hole under the pole. Place the weight on the floor and hit the top end of the pole gently with a hammer. The glue will gradually seep out around the pole and the pole will finally come to rest on the floor of the hole. If the pole has some play in the hole, you will need to hold the pole vertically until the glue starts to cure and harden.

   

Next, with 1/4- inch thick Plexiglas, cut a rectangle the size and shape of the lead weight bottom. Cement the Plexiglas with 5-minute epoxy to the base of the lead weight opposite the pole (Figure 8). The Plexiglas provides an absolutely flat contact with the polished granite surface thus eliminating any rocking that might occur. The lead weights are not smooth on the bottom and without the Plexiglas will not lay flat on the table.

   

 

Figure 9: Short rod for attaching optical holder to table mount.

 

 

Each table mount will have a short rod (Figure 9) attached to the mount with a clamp. Some of these rods will have optical holders attached to them. The short rods need only be about 3 inches long, and are cut from the 1/2- inch diameter aluminum poles. After the rod is cut to length, place it vertically in a vise. On the top center end of the rod, drill a hole about 1 inch deep with a 9/64 inch bit. Next, with a 8-32 tap, make threads in the drilled hole with clockwise turns. Use one-half to one turn at a time to minimize breaking the tap. Then reverse the direction about one turn, then continue forward. Keep going forward and reversing until the threads are about 3/4 inch deep. As the threads are created, bits of aluminum will fall to the bottom of the hole and will need to be dumped out when you finish tapping the hole.

   

After the hole is drilled and tapped, screw a 1½ inch long, 8-32 bolt into the hole, until it is tight. Take the rod and place it horizontally in a vise and with a hacksaw, cut off most of the bolt head leaving a 1/4- inch length. With a file, smooth the end so that you can screw on a nut. You are now ready to add an optical holder to the end of the rod.

 

Figure 10: Two types of clamps: top clamp is home
made, bottom clamp is purchased from a supply house.

 

 

 

The clamps (Figure 10) used on all the table mounts and rods can be purchased from scientific supply houses or can be made (it's just as cheap to buy them as make them) and should accommodate 1/2" diameter aluminum poles. You can make your own by using a drill press, or a Machine(Hologram Printer) shop can do them for you. It is important that the holes be drilled exactly perpendicular to help facilitate right-angle alignment between rods. A Machine(Hologram Printer) shop made my clamps for me. A solid piece of aluminum was used with the dimensions of 1-3/4 inches x 1 inch x 1 inch. Two 5/8- inch diameter holes were drilled in the aluminum at right angles to one another, with the center of each hole 1/2 inch from each end. Poles and rods are clamped into the holes with 1/4-20 thumb screws on each end screwed into tapped 1/4-20 threads.

 

 

 

 

The laser needs two mounts (Figure 3b). Two clamps are located on each pole with a connecting rod running between the two clamps and mounts. The laser sits between the two mounts. At the center of the connecting rod is another clamp that has a 2 -inch aluminum rod with a 1/4-20 bolt seated in the end of the rod (drill and tap 1/4-20 threads into this rod using a 13/64 inch bit). This bolt screws into a 1/4-20 hole in the bottom of the laser housing. Most small He-Ne lasers are supplied with this 1/4-20 hole. Alternatively, the laser can be placed on bricks on top of the optical table to achieve the 8-10 inch beam height required in the described optical arrangements of Part II.

 

Figure 11: Optical holder.

 

 

If you purchase a circular-gradient beamsplitter, a table mount will come with it (Figure 4a). Otherwise, a homemade optical holder (Figure 11) can be used to mount your slide/prism, linear variable beamsplitter, directional mirrors, and diverging lenses (Figures 4b, 4c, 5, and 6a,b respectively). Two pieces of Plexiglas are used to sandwich the optical component between them using two bolts with wing nuts. The two Plexiglas pieces are 3 inches long by 1/2 inch wide by 1/4 inch thick, with holes drilled in each end to accommodate 8-32 bolts with a spacing between bolts of 2 inches. Foam rubber is glued to the inside of each Plexiglas piece to cushion the clamping force of the bolts. On one of the pieces of Plexiglas, halfway along its length and centered, drill and tap a thread for a 8-32 bolt. The hole screws onto the end of the 3- inch rod that is clamped to the table mount (Figure 5).

   

In order to mount the parabolic mirror, you may want to purchase the telescope mirror mount made for this mirror (Figure 7b). Two holes are drilled, opposite one another, in the mirror mount for 1/4-20 bolts. Two 3-inch long, 1/2-inch diameter rods are tapped for 1/4-20 bolts, and the rods are attached to the two holes in the mount with 1/4-20 nuts. Two table mounts, each with a clamp, are attached to the rods, one on each side of the mount. An additional 1/2-inch diameter aluminum rod is added between the two mounts to prevent torquing.

 

 

The Hologram Plate Holder

 

Figure 12: Plate holder showing front side (left) and back side (right).

 

 

This system utilizes a 4 x 5 inch Hologram plate holder (Figure 12). Three aluminum bars need to be cut for this. The horizontal bar is 7 inches long by 3/4 inches wide and 1/8-inch thick. The two vertical bars are 5 inches long, and have the same width and thickness as the horizontal bar. The vertical bars are attached to the horizontal bar, one on the very end and the other 3/4 inches from the end, with 8-32 bolts and nuts. The inside spacing between the vertical bars should be no greater than 4 3/4 inches. This provides a 1/8-inch surface on each vertical bar for the 4 x 5 inch photographic plate to rest against. The plate also rests on the top edge of the horizontal bar. The remaining 3/4 inches of the horizontal bar outside one of the vertical bars is an area used to mount a rod which will hold the plate holder to a table mount with a clamp. Using a rod (Figure 9), attach it to the horizontal bar shown in Figure 12.

   

The plate is held tight to the vertical bars with spring clamps. Holes are drilled in the vertical bars, two on each bar, a distance of 1-inch from the top and bottom. Place an 8-32, 1-inch-long bolt through each hole with a window screen clamp attached to its head. On the other side of the vertical bar, put a low-tension compression spring on the bolt and screw on an 8-32 nut. The spring clamp allows you to pull the clamp outward from the bar and place it on the plate.

 

 

The Object or Scene

 

 

 

 

The object or scene should be three dimensional in order to achieve a three-dimensional image and should be the same size or smaller than the plate area.

 

 

 

 

PaRT 22222222222222

 

 

The images found in display holograms are visually striking because the images are completely three dimensional. This means that the images have complete depth-of-field and parallax when viewed directly with the unaided eye. The images actually float in space in a location behind, straddling, or in front of the Hologram.

   

This section describes how to use the holography system built in Part I. The optical table and other components must first be tested for vibrations using a Michelson interferometer. Using the Michelson interferometer, you will learn to analyze the various movements in the interference patterns to isolate the various causes of the vibration or movement. Next, you will produce a simple transmission and reflection Hologram while learning film/plate types, taking exposure readings, measuring Hologram density, and chemical processing procedures. You will then produce a multibeam transmission Hologram and a multibeam white-light reflection display Hologram. Finally, you will learn how to display your holograms.

 

 

 

 

Conclusion

 

Once you have built your holography system, you will be ready to move on to Part II: Making Transmission and Reflection Holograms. If at anytime you need help with this project, please send me email and I will be happy to respond.