Creating Transmission and Reflection Holograms. Introduction

Transcription

1 Creating Transmission and Reflection Holograms Introduction You will now learn how to use the holography system you built in Building a Holography System to make transmission and reflection holograms. The optical table, components, and the environment must first be tested for vibrations using a Michelson interferometer. You will be using components you've built to create an optical setup for the Michelson interferometer in the next section Testing for Vibrations. Using the Michelson interferometer, you will learn to analyze the various movements in the interference fringe pattern to determine what is causing the fringe pattern to move. With this information, you can eliminated all movements and insure a successful hologram. Next, you will learn: what additional supplies you need. what recording plate types are available and which one you should buy. what processing chemicals you need to buy and how to mix them. and the processing procedure to develop the hologram after it is exposed. I am covering this information prior to setting up your first optical arrangement so that you can obtain these items ahead of time and have them ready for use when you need them. Next, you will set up your first transmission hologram arrangement while learning: how to determine the orientation of the laser's beam polarization and how to control it, how to figure out your exposure time for the hologram, how to record and process (develop) the hologram, and how to test the hologram's exposure density to make sure you're getting the brightest hologram image possible. The following illustration shows a single-beam transmission hologram setup. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 1

2 Once you have produced your first transmission hologram, you will learn how to make a simple reflection hologram followed by a multi-beam transmission hologram, and a multi-beam white light reflection display hologram. Finally, you will learn how to display and light your white light reflection display hologram. I strongly recommend you read through this whole document before implementing it. For assistance in producing these holograms, please contact Stephen W. Michael (smichael@3dimagery.com). Testing for Vibrations I recommend that you read this whole section on Testing for Vibrations before you proceed with setting up the interferometer. At the end of this section are two important notes: one dealing with fine adjustments and the other dealing with a technique called retro-reflection. The first optical arrangement you will set up is a Michelson interferometer as shown in Figure 14a. The interferometer produces a bulls-eye pattern on the screen S called an interference fringe pattern. Its purpose is to visibly detect, with your eyes, any vibrations that occur on the table surface that are emanating from the floor and any movements that can occur in an optical setup's components (the black fringes, as shown in Figure 14g, move if there are vibrations and/or component movements present). Once you understand what causes the interference fringe patterns to move, you will be able to avoid these vibrations and movements during your exposures. In a multi-beam optical setup, all components from the beamsplitter to the object scene and plate holder (including the beamsplitter, object scene, and plate holder), cannot move relative to each other during the exposure. In a single beam optical setup, no movement can occur between the object scene and the plate holder. The bottom line: if the fringe pattern is not moving, you'll have a successful hologram. Because of limited table space, having an interferometer set up during your exposures is not practical. You will now set up the Michelson interferometer. Figure 14a: Michelson interferometer: laser (L), diverging lens (DL), beamsplitter (BS), mirrors (M1 & M2), and screen (S). Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 2

3 Viewing the optical arrangement in Figure 14a, position the laser L at one end of the table's longest length and centered and place the laser high enough on the table so its output aperture beam is at a height of 9 inches (22.9 cm) above the table surface. The beam will travel to a beamsplitter BS where it is split into two beams (a 50/50 plate beamsplitter works great for this setup and the coated side of the beamsplitter should face the incoming beam). Leave the diverging lens DL out of the setup for now. One beam is transmitted through the beamsplitter to mirror M2 and the other beam is reflected at 90 degrees to the right to mirror M1. The distance, or path length as it is called, between each mirror and the beamsplitter should be almost the same, to within 1/8 inch (0.32 cm). If you make them both exactly the same, you're bulls-eye pattern may be distorted. These path lengths can be determined using a tape measure and should be as long as possible for the table size. The interferometer's sensitivity increases the further the mirrors are from the beamsplitter. Both mirrors should then reflect their beams back to the beamsplitter and strike the beamsplitter at the original incident beam's position. Part of mirror M2's reflected beam will then be reflected by the beamsplitter to the screen S and part of mirror M1's reflected beam will be transmitted through the beamsplitter to the screen S. Two beam dots should be visible on the screen as shown in Figures 14b and 14c. The screen S is a piece of 4 inch x 5 inch (10.16 cm x 12.7 cm) white mounting board placed in the plate holder. Note: Make sure the center of the beamsplitter, both mirrors, and the screen are 9 inches (22.86 cm) above the table. Use the retro-reflection technique to insure the beam is 9 inches (22.86 cm) above the table throughout the setup. Your goal is to superimpose (overlap) these two dots so that the center of the interference fringe pattern (center of the bulls-eye) is visible as shown in Figure 14g. The dots can be closely superimposed by grossly moving mirror M1 slightly up and down, and/or sideways by adjusting the mirror mount in its connector and/or moving the lead base of the table mount, respectively. It is best at this point to have just one beam dot barely overlapping the other beam dot or having their edges touch. Try to get the two beams as horizontally level to each other as you can. In Figure 14b, the left dot is slightly higher than the right dot. They should have been more level. This is not critical, but it will make your final adjustments a bit easier. You'll be refining this momentarily using a fine adjustment technique. Figure 14b: Two beam dots on screen with room lights on. Figure 14c: Two beam dots on screen with room lights off. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 3

4 Once you get the two dots somewhat overlapped as discussed above, you will now need to magnify the dots to see the fringe pattern. You do this by using a diverging lens (DL). A plano- or double-concave lens with a -15 mm focal length and 12 mm diameter or a 10x microscope objective is suitable for this purpose. There are two possible positions in the optical setup where you can place the lens: either between the beamsplitter and the screen, or between the laser and the beamsplitter. You will start by placing the lens between the beamsplitter and the screen. This position will enlarge the dots significantly on the screen S and allow you to see the fringe pattern easily as shown in Figures 14d and 14e. By enlarging the dots this way, you can easily adjust mirror M1 to help you completely overlap the two dots (later on, the final position of the diverging lens will be placed between the laser and beamsplitter to de-magnify the fringe pattern so you can see the whole, centered bulls-eye fringe pattern). Figure 14d: Magnified beam dots with fringe pattern, room lights on Figure 14e: Magnified beam dots with fringe pattern, room lights off Because of the huge magnification of the fringes with the lens at this position, it is not possible to see the whole bulls-eye pattern. The fringe patterns you re seeing in the above figures are the outer edges of the bulls-eye pattern, not the center of the bulls-eye pattern. Again, adjust mirror M1 to get these two dots more overlapped (this is the moment to use the fine adjustment technique covered in the note on fine adjustments covered towards the end of this section on vibrations). As you get them more overlapped and approach the center of the bulls-eye pattern, the fringes become fatter and less numerous as shown in Figure 14f. Once you reach this point, let the table and components settle down for a few seconds so the fringe pattern is not moving or barely moving, then touch the table and watch the fringes move. You'll notice how very sensitive the interferometer is when the table is touched. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 4

5 Figure 14f: Fringes closer to the center of the bulls-eye pattern. To see the whole bulls-eye pattern, you need to place the diverging lens between the laser and the beamsplitter, making sure the diverging beam passes through the beamsplitter and is reflected back to the beamsplitter from both mirrors to the screen. You can move the lens around until you see the divergent beam surrounding the beamsplitter on the screen as shown in Figure 14g. You should now see the whole bulls-eye pattern within the beamsplitter's shadow on the screen. If you don t, move mirror M1 until the bulls-eye pattern is centered using the fine adjustment technique. With the whole bulls-eye pattern visible, it is easier to analysis what is causing the fringe pattern to move, as discussed next. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 5

6 Figure 14g: Bulls-eye interference fringe pattern centered. The interference fringe pattern can be used to analyze any vibrations occurring on the table surface and/or any movement of the optical components. There are three types of fringe movement: If the pattern moves rapidly then settles down, the table is receiving ground vibrations. These types of vibrations may be caused by outside moving vehicles, people walking in an adjacent room, elevators, a running dishwasher, etc. Just stomping your foot on the floor will cause this type of vibration. It is best to run this analysis in the late evening when the surrounding environment is more likely to be quieter. This is also the best time to make your exposures. If the fringe pattern moves slowly back and forth, it is due to air currents in the room moving over the components. Make sure all air conditioning units are turned off. If you have central air conditioning, either turn off the thermostat or block the input vent to the room at least 30 minutes before your exposure. If you have a door to this room, make sure it is closed during your testing and during your exposures. This type of pattern movement may also be caused by optical components that are not locked tightly into position. Make sure all thumb screws on the optical mount connectors are tightened as well as the wing nuts on the mounts for lenses and mirrors. A fringe pattern that moves slowly in one direction is due to thermal expansion or contraction caused by adjusting the optical components (usually the mirrors or lenses) and transferring body heat through your fingers to the components. The thermal effects usually dissipate rapidly (within a minute or two). This type of fringe movement can also be caused by mechanical creeping of the mounting connectors or Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 6

7 optical mounts. Make sure these components are tightened. Gradual room temperature changes do not affect the components or table. Please refer to the Testing for Vibrations section on the web site to view movies showing the real time moving fringe patterns of these three types of fringe movements. By studying the movements of your fringe patterns and relating the movements to causes in your environment and components, you'll get a good feel for what the most favorable conditions will be when making your exposures. The only thing I can't control is traffic driving past my house. This is why I usually make my exposures in the evening between 10 pm and midnight. The bottom line is: the quieter the table surface, the brighter the hologram. Note: Fine Adjustment To get the center of the bulls-eye pattern exactly in the center of the beamsplitter's shadow on the screen takes some practice and fine tuning. Seeing the center of the bulls-eye pattern in the center of the beamsplitter's shadow on the screen means you have both dots exactly overlapped. Make sure the diverging lens has been placed between the laser and beamsplitter. Figure 14g shows how the bulls-eye pattern is no longer magnified with the lens in this position and you can see more of the bulls-eye pattern. To get the bulls-eye centered, I work with just one of the mirrors and its mount. Referring to the setup in Figure 14a, I would work with mirror M1. The reason I work with this mirror is because if you move the mirror right or left, the pattern on the screen moves the same direction. This is also true when moving the mirror up or down. The reason for this is because the reflected laser light from mirror M1 goes straight through the beamsplitter to the screen. Because the reflected beam from mirror M2 is reflected (not transmitted) to the screen from the beamsplitter, right and left positions switch. So if you move M2 right, the screen pattern moves left. This causes a bit of confusion and who needs confusion with everything else you're doing here. Before you start to move mirror M1, first look at the curvature of fringe pattern on the screen. The center of the bulls-eye is in the direction of the fringe pattern's concave curvature. As an example, if the concave curvature is to the right, so is the center of the bulls-eye as shown in Figure 14h. Additionally, the width of the fringes gets thicker in that direction. Figure 14h: Concave curvature of the fringe pattern is to the right. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 7

8 To move the center of the bulls-eye to the left, you need to move mirror M1 to the left. To move the mirror gently and slightly, tap the front right side of the lead weight slightly with your index finger as shown in Figure 14i. Figure 14i: Moving mirror M1 left with index finger. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 8

9 Repeat this tapping gently until you see the center, or portion of the center, of the bulls-eye as shown in Figure 14j. Continue tapping until the bulls-eye is completely centered as shown in Figure 14g. If the concave curvature started out to the left, then gently tap the front left side of the lead weight to move the bulls-eye pattern to the right and center. Figure 14j: Center of bulls-eye moving left with tapping. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/2017 9

10 If the concave curvature is facing upward, you need to move the mirror itself downward to move the center of the bulls-eye downward. To achieve this, I grab the acrylic base of the mirror mount by its ends with my thumb and index finger and rotate the whole mirror mount clockwise downwards around the 8-32 short rod bolt as shown in Figure 14k. This is a very slight rotation. If the curvature is facing downward, you need to rotate the mirror mount upward or counterclockwise. Again, this is a very slight rotation. Since you're rotating the optic mount counterclockwise, you're actually loosing the mount from the 8-32 bolt in the short rod. You don't want to loosen the mount from the bolt so much that it is no longer rigidly attached to the bolt. Loosening the connector that holds the short rod to the table mount pole won't work since that is a gross adjustment. Figure 14k: Rotating mirror up or down. Sometimes the concave curvature faces a diagonal direction towards a corner of the beamsplitter shadow. In this case, you would have to make both up or down and left or right movements of the mirror. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

11 Note: Retro-reflection All of the optical setups on this site will almost always have the laser's output aperture beam at 9 inches (22.9 cm) above the table surface. There are two reasons for this: in the multi-beam white light reflection hologram setup, the reference beam will need to impinge on the recording plate from below the 9 inch (22.9 cm) high plane at an angle of 56 degrees. If the center of the recording plate was just above the table surface, this would not be possible. in all the optical setups, the laser beam should always be parallel with the table surface and with each other to make sure that the polarized direction of the laser's beam is the same for all the beams traveling around the table. The importance of the polarized direction of will be discussed later. To insure that all the beams are parallel with the table surface as they traveling around the table from one component to the next, you will use a technique called retro-reflection. Looking back at Figure 14a, as you position mirror M2, adjust the reflected beam from M2 back through the beamsplitter to hit the laser aperture just 3-4 mm to its right or left (but not back down the laser tube) as shown in Figure 14L. Do the same with mirror M1. This will guarantee that the all the beams are parallel with the table surface and 9 inches (22.9 cm) above the table. I'll cover retro-reflection in more detail when you set up your first single-beam transmission hologram arrangement. Figure 14L: Retro-reflecting laser beam back to laser aperture. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

12 Supplies, Plates, and Processing Before you start setting up your first transmission hologram arrangement, there's a few more items you need to acquire and procedures you need to learn about so you're ready to record and process your hologram once your arrangement is set up. This section covers: what additional supplies you need what recording plate types are available and which one you should buy what processing chemicals you need to buy and how to mix them and the processing procedure to develop the hologram after it is exposed. Please read completely all of the information in this section before continuing. Please refer to the Resources link for suppliers, items' specifications, and costs. Additional Supplies Needed Now for Single-beam Setups You will need three safelights. One safelight is positioned over the optical table where you load the plate into its holder, one over the chemical processing area, and one over the area where you remove the plate from its box. These safelights allow you to work with your plates and see what you're doing instead of working in total darkness. There are lots of safelights available, but they're usually expensive and may not provide the filter (dark green) required for red sensitive plates/films (He-Ne laser light). Electroluminescent night lights work very well when covered with two layers of Kodak's Wratten Filter #58 Green. You can get two layers of the green filter out of one package for the night light. Just scotch tape the filters over the luminescent portion of the night light. Keep the safelights at least 6 feet ( cm) away from any undeveloped photographic plates or film. Extension cords work well for positioning the night lights. Surgical gloves (non-sterile, talcum-free) for handling plates and for processing since you will be touching the chemicals directly. They also prevent finger prints from getting on the plates. Protractor, half sphere shape, not circular. 12 foot (3.66 meter) tape measure. A small level. Some string or twine. 1/2 inch (1.27 cm) wide black masking tape. Light proof black box or envelope to protect 4 inch x 5 inch exposed plates and film when transporting to processing area. Photography polarizing filter with an indicator show direction of polarized lines or cheap clip-on polarized glasses. Acetone, Q-tips, and photographic lens cleaning paper. Two 4 inch x 5 inch x 1/16 inch (10.16 cm x 12.7 cm x 0.16 cm) glass plates such as those used in picture frames to sandwich film for test exposures. One 1 inch x 1 inch x 1/8 inch (10.16 x 12.7 cm x 0.32 cm) front surface mirror with enhanced aluminum coating and its mirror mount. One 5 inch x 7 inch x 1/4 inch (10.16 x 12.7 cm x 0.6 cm) front surface mirror with enhanced aluminum coating and its mirror mount. Black mounting board for masking extraneous beams and reflections. White mounting board for use as a screen in the plate holder. Volt-ohm-meter (VOM) with silicon solar cell for measuring beam intensity ratios, exposure times, and exposure density. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

13 For processing your plates, you will need: o Five 5 inch x 7 inch (12.7cm x 17.78cm) processing trays for the developer, stop bath, bleach, photoflo, and washing. o thermometer. o stirring rod. o scale, 200 gm capacity for measuring out chemicals. o 2000 ml flat bottom flash for mixing chemicals. o magnetic Mini-Stirrer with stir bar for mixing chemicals. o a 2 ml pipette and dispenser for measuring out sulfuric acid. o wax paper for weighing the chemicals on. o 5 one-quart (one liter) bottles for parts A and B of the developer (each separate), stop bath, bleach, and photoflo. o a 1-liter plastic graduated measuring cup for mixing chemicals. o a funnel that will fit into the quart bottles for pouring the stop bath, bleach, and photoflo back into their respective bottles. o paper towels. Additional Supplies Needed Later for Multi-beam Setups Six more 1 inch x 1 inch x 1/8 inch (2.54 cm x 2.54 cm x 0.32 mm) front surface mirror with enhanced aluminum coating and their mirror mounts. Three more 5 inch x 7 inch x 1/4 inch (12.7 x 17.8 cm x 0.6 cm) front surface mirrors with enhanced aluminum coating and their mirror mounts. A second 4 inch x 5 inch (10.16 x 12.7 cm) plate holder. This plate holder will be needed when you create your final white-light reflection display hologram. Holography Plates and Films Holography plates and films are available through Integraf. Integraf supplies the following two plate and film types in sizes 2.5 inch x 2.5 inch (2.54 cm x 2.54 cm), 4 inch x 5 inch (10.16 x 12.7 cm), and larger: PFG-01, wavelength sensitive 633 nm (red), exposure sensitivity 80 microwatts/cm 2. VRP-M, wavelength sensitive 532 nm (green), exposure sensitivity 75 microwatts/cm 2. Since you are using a He-Ne red laser with an output wavelength of 633 nm, you will want to use PFG-01 plates and films. The exposure sensitivity has been included here for you to use when calculating your exposure time if you decide to use a power meter to measure your beam intensities. Processing Chemicals and How to Prepare Them The following table shows how to mix one liter (1.06 quarts) of developer A, developer B, stop bath, bleach, and photoflo. Mix the developers and bleach at 100 degrees F (38 degrees C), then allow to cool to room temperature of ~70 degrees F (21 degrees C) for processing. The stop bath and photoflo can be mixed at room temperature. Use the 1-liter measuring cup and stirring rod for mixing the chemicals. Wash the cup thoroughly after mixing each chemical with hot running water. After mixing each developer, the stop bath, bleach, and photoflo, place them in their own one quart bottles, label and date them. I label the two parts for the developer Pyrochrome for Part A and Sodium Carbonate for Part B. I label the remaining three Stop, Bleach, and Photoflo. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

14 All chemicals can be dangerous in one way or another. Please follow the safety considerations written on the label of each chemical container. Wear rubber or surgical gloves, dust mask, apron, and safety goggles when mixing chemicals. This web site and its author are in no way liable for your use of these chemicals. Here is a list of the chemicals you will need to purchase: Pyrogallol Sodium carbonate Kodak indicator stop bath Potassium dichromate Concentrated sulfuric acid Photoflo Mixing Chemicals for Processing Pyrochrome Developer Part A Part B start with 750 ml distilled water at 100 F (38 C) start with 750 ml distilled water at 100 F (38 C) add 10 gm Pyrogallol and stir until dissolved add 60 gm sodium carbonate and stir until dissolved add distilled water at ~70 F (~21 C) to make one liter (1000 ml) add distilled water at ~70 F (~21 C) to make one liter (1000 ml) Combine equal parts A and B just before developing plate in a 5" x 7" development tray. Use 6 oz of both A and B (totaling 12 oz). Used developer has an 8 hour tray life if covered. Two day life in glass or plastic bottle. 12 oz. can process four 4" x 5" holograms. Kodak Indicator Stop Bath 1 part stop bath to 32 parts distilled water mixed at ~70 F Bleach start with 750 ml distilled water at 100 F (38 C) add 2 gm potassium dichromate and stir until dissolved add 2 ml concentrated sulfuric acid. Stir well. add distilled water at ~70 F (~21 C) to make one liter Photoflo add 1 part Photoflo to 200 parts distilled water at ~70 F. Stir well. The freshly mixed color of the stop bath is bright yellow and the color of the bleach is a bright orange-yellow. Add 10 ounces to each of their developing trays. The stop bath and bleach can be used many times for many holograms, so when you've finished your processing for the evening, you can pour these solutions back into their quart bottles. When the stop bath starts to go bad, it will turn a dark yellow-orange with a slight green tinge. When the bleach starts to go bad, it will turn a dark orange with a slight green tinge. You should label each of the developing trays on the side with the words that indicate that solution's function (developer, stop, bleach, photoflo, and wash) and use the same tray each time for that solution. Note: Use the pipette and dispenser to pull 2 ml of sulfuric acid out of its bottle. Never pull acid into the dispenser. When finished with the pipette, remove it from the dispenser and run hot tap water through it from the end that went into the dispenser for one minute. Also wash the outside of the pipette. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

15 Additional note: make sure you are purchasing 98% (concentrated) sulfur acid. Photographer's Formulary sells only 49% sulfuric acid. If you buy from them, then you need to use 4ml of sulfuric acid per liter. Processing Procedures All tray solutions should be at room temperature while processing (~70 F plus or minus 3 ) as well as washing the plate in a sink between certain processing steps. Always wear surgical gloves during all the steps of the processing procedure. The gloves will protect your hands from the processing solutions. If you notice that solutions are leaking into a glove, remove it, wash your hands, and get a new glove. All of the chemicals in their diluted form in their solutions won't really hurt your hands for short periods of time. If a glove leaks while using the developer, your fingers may turn slightly brown but this will go away after a day or so. I rarely have a leakage problem with my gloves. Note: The plus or minus factor of 3 is optimal, but not critical. During the summer, I'll have tap water temperatures up to, and sometimes above, 80 F (27 C) when my tray temperatures are at ~70 (~21 C). This has never caused me a problem with reticulation, a fine pattern of wrinkling in the emulsion caused by a sudden change in temperature from the tray temperature to the tap water washing temperature. Processing Steps for Plates and Films Step Processing Time (minutes) Agitation 1. Pyrochrome developer 5 Continuous 2. Stop bath 1 2 Continuous 3. Wash in sink tray 5 Running tap water 4. Bleach 1, 3 see footnote 2 Continuous 5. Wash in sink tray 5 Running tap water 6. Photoflo 2 None 7. Dry (room temperature) None 1 During development, the stop bath and bleach may leave an opaque, white residue on the plate. A thoroughly water soaked ordinary paper towel is used to gently wipe both sides of the plate while holding the plate under water in the washing tray with running tap water. Hold the plate firmly by its edges on one end of the width of the plate while you wipe one half of one side of the plate. Then turn the plate around and repeat the wiping on the other half of the same side of the plate. Then flip the plate over and repeat the whole wiping process again. Don't let the plate move against the bottom of the tray while wiping because this may scratch the emulsion. The emulsion is surprisingly resistant to scratches while wiping its surface. 2 The plate is left in the bleach until the emulsion (all dark areas) has cleared. You can go for a minute or so past this point, but don't leave the plate in the bleach to much past its clearing point. 3 When you're through using the bleach, you'll notice silt in the tray solution. This silt is silver that was removed from the emulsion as the plate cleared. Don't concern yourself with this. Just pour the bleach back into its bottle and then the next time you pour out the bleach into its tray, pour it in gently and the silt will remain at the bottom of the bottle. Even if some silt gets poured back into the tray, it won't affect anything. All plates and films should be air dried whether they are transmission or reflection holograms. If you try to dry them faster, the emulsion will shrink and shift the color of the hologram. This is especially critical with reflection holograms. I air dry my plates by just leaning its top edge against a surface and placing a paper towel under its bottom edge. Film can be air dried by running a piece of string between two points and using a plastic clothes pin over the string and grasping one corner of the film. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

16 Note: Agitation There's a couple ways you can agitate the plate while it is in the developer, stop bath, and bleach. You can rock the tray gently left and right for 30 seconds, then tilt an adjacent side of the tray up and down for 30 seconds, and repeat this until the indicated processing time is up. The other way is to grab the plate with your thumb and middle finger on opposite sides of the plate and move the plate back and forth in the solution. The agitation should not be very vigorous. Rocking the tray left and right or using your fingers back and forth should take about 3-4 seconds for a cycle. A cycle being one left and right rocking or one back and forth with grabbing the plate. If you're using film and you want to use your fingers, grab the film by a corner and move it back and forth for one cycle. Creating a Single-Beam Transmission Hologram Producing a single-beam transmission hologram will help you achieve four goals: You will become familiar with setting up an optical arrangement for recording a hologram. You will learn the step-by-step recording procedure for obtaining a successful, high-quality hologram. You will become familiar with the chemical processing steps involved. You will be successful in producing a hologram that will build confidence in your abilities to create more complicated multi-beam holograms for display purposes. Note: All of the optical arrangements discussed from this point forward are based on a 4 foot x 6 foot ( cm x cm) optical table and the holograms produced will be 4 inches x 5 inches (10.16 cm x 12.7 cm) in size. All of the optical arrangements are scalable. All components in an illustrated optical arrangement are not necessarily to scale for the sake of clarity. All table mounts and optical mounts are not included in the illustrations of optical setups for the sake of clarity. Determining and Controlling Polarization Determining and controlling the orientation of the laser's beam polarization is important for two reasons: it eliminates extraneous interference patterns on the recording plate or film that deteriorate the holographic image, and it maximizes the brightness of the holographic image in all the multi-beam optical setups I will describe. The optical arrangements for a single-beam transmission hologram, single-beam reflection hologram, and a multi-beam transmission hologram will have the reference beam incident on the recording plate from the side and parallel with the table surface. A multi-beam reflection display hologram will have the reference beam incident on the recording plate from overhead or underneath. If your reference beam is incident from the side, your beam's polarization needs to be horizontal to the table surface as shown in Figures 15a and 15b. If your reference beam is incident from overhead or underneath, your beam's polarization needs to be vertical to the table surface as shown in Figures 15c and 15d. Arrows indicate beam propagation direction. Note: The orientation of the beam's polarization is most important when using recording film sandwiched between two glass plates. The laser light can reflect internally between the two glass plates, causing extraneous interference fringes on the film which degrade your holographic image because you have to look through these fringes to see the image. This can even happen with a recording plate where there can be internal reflections between the inside sides of the glass plate supporting the emulsion. The correct polarization orientation will help minimize or eliminate these extraneous fringes in combination with using a reference beam incident angle of 56. This is called Brewster's angle and will discussed later. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

17 Figure 15a: Horizontal polarization. Reference beam incident from the side and polarization parallel with the table. Figure 15b: Close-up of horizontal polarization. Reference beam incident from the side and polarization parallel with the table. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

18 Figure 15c: Vertical polarization. Reference beam incident from above the plate and polarization perpendicular with the table. Plate is semi-transparent to show cone. Figure 15d: Close-up of vertical polarization. Reference beam incident from above the plate and polarization perpendicular with the table. Plate is semi-transparent to show cone. As I mentioned in the Laser section of Building a Holography System, your helium neon laser should be linearly polarized. Because linear polarization oscillates at right angles to the propagation direction of the laser beam, you can visualize these oscillations as being a simple plane as shown in Figures 15a-d. The very first thing you want to do before you start setting up your first optical arrangement is to find out the orientation of your laser's polarized beam. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

19 Determining the Laser's Polarization Orientation Linear polarized helium neon lasers come encased in either rectangular shaped metal casings or cylindrical metal casings. Usually the top of the laser casing has a label with the company's name and other information and on the bottom of the casing is a 1/4-20 threaded hole for mounting. See Figure 3a and 3b in the Laser section of Building a Holography System. If the manufacturer has orientated the laser tube correctly inside the casing, the laser's beam polarization orientation will be either vertical or horizontal when the label is on top. With my first 5 mw helium neon laser, built 8 years (1970) after lasers were invented, the tube was installed so the polarization orientation was at 4 o'clock instead of noon/six for vertical or 3/9 for horizontal. Since that time, I've had two 35 mw He-Ne lasers, one with horizontal polarization and one with vertical polarization. It looks like manufacturers now understand the importance of the orientation. The least expensive way to find out the orientation is to buy clip-on polarizing eyeglass lenses at a drug store (around $16). These clip-on eyeglasses have their polarizing lines running vertically in each lens. Set the laser on your table with the label facing up, turn it on, and point the beam at a piece of white mounting board. Insert one of the lenses of the eyeglasses into the beam so the beam passes through the lens perpendicularly and so the lens is orientated as you would wear it, that is, with the polarizing lines running vertically. Now rotate the lens through 90 degrees and observe the beam on the white card to see if the beam gets darker or brighter. If the laser's polarization is vertical, the beam will be brightest when you first insert the lens and get darker as you rotate the lens towards 90 degrees. If the laser's polarization is horizontal, the beam will be dark when you first insert the lens and get brighter as you rotate the lens towards 90 degrees. Continue rotating back and forth until you find the brightest beam and mark that angle on the output aperture of the laser with a permanent marker. If your beam is other than vertical or horizontal, you'll need to rotate the laser along its length to make it's beam vertical or horizontal and mount the laser to its table mounts accordingly. Controlling Polarization Orientation Let's now say that your laser was built correctly and it's polarization was chosen to be vertical instead of horizontal, with the laser's label facing straight up. With this first single-beam transmission hologram arrangement, you'll be bringing the reference beam in from the side, so your polarization orientation needs to be horizontal. There are two ways you can change the beam's polarization orientation from vertical to horizontal: 1. You can mount the laser on its side which is pretty easy to do. 2. If you don't want to mount your laser on its side, you can arrange two mirrors close together so that the reflected beam's polarization orientation from the second mirror (mirror 2) is rotated 90 degrees from the incident beam's orientation on the first mirror (mirror 1). This mirror combination setup, shown in Figure 15e, shows a vertically orientated beam being converted to a horizontally orientated beam. The setup is exactly the same for converting a horizontal beam to a vertical beam. I will refer to this twomirror setup as a polarization rotator. Make sure you use separate table mounts for each mirror because it's much easier to align and adjust each mirror separately. It's impossible to mount both mirrors on the same table mount pole and align them properly. Additionally, the second mirror should be 9 inches (22.86 cm) above the table where all the other components downstream in the setup will be. This means that the laser and the first mirror will need to be at a lower height above the table. If you place the first mirror at 7 inches (17.78 cm) above the table, then the laser aperture should be at 7 inches (17.78 cm) also and the first mirror should be retro-reflected to insure the beam is parallel with the table surface. As a side note, all optical recording setups will assume a laser with a horizontal polarization since we will always be impinging on the recording plate from the side with the reference beam. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

20 Figure 15e: Converting vertical orientated polarization to horizontally orientated polarization. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

21 Setting Up A Single-Beam Transmission Hologram Arrangement A single-beam transmission hologram is a very simple optical arrangement and requires less system stability than multi-beam arrangements. It's called a transmission hologram because the reference beam exposes the photographic plate on the same side as the object scene's reflected light exposes the plate, as shown in Figure 16a. The processed hologram image is reconstructed when the same reference beam light "transmits" through the hologram to the viewer's eyes as shown in Figures 16j in this section and 18a in the section on Recording and Processing. Figure 16a: Single-beam transmission hologram setup. Figure 16a illustrates what the recording arrangement will look like when it's set up. Refer to this illustration as you set up the arrangement. As a quick overview description of this setup, the laser's beam travels to a mirror which reflects the beam through a diverging lens and illuminates the photographic plate holder and object scene. A diverging beam from the lens is not shown in this illustration for the sake of clarity but will be shown in a later illustration. So let's get started. Place the laser on the corner edge of the table's long length facing the table's opposite corner and turn it on (refer to Figure 16a). Make sure the laser, or beam, is orientated to give you a horizontally polarized beam. For the sake of simplicity, let's say the laser was manufactured with the beam polarization already horizontal. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

22 Point the output aperture beam of the laser in the direction where the mirror will be placed. Make sure the beam height at the output aperture is 9 inches (22.86 cm) above the table using your tape measure to check the height. Since your tape measure is probably made of metal, make sure you keep the tape perpendicular to the table surface so none of the laser beam gets reflected and directed towards your eyes. Also attempt to make the beam as parallel to the table surface as you can by tilting the front of the laser up or down. Now place a table mount on the table where the mirror will be placed and tie a piece of string on the table mount pole 9 inches (22.86 cm) above the table (you can improvise instead with a piece of tape, marker pen, whatever). Adjust the laser either right or left and/or tilt the laser up or down so the beam hits the string. Re-check your height measurements at the aperture and at the string and make adjustments until both heights are around 9 inches (22.86 cm). This height is not super critical. Just get the height of the beam at the aperture and string the same as close as you can. Now replace the table mount with the table mounted mirror as shown in Figure 16a. Face the first surface side of the mirror towards the laser and make sure the beam is hitting the center of the mirror as close as possible. Use the retro-reflection technique to adjust the reflected beam going back to the laser so that the beam dot is aligned horizontally to the right or left of the laser output aperture. Once this is done, your mirror is pretty much perpendicular to the table and will reflect a 9 inch (22.86 cm) high beam wherever you point it across the table. This will insure that your beam maintains a horizontally polarized orientation all the way from the laser to the recording plate and object scene. Leave the diverging lens out of the arrangement for now. Now place two table mounts for the plate holder and one table mount for the object scene on the table in the approximate positions shown in Figure 16a. Drop one connector on each of the table mount poles for the plate holder and let them just rest at the base of the poles. Do the same for the object scene table mount except drop two connectors on the pole instead of just one. You will need these connectors later to connect the plate holder table mounts to the object scene table mount so they are connected together and can't move relative to each other. Now connect the plate holder to its two table mounts making sure its level and connect the object scene mount to its table mount. The center of the plate holder and the center of the objects in your object scene should be 9 inches (22.86 cm) above the table. Position the plate holder and object scene as shown in Figure 16b. They should be as close together without the object scene mount or objects casting a shadow on the plate. Place a piece of 4 inch x 5 inch (10.16 cm x 12.7 cm) white mounting board (as a screen) in the plate holder so you can see if shadows are being casted. Referring again to Figure 16b, the plate holder should be perpendicular to the table and be level. You can use a small level to check this. The plate holder should also be facing the incoming beam at a 56 degree angle to the plate's normal. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

23 Note: Why a 56 degrees incident angle on the plate? This angle is called Brewster's angle. If you're using film sandwiched between two glass plates instead of a photographic plate, the laser light can reflect internally between the two glass plates, causing extraneous interference fringes on the film, degrading your image. Brewster's angle eliminates this problem. Once the plate holder is in position, readjust the position of the object scene as close to the plate as possible without casting a shadow on the plate. I've been able to get the sphere as close as 3/4 of an inch (1.9 cm). The incoming beam should be at a midpoint between the center of the plate holder and the center point between the two objects in the scene which is approximately where the beam is in Figure 16b. You'll refine the beam's position later when you diverge the beam. Figure 16b: Top view of plate holder and object scene. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

24 Next, lock the plate holder and object scene together using two aluminum poles and the connectors you previously dropped on to the table mount poles as shown in Figure 16c. With the plate holder and object scene now locked together, you can move them together on the table if needed and you can still change the height of each independently if needed. Figure 16c: Plate holder and object scene locked together. It is now time to place the diverging lens in the path of the beam. A double concave or plano-concave lens with a -15 mm focal length and 12 mm diameter or a 10x microscope objective works fine in this setup. Figure 16a shows the starting position for the lens. This position would be about 43 inches ( cm) from the center of the plate holder and object scene. The position was determined using the equation in Figure 16f. Once the lens is in this position, you will need to adjust the lens left or right and/or up or down to get the objects in the scene and the white screen in the plate holder illuminated. To help you do this, place a piece of white mounting board, of the appropriate size, behind the plate holder and object scene so you can see their shadows casted on the board as shown in Figure 16d. This will give you a good idea how you should move your lens so your plate and objects are illuminated properly. I usually turn the rooms lights off while adjusting the lens, making it easier to see the shadows. I also add some ambient lighting, like a flashlight shining on a white wall or white card, so I can see the lens mount for adjusting and so I'm not knocking things over or banging into things. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

25 Figure 16d: Plate holder and object scene casting shadows on screen. Once you get the lens positioned to properly illuminate the plate and scene, it should look something like Figure 16d and 16e. The intensity of the beam on the white screen should be uniform across the whole screen. Figure 16e: Plate holder and object scene illuminated properly with diverging beam. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

26 Determining Where to Put the Diverging Lens No matter what optical setup your doing, at some point during the set up you have to decided what focal length lens to use and where to place it in the beam whether it be a single-beam or multi-beam setup. Usually you want to know how far away the lens needs to be to cover the recording plate and object scene uniformly such as in the setup you're now arranging. I have come up with a couple formulas that you can use based on actual measurements I've made on several types of setups. The first formula shown in Figure 16f is specific to a single-beam transmission hologram. This formula gives you the distance (L) the lens needs to be from the plate holder/object scene based on the focal length (F) of the lens, the total width of the plate and object scene (W (p/o) perpendicular to the incoming beam, a coefficient number (1.274), and the diameter of the laser beam D (b) at the laser's aperture. Figure 16f: Formula to calculate distance of lens from plate/object for a single-beam transmission hologram. Figure 16g illustrates this relationship graphically from a top-view perspective. The Rule: the shorter the focal length F of the lens, the shorter the distance L needs to be. Figure 16g: Illustrated relationship between the focal length of the diverging lens and the distance L. Let's look at an example. Using Figure 16e, we will determine where a diverging lens should be placed with a focal length of -0.6 inches (-15 mm lens or 10x objective), an object scene/plate holder width of 4.5 inches (11.43 cm), and a laser beam diameter of 0.08 inches (0.2 cm) measured at the laser aperture as shown in Figure 16h: Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

27 Figure 16h: Example calculating the distance of lens from plate/object scene for a single-beam transmission hologram. The diverging lens should be placed at least 43 inches ( cm) away from the plate holder and object scene to have fairly uniform illumination. The location of the lens in Figure 16e is pretty much where it should be. When doing this calculation, ignore the minus sign of the lens. For a single-beam reflection hologram setup, a slightly different formula is used because the object scene is not included in the W factor of the formula. Here, the W factor is just the diagonal length of the photographic plate (6.4 inches [16.26 cm] ) since the object scene is directly behind the plate. This is also true in a multi-beam setup since the plate's incoming beam is completely separate from the beam for the object scene. Use the diagonal length of the plate for W. In Figure 16i, I have listed the focal length of various microscope objectives and their equivalent plano-concave or double concave lenses focal lengths and diameters as well as today's costs. Microscope Objective Plano or Double Concave Lens Magnification Focal Length (mm) Cost Diameter (mm) Focal Length (mm) Cost 5x 25.5 $ $ x 16.5 $ (Plano) $ x 8.8 $ $30 40x 4.5 $ $30 60x 3 $195 n/a n/a n/a 100x 2 $255 n/a n/a n/a Figure 16i: Available microscope objectives and concave lenses. Microscope objectives are more expensive than plano-concave or double concave lenses, but finding lenses with focal lengths shorter than 9 mm is very difficult but not impossible (you can get lenses with -6 mm focal lengths, but their diameters are 6 mm and can be difficult to mount and get a 2 mm laser beam through, but not impossible). All simple lenses should have MgF 2 anti-reflection coatings. Also, if you plan to make holograms larger than 4 inches x 5 inches (10.16 cm x 12.7 cm), you will need to use microscope objectives instead of lenses to achieve the illuminating coverage of the larger plates. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

28 During the exposure of the scene shown in Figure 16e, the hologram is created by the light interference between the light rays reflected from the objects to the plate and the light rays passing by the objects directly to the plate. The light rays reflected from the objects are considered the object beam, as in a multi-beam arrangement, and the light rays passing the object and going directly to the plate are considered the reference beam. In this setup, I want you to be successful, so choose objects for the scene that are white and rigid. A small white figurine, small white car, or small white chess pieces are good examples. Having two objects will enhance the parallax effect but not required. The castle and unicorn on the Introduction page of Building a Holography System were made on 4 inch x 5 inch (10.16 cm x 12.7 cm) plates. Figure 16j shows how my two geometric shapes will look like in the exposed and processed hologram from two different perspectives showing parallax. Figure 16j: Processed single-beam transmission hologram showing parallax. Now that you have your first optical arrangement setup and ready to record a hologram, I'll next show you how to calculate your exposure time needed during the recording process. Exposure Calculation Technique In the single-beam transmission hologram setup, using a 5 mw helium neon laser with an output power of 5 mw at the aperture, a 10x microscope objective placed 43 inches ( cm) from the plate holder, and a PFG- 01 plate, the exposure time would be 20 seconds. This will give you an emulsion exposure density of 1.0, the optimal density for a bright transmission hologram. The acceptable density range for a bright transmission hologram is 0.8 to 1.2. I actually used the setup in Figure 16a to achieve these results. If you can afford a handheld power meter ($525 from Edmund Optics), I recommend you get one. Using a power meter allows you to measure the intensity of a laser beam, diverged or undiverged, directly in watts/cm 2 - seconds. The meter has a push button which allows you to change the wavelength of the laser light you're measuring. In our case, nm (nanometers). The actual setting on the meter would be 633 nm since the meter doesn't give fractions of a wavelength. There is also a slide-into-place filter for measuring power levels greater than 10 mw. Power readings are given in watts/ cm 2 -seconds, milliwatts/cm 2 -seconds, and microwatts/cm 2 -seconds. Let's look at an example of using this meter in figuring out your exposure time for the single-beam transmission hologram. Set the meter to watts, wavelength set at 633 nm, and the filter not covering the sensor. Place the sensor area of the meter at the center of the plate holder facing the incoming diverging reference beam square on, that is, perpendicular to the beam. Take four separate readings and average them. Let's say the average is 4 µw/cm 2 -seconds. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

29 Because the reference beam is incident on the plate at 56 degrees, you must take the cosine of 56 and multiple this number (0.5592) by 4: x 4 µw/cm 2 -seconds = 2.24 µw/cm 2 -seconds. This is the real power level (intensity) of the reference beam at the plate. Now you divide the power sensitivity of the PFG-01 recording plate by the calculated power reading at the plate holder: 80 µw/cm µw/cm 2 -seconds = 35.7 seconds. Your exposure time will be 36 seconds because you should always round up. If you don't want to spend this much on a power meter, you can use a volt-ohm-meter (VOM) connected to a small solar cell to measure the light level at the plate holder in volts and use this measurement to indirectly calculate the exposure time. The sensitivity of holography plates and films are rated in microwatts per centimeter squared, not in ISO or ASA ratings. If you have a photographic light meter that measures lumens, you can convert lumens to watts. You will not be using this exposure calculation technique until the next section on Recording and Processing. It's presented here so you can become familiar with the technique and refer back to it when you need it during your recording. Here, I'll be telling you to make an exposure and to process the plate, but I'm actually referring to what you'll be doing in the next section on Recording and Processing. You will not be making the real exposure and processing at this time. Figure 17a: Meter and solar cell connected. Using a VOM, connect its leads to the solar cell leads with positive to positive and negative to negative as shown in Figure 17a. Red is usually positive and black (or other color) is negative. Set the meter to its lowest direct current (DC) voltage range. If a negative sign shows up, reverse the leads. It really doesn't matter if you're getting a negative or positive reading. The amount of voltage should be the same. Place the solar cell at the center of the plate holder (white card removed) facing and perpendicular to the incident beam. Take note of the voltage reading, with the room lights off, and write it down. You will need a small flashlight to read the meter in the dark. Make sure the flashlight's light does not strike the solar cell. Expose the plate for 20 seconds. Process the plate only through the stop bath step, then wash and air dry. Do not bleach the plate. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

30 You're now going to measure the density of the dried hologram. With the room lights on, place the VOM on a table top with the solar cell attached and place the solar cell face up on the table top so the room lights illuminate the solar cell. Take a reading in volts of the ambient room light as shown in Figure 17a. We'll call this Intensity One (I 1 ). Write the voltage down on paper. Next, place the dried, unbleached, exposed plate on top of the solar cell with the emulsion facing up (the emulsion side of the plate is less reflective than the glass side). Take a second voltage reading as shown in Figure 17b. We'll call this Intensity Two (I 2 ). Write this voltage down on paper. If you've used film instead of a plate, place the film down on the cell with the emulsion up and hold the film's four corners down with your two gloved hands to keep the film as flat as possible. If you were to use a clear glass plate to keep the film flat, the glass would reflect some of the light and give you an inaccurate reading. Figure 17b: Meter and solar cell with plate on top. Now divide the first voltage I 1 by the second voltage I 2. The resulting number is a ratio and does not have units. Using a Common Logarithms of Numbers table, look up this number and the corresponding log of that number is given and is your plate density. Or use a calculator to find the log of the ratio. The equation is: As an example, if I 1 is 24 volts and I 2 is 3 volts, then I 1 divided by I 2 is 8. The log of 8 is 0.9. This puts you within the density range of a properly exposed transmission hologram. If you get a voltage reading of greater than 9.91 (which is the largest voltage number in the log table), you need to do some simple arithmetic to achieve the proper density reading. For example, if your dividend between I 1 and I 2 is 11 volts, find the log number for 9.91 volts and write it down (.9961). Next, subtract 9.91 from 11 giving you Look up 1.09 as your voltage reading in the table and you will get Add.9961 and.0453 and you get You're within the density range of 0.8 and 1.2. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

31 If your density is below this range, you need to increase your exposure time. If the density is above the range, you need to decrease your exposure. If your first exposure didn't fall within the range, keep making different exposure times until you've achieve a density within the range. Keep everything constant and only change the exposure time. Once you've achieved an exposure density within the acceptable range, you can use the original voltage measured at the plate holder as a reference for future setups and measurements with the VOM and solar cell at the plate holder. For example, if your original measurement at the plate holder was 6 volts and your exposure time was 20 seconds and you achieved a plate density of 1.0, then a voltage reading of 6 volts at the plate holder of your next setup will give you a density of 1.0 for a 20 second exposure. If your voltage reading at the plate holder is 3 volts in a new setup, then your exposure time should double to 40 seconds. The relationship between the voltage reading and the exposure time is linear. I've included the Common Logarithms of Numbers table here for your convenience. You may have noticed that you did not take the cosine of 56 degrees and multiply that number by your voltage reading of the reference beam with the solar cell facing the beam directly. That's because this is an indirect method of finding your exposure time. With a power meter, you do have to take the cosine of 56 degrees of your power meter reading to get an direct accurate power reading in watts for making your exposure time calculations. You do have to use the cosine of 56 degrees of your solar cell reading of the reference beam when comparing the beam intensity ratios between the reference beam and reflected object scene light at the plate. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

32 Common Logarithms of Numbers Voltage Reading Ratio Log Number Voltage Reading Ratio Log Number Voltage Reading Ratio Log Number s Note: I suggest you do all your exposure testing with film instead of plates since film is cheaper. You will need to get two glass plates for sandwiching the film and use the 56 degree reference beam angle. I've found that sometimes the size of the 4x5 film can be slightly larger than the size of the 4x5 sandwiching plates. You can find out if this is true by sandwiching the film (under safelight conditions) and feel the edges of the two plates to see if you can also feel the edge of the film on two sides. If you do feel the film, you need to cut off about 1/16 inch (0.16 cm) of film on one 4 inch (10.16 cm) side and the same for one 5 inch (12.7 cm) side. A rolling cutting board works great for this (not a chopping cutting board), but scissors work well also. If you let Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

33 the film extend beyond the glass plates, the film could get squeezed when placed in the plate holder, causing unwanted wrinkles in the film which will warp your image. Recording and Processing This section should be read through completely before implementing. After reading this section, it would be a good idea to review the section on Processing and the section on Exposure Calculation Techniques before implementing this section. Now that you have your single-beam transmission arrangement set up, you are ready to make your first hologram and measure its density. Each bullet listed is a step in the recording and processing procedure and should be followed in the sequence listed. The Recording and Exposure Turn off your air conditioning or heating systems. This needs to be done at least 30 minutes before you make your first exposure. Turn on the laser and let it warm up for 30 minutes if it is not already on. Turn on your safelights. Practice inserting and removing one or two sandwiched 4 inch x 5 inch x 1/16 inch (10.16 x 12.7 cm x.16 cm) glass plate(s) in the plate holder under safelight conditions depending on whether you're going to use a plate (one glass plate) or film (two glass plates). Note: Wear surgical gloves when practicing. Remove the gloves once you're through practicing. You can reuse gloves if you know that you haven't touched anything that would leave fingerprints on your plate or glass plates like touching your skin, eating food, drilling holes, those sorts of things. After processing, throw the gloves away. Make a simple shutter using a piece of black mounting board whose size is about 4 inches (10.16 cm) wide and about 12 inches (30.48 cm) tall. Note: Practice leaning this against the front of the laser to cover the aperture, then tilting the board away from the laser, then gently lifting the board while still blocking the laser beam, then waiting about 60 seconds, then lifting the board higher so the beam can expose the photographic plate, then lower the board to block the beam again after the appropriate exposure time, and finally rest the board on the table and lean the board against the laser again. When you're through practicing, remove the shutter. Recheck that your plate holder and object scene are properly illuminated. Components can settle over time and throw the illumination slightly off. Put the white card in the plate holder during this recheck. Do this check with the room lights off. Remove the white card from the plate holder and using your VOM and solar cell, with the room lights off, take a voltage reading at the plate holder as described above in the section Exposure Calculation Techniques and write that voltage down. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

34 You're now ready to make your exposure. Turn on the room lights. Put the shutter in place, blocking the laser beam. Set out the PFG-01 plate box (or film box if you're using film) on a table surface underneath one of the safelights. The safelight should be at least 6 feet away from the location of the plate box. Put on a new pair surgical gloves. Turn off the room lights so only the safelights are on. Remove a recording plate from its box and insert the plate into the plate holder. Note: It's important to know which side of the plate the emulsion is on. As you face the plate box, make sure the Integraf label is right side up so you can read it from where you are standing. If their plates are loaded in the box consistently in the same direction, then the emulsion will face to your left. Remember this side of the plate as you remove it from the box and place it in the plate holder. The emulsion should be facing the object scene. With film, you can tell which side the emulsion is on the film because the film curves slightly towards the emulsion side. During the washing phase of the development process, you can determine the emulsion side of the plate. Lift the plate out of the washing tray water. The water will flow off the plate slower on the emulsion side than on the glass side. We assumed that the emulsion was on the left side of the plate as it sat in the plate box and you've kept this side up while processing. If the up side shows the slower water flow, then our assumption was correct. You can now place an arrow on the outside of the plate box facing to the left with a black marker. If our assumption was incorrect, then the arrow should face to the right. You will find that all the plates' emulsions will face in the same direction inside the plate box. After the plate is in the plate holder, go back to the plate box and make sure the box lid is back in placed and locked into the closed position. Note: The box has sliders on the right and left sides of the box and once the sliders are in place, the lid is sealed tightly. You want to do this at this time because later on you will be turning the room lights on and you want to make sure the box is closed and sealed tightly. Additionally, when I'm through with the plate box for the day, I run a piece of 1/2 inch (1.27 cm) wide black masking tape along the seam where the lid seals with the lower part of the box just to insure no room lighting can leak in over time. You're now going to leave the room for 15 minutes while the whole recording system settles down. Make sure you have a watch that has a second hand so you can make the exposure for the appropriate time. After 15 minutes, you can re-enter the room, opening the door slowly and gingerly, and closing the door softly. Walk gingerly over to where the shutter is located making sure you don't touch the optical table as you do so. Stand there for 2 minutes. Try not to breathe heavily towards the table and don't shift your footing. Changing your standing position will send vibrations to the table. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

35 After 2 minutes, tilt the shutter away from the laser and gently lift the shutter off the table keeping the laser beam blocked. Wait in this position for 60 seconds. Now lift the shutter out of the beam and make the exposure for 20 seconds. Do not touch the optical table during the exposure and try not to breathe heavily towards the table or components. After 20 seconds, block the beam with the shutter without touching the table, and then set the shutter on the table and lean it against the laser making sure it will stay in place. You're exposure is complete. Remove the plate from the plate holder remembering that the emulsion, assumingly, is facing the object scene. Place the plate in a light proof black box with the emulsion up to prevent possible scratches. These light proof black boxes can be purchased from professional photo stores. Recheck that your plate box and light proof black box with the exposed plate are closed and light proof. You can now turn the room lights on. Processing Set up four 5 inch x 7 inch (12.7cm x 17.78cm) processing trays for the developer, stop bath, bleach, and photoflo. The table you process on should have a surface that is not porous and can be wiped down with a sponge. It's inevitable that you will spill some solutions on the table when agitating and transferring the plate between trays. If you want, you could place each tray in their own larger aluminum basting pan to catch spills. I highly recommend an apron or lab coat during processing. The developer will stain your clothing and is not removable. Wear old clothing. Mix 6 ounces (177 ml) of Pyrochrome Part A developer with 6 ounces (177 ml) of sodium carbonate Part B developer in your one liter plastic graduate measuring cup. Pour Part A into the graduate first, then add Part B. Stir. Pour the solution into the processing tray labeled Developer. Rinse the cup and stirrer in hot water thoroughly and place on paper towel to drain and dry. Pour 10 ounces (296 ml) of the stop bath solution into its tray labeled Stop using the measuring cup. Rinse the cup thoroughly before using it in the next two steps. Pour 10 ounces (296 ml) of the bleach solution into its tray labeled Bleach using the measuring cup. Pour 10 ounces (296 ml) of the photoflo solution into its tray labeled Photoflo using the measuring cup. Get your light proof box containing the exposed plate and bring it into the area where you are processing. Place the box away from the area you are processing so solutions don't splash on to the box. Turn off the room lights. Make sure the safelights are still on and you're still wearing gloves and other protective items. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

36 Remove the exposed plate from its black box and lower it into the developer tray gently, keeping the emulsion side up. When the 5 minutes is up, pull the plate out of the developer, let it drain into the developer tray until it's just dropping a few drops of solution, then place the plate into the stop bath gently, emulsion up, and continue the agitation technique for 2 minutes. You can now turn on the room lights. Follow the processing procedure all the way through the stop bath and wash, but no further. While washing, check for white residue and remove if present. With fresh stop bath, the white residue is usually not present. After the wash, soak the plate in Photoflo for 2 minutes with the emulsion up and then air dry the plate for 6 hours at around room temperature (~70 F) [~21 C]. Density Check With the plate completely dry, use the exposure calculation technique to check the plate density. If the plate density is within the range, I want you to go ahead and finish the processing procedure. To do this, soak the plate in running tap water for about 30 seconds at around 70 F (21 C), allowing the emulsion to become saturated with water. Now move to the bleaching solution and continue the processing procedure through the bleach, tap water rinse (check for white residue), Photoflo, and finally air drying. Allow the plate to dry for 6 hours with the emulsion side of the plate facing outward. Note: As I mentioned a little earlier, while you're doing the first wash, check to see if the side of the plate you've been keeping up is the emulsion. If it is, our assumption about plate placement in its box was correct - the emulsion side of the plate faces to the left in the box. If it turns out that the emulsion is actually facing down, then in the next paragraph where you view the hologram, the glass side of the plate will need to face the object scene. With you now knowing which side of the plate has the emulsion, all future exposures should have the emulsion facing the object scene. Viewing Your First Hologram You are now ready to view your first hologram. Take a piece of black mounting board and place it in front of the object scene facing the plate holder so you can't see the object scene from the plate holder's perspective. Remove the shutter. Since you now know what side of the plate the emulsion is on, hold the plate behind the plate holder in the reference beam with the side of the plate that faced the object scene during the exposure. If you see the image of your object scene as shown in Figure 18a, you can now place the plate in its holder and view it for hours. If you don't see an image, you need to rotate the plate 180 degrees around it's vertical axis to see the image. If you still don't see an image, you may have misjudged which side of the plate was facing the object scene. Rotate the plate 180 degrees around its horizontal axis and look for the image again. If it's still not there, rotate the plate 180 degrees around it's vertical axis and see if the image is there. If it's still not there, then vibrations and/or movement occurred during the exposure and you'll need to try again. But with this setup, I'm 99.99% confident you'll have an image. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

37 Figure 18a shows an illustration of what you should see (your object scene will probably be different than the one shown in the illustration). The image in Figure 18a is located in the same position as the original object scene. This image is called a virtual image (like the image you see of yourself in a mirror). Figure 18a: Transmission hologram virtual image. Now take the hologram out of the plate holder and rotate it 180 degrees about the Z axis (vertical axis) so that axes X and X - switch positions and Y and Y - switch positions. Figure 18b below shows what the image will look like now. You will see the image floating in space between the plate and yourself. This is called a real image because it is actually being focused into space and can be seen on a screen. Using a piece of white mounting board as a screen, you can focus different parts of the image on the card by moving the card through the 3D image. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

38 Figure 18b: Projected transmission hologram real image. You may notice something unusual about this real image while viewing it floating in space between you and the plate. If you move left to right to view parallax in the image, the image moves with you and its parallax cannot be observed. Also, the foreground of the original image has become the background and the background has become the foreground. Left and right parts of the original image have also switched positions. This type of image is called a pseudoscopic real image and is the inverse image of the original scene and the virtual holographic image. You will use this real image to produce a multi-beam white light reflection display hologram in which the image will revert back to its normal orientation (called orthoscopic). You will also notice that the pseudoscopic real image is greatly magnified. This magnification is caused by the fact that you used a diverging beam during this recording. When you produce a multi-beam hologram, you will use a collimated (the leading edge of the beam is flat instead of curved) reference beam which will keep the virtual and real images at a 1:1 (1 to 1) magnification. Transmission holograms have to be reconstructed with laser light. If you tried to reconstruct the image with the sun or a 100 watt clear envelope light bulb, your image would be a rainbow of colors and blurry. Transmission holograms take the full spectrum of the sun or light bulb and creates an image for every color in the visual spectrum therefore creating overlapping images that produce a rainbow blur. Note: Why do we have the reference beam incident on the recording plate at an angle to the plate's normal? When the hologram image is reconstructed with the original reference beam, the beam would be visible as part of the holographic image if it wasn't incident at an angle to the plate's normal and you don't want that. The angle is "off-axis" enough so you don't see the reconstructing beam in the image. Your final white light multibeam reflection display hologram will be illuminated (reconstructed) using a white light lamp and you certainly don't want to see that lamp as part of your fantastic image! Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

39 Creating a Single-Beam White Light Reflection Hologram Figure 19a illustrates the optical arrangement for a single beam reflection hologram. This setup is identical to the single-beam transmission hologram arrangement except for the positioning of the plate holder and object scene. The plate holder should be perpendicular to the incident beam and tilted at an angle of 56 degrees towards the incident beam creating an overhead reconstruction angle for viewing the finished hologram. The object scene should be placed directly behind the plate holder on the opposite side from the incident reference beam and almost touching the plate. Figure 19a: Single beam reflection hologram setup. Figure 19b shows the diverging laser beam from the lens to the plate holder and Figure 19c shows a close-up view of the plate holder and object scene positioning, and reference beam incident angle. Figure 19b: Plate holder and object scene illuminated properly with diverging beam. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

40 Figure 19c: Plate holder and object scene positioning and reference beam incident angle. During the exposure, the laser beam will pass through the hologram plate, illuminate the object scene, and the scene will reflect its light back to the plate. At the same time, the beam passing through the plate serves as the reference beam. The elimination of vibrations and component movement during the exposure is even more critical during a reflection hologram recording. Because the angle between the reflected light from the object scene to the plate and the incident angle of the reference beam totals 124 degrees, the spacing between fringes in the interference pattern in the emulsion is much narrower (finer) than a transmission hologram and must not move during the exposure. To help prevent any movement between the object scene and plate holder, you must once again connect the table mounts together as you did for your single-beam transmission hologram. The recording and processing procedure for this type of hologram is the same as for the transmission hologram except that you want to end up with an exposure density of 2. So again, you want to check the hologram's density after you've developed the plate through the developer and stop bath, washing, photoflo, and drying. Once you've found the right exposure time for a density of 2, you can continue the development process with a 30 second soaking of the plate, bleaching, washing, photoflo, and drying. Again, make sure the emulsion faces the object scene. A reflection hologram is viewed differently from a transmission hologram as shown in Figure 19d. The person viewing the hologram is now on the same side of the hologram that the reconstructing light source is on. This is called a reflection hologram because the reconstructing light is reflected from the plate to the viewer. This is a white-light hologram and can be viewed with sunlight (best), a clear light bulb with a vertical filament, or a flashlight. Because the fringes are more narrowly spaced, they filter out all but one wavelength of the white light, giving you a monochromatic image. At a 56 degree reconstructing angle, its color will be golden or reddish since you're using a red laser. You can even change the angle of reconstruction to get other monochromatic colors. You'll remember that the plate was titled towards the beam at a 56 degree angle, but the object scene was level with the beam. This means that when you view the hologram, the plate will not be tilted towards you but vertical and you'll need to look downward into the plate to see the image. This is an overhead reconstructed Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

41 hologram because when you hold the plate vertical in front of your eyes, the reconstructing light will be incident on the plate from overhead or over your shoulder. Figure 19d: Reconstructing reflection hologram virtual image. You could have placed the object scene up higher and centered and perpendicular to the recording plate during the exposure as shown in Figure 19e. Then the image would be centered in the middle of the finished hologram and you wouldn't have to look downward into the hologram to see the image. Since that would have been a slightly more difficult setup, I went with an easier setup for your first time reflection hologram. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

42 Figure 19e: Plate holder with object scene positioned high and perpendicular to the recording plate. The final hologram will be clear and you will be able to see background objects through the hologram as well as the image. The image will be less bright under these conditions. To increase the brightness of the image, place the plate, emulsion up, on a piece of paper towel that is on a large area of newspaper outdoors and spray the plate with 2 to 3 thin coats of quick-drying flat black enamel paint. Do not over spray during each coat. It's better to use multiple thin coats than one or two thick coats. Use a left to right, right to left sweeping motion when spraying. Let the paint dry for 4 hours. Now, when you reconstruct the image in white light, it will be much brighter. You can tell the emulsion side from the glass side of the plate (or acetate side if using film) because the emulsion side has a dull finish when inspected under white light whereas the glass or acetate side is shinny. Painting the emulsion side of the hologram using the above spraying technique should not affect the fringe spacing in the hologram. If it does and shifts the color, then face the emulsion away from the object scene during the recording so you can paint the glass side of the plate. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

43 Creating A Multi-Beam Transmission Hologram Figure 20a shows the optical arrangement for a multi-beam transmission hologram with a single object beam. It is called a multi-beam hologram because a beamsplitter splits the laser's beam into two separate beams: a reference beam and an object beam. Figure 20a: Multi-beam transmission hologram setup with a single object beam. Multi-beam setups have numerous advantages over single-beam setups: You have more control over illuminating your object scene. You can control the light intensities of each beam separately allowing you to fine tune the brightness of the final image. You can have overhead or underneath reference beams for more practical display arrangements. You can control the final position of the holographic image to be projected out in front of the plate, behind the plate, or straddling the plate. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

44 I will first discuss a multi-beam transmission hologram setup where the object scene is illuminated using one object beam. This will be followed by the same setup where the object scene is illuminated using two object beams. Referencing Figure 20a, I will discuss: The two separate paths the laser beam travels from the beamsplitter to the plate. Certain optical components and how they should be used in controlling the beam in each path. Paths Traveled by the Laser's Beams The output beam of the laser should be 9 inches (22.86 cm) above the table as well as the centers of all the mirrors, beamsplitter(s), parabolic mirror, diverging lenses, plate holder, and object scene. The laser's beam polarization should be oriented horizontally since the reference beam will be incident on the plate from the side. We have already assumed horizontal polarization when we set up the single-beam transmission hologram arrangement. The beam from the laser travels to directional mirror M1 which directs the beam to the beamsplitter BS. Be sure to use the retro-reflection technique whenever you're placing a mirror or beamsplitter into the setup. Also make sure the beam is hitting the optical component at its center. The beamsplitter splits the laser beam into two beams. The beam transmitted through the beamsplitter will be the reference beam R and the beam reflected from the beamsplitter will be the object beam O. When placing the beamsplitter on the table, make sure the reflective coated side of the beamsplitter faces the incidence beam from the laser and reflects the object beam at a 90 angle from the reference beam passing through the beamsplitter. Let's look at the path of the reference beam first. The reference beam R continues to mirror M2 which directs the beam through diverging lens DL1. Diverging lens DL1 diverges the beam to the parabolic mirror PM which then directs a collimated beam to mirror M3 which in turn directs the collimated beam to the plate holder PH at an incident angle of 56 degrees. For the sake of clarity, only the center-axis part of the beam is shown, not the actual diverging or collimated beams. A collimated beam is parallel light rays, not diverging or converging light rays. M3 is a 5 inch x 7 inch x 1/4 inch (12.7 cm x cm x 0.6cm) mirror. Leave DL1 out of the setup for now. At the same time, the object beam O travels from the beamsplitter BS to mirror M4 where it is directed to mirrors M5 through M7. Mirror M7 directs the beam to the object scene OS. Diverging lens DL2 produces a diverging beam that uniformly illuminates the object scene OS. The object scene OS then reflects laser light directly to the plate holder PH. M7 should be at least a 5 inch x 7 inch x 1/4 inch (12.7 cm x cm x 0.6cm) mirror. Leave DL2 out of the setup for now. Note: Any time you are using a mirror to direct a diverging or parallel beam, the mirror must always be larger than the diameter of the beam. Otherwise, the mirror size becomes the limiting size of the beam further down the beam's path. Beamsplitter's Function The function of the beamsplitter is not only to split the laser's beam into two beams but also to adjust the beam intensities of the reference and object beams at the plate holder so that the intensity ratio of the reference beam to the object beam is in the range of 2 to 1 (2/1) and 1.5 to 1 (1.5/1). Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

45 If you're using a circular variable gradient beamsplitter, adjusting the beam ratios is straight forward, which I'll be discussing shortly. If you're using the 50/50 fixed beam ratio beamsplitter, you may need to use the extra glass plate technique I discussed in the section Beamsplitter under Building a Holography System. Parabolic Mirror and Diverging Lens Relationship The parabolic mirror PM and diverging lens DL1 have a special relationship. It is necessary to create a collimated beam of light reflected from the parabolic mirror to mirror M3 and finally to the plate. This is very important to the size of the pseudoscopic real image which will be used as the object scene in the final multibeam white light reflection hologram. If the beam is collimated, the magnification of the real and virtual images will be 1x (the same size). If the beam is not collimated, the real image will be magnified, too large, and distorted. To achieve this collimated beam, the focal point of the diverging lens DL1 should be placed at the focal point of the parabolic mirror (the focal point of the parabolic mirror is written on the edge or back of the mirror). A 10x or 20x microscope objective or a plano- or double-concave lens with a -15 mm or -9 mm focal length, respectively, in combination with a 6 inch (15.24 cm) diameter telescope parabolic mirror with a 24 inch (60.96 cm) focal length, will provide an excellent collimated reflected beam from the parabolic mirror. The beam will have a diameter of 6 inches (15.24 cm) and will have uniform intensity across its diameter. The plate will have the same uniform intensity across its diagonal which is 6.4 inches (16.26 cm). The beam does not cover the corners of the plate completely--only 1/4 inch (0.64cm) at each corner is lost which is not significant. There is another method for producing a collimated beam without using a parabolic mirror. With this method, you want to place the diverging lens in the reference beam at a distance from the plate holder at least 10 times the diagonal size of the plate. If you're using a 4inch x 5 inch (10.16 cm x 12.7 cm) plate, its diagonal distance is 6.4 inches (16.26 cm). Therefore, the diverging lens should be placed at least 64 inches ( cm) from the plate. This means that you would need to replace mirror M2 and the parabolic mirror PM with 5 inch x 7 inch x 1/4 inch (12.7 cm x cm x 0.6cm) mirrors and probably place DL1 somewhere between the beamsplitter BS and mirror M2. Two 5 inch x 7 inch x 1/4 inch (12.7 cm x cm x 0.6cm) mirrors are a lot less expensive than a 6 inch (15.24 cm) diameter parabolic mirror. Note: If you're using a parabolic mirror, the angle between the incident and reflected beam at the parabolic mirror PM in Figure 20a should be as small as possible when reflecting the beam to mirror M3 (mirror M3 should not block the light from mirror M2). This will minimize aberrations in the reflected beam from the parabolic mirror, and thus minimize aberrations in the real image. This, in turn, will allow the holographic image in the final white light reflection display hologram to be projected further out in front of the plate without causing distortions in the image. Object Beam Components Now we will look at the optics in the object beam. Notice in Figure 20a that the object beam has a total of 4 directional mirrors M4, M5, M6, M7 in its path between the beamsplitter and the object scene. This configuration is set up so that the object's beam path length can be changed conveniently, using mirror M5, to match the reference beam's path length. In a multi-beam transmission setup, always try to adjust the object beam path length to match the reference beam path length, not the other way around. The bottom line is that the distance of the reference beam between the beamsplitter and the plate holder's central point, and the distance of the object beam between the beamsplitter and the plate holder's central point should always be equal, to within 1/2 inch (1.27 cm). The closer their path lengths are equal, the brighter the image. Measuring beam path lengths will be discussed a little further on. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

46 The diverging lens in the object beam should have a focal length that will create a diverging beam that uniformly illuminates the object scene. You can try a 10x or 20x microscope objective or a plano- or doubleconcave lens with a -15 mm or -9 mm focal length. Mirror M7 should be a 5 inch x 7 inch x 1/4 inch (12.7 x cm x 0.6cm) mirror so that the diverging lens can be placed between mirrors M6 and M7 to achieve a larger diameter beam at the object scene. Again, you can use the divergence equation in Figure 16f to calculate the lens distance from the object scene. You can experiment with illuminating the object scene now, but when you measure path lengths, leave DL2 out of the setup. The object scene should be centered when looking through the plate holder. It should face the plate holder squarely. The plate holder itself should be perpendicular to the table surface. The front of the object scene should be a predetermined minimum distance from the plate holder in order to allow the recording reference beam a clear path to the plate at a 56 degree angle without projecting any part of the object scene as a shadow on the recording plate. The graph in Figure 20b relates the reference beam angle to the minimum distance that the front of the object scene can be from the plate. Using this graph and an incident angle of 56 degrees, the closes you should have the object scene to the recording plate is 5 inches (12.7 cm). This will give the widest possible viewing angle when viewing the image in the white light reflection display hologram discussed later on. In addition, because you will be using the projected real image of your finished multi-beam transmission hologram as the object scene in your multi-beam reflection hologram setup, this minimum distance becomes important once again. With your recording reference beam being collimated, and since you will use this collimated reference beam to reconstruction and project the real image from your multi-beam transmission hologram, then the projected real image will be the same distance from the plate that the original object scene was from the plate when it was recorded. This will allow the reconstructing reference beam to pass through the transmission hologram and not pass through any part of the projected real image. This is important because you will be placing a second recording plate in the middle of the projected real image when recording your multibeam reflection hologram and you don't want the reconstructing beam, that passes through the transmission hologram, to illuminate the second plate in any way. I'll cover this again when you setup your multi-beam reflection hologram. Figure 20b: Relationship of reference beam angle to minimum object distance from plate. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

47 Measuring Path Lengths With all of your optical components, plate holder, and object scene in place (minus the diverging lenses) and the reference beam incident on the center of the white screen in the plate holder and the object beam is incident on the center of the object scene, you are ready to measure the path lengths of the reference and object beams. Using a tape measure, start by measuring the reference beam from the beamsplitter to mirror M2. Place one end of the tape measure above the beamsplitter where you see the laser beam leaving the beamsplitter and measure the distance to the laser beam dot on M2 from above the dot. Do not touch any of the optical surfaces with the tape measure. Try to be as accurate as possible, but don't lose sleep over it! If you're within 1/8 of an inch (0.32 cm), you're doing great! Note: Two people doing the measurements helps tremendously, especially for long distances. If you're by yourself, you can lay the approximate tape length out on the table surface, go to the beamsplitter, look down on the top of the beamsplitter and move the front end of the tape under the beam dot. Then go over to mirror M2 and again, looking down on the top of the mirror, move the tape under the dot and read the distance. Write all the distances in the reference beam on a piece of paper in the order you measured. Continue measuring the distance from M2 to PM, from PM to M3, and then from M3 to the center of PH. Add all these measurements together to arrive at the total path length of the reference beam. Do the same with the object beam starting at the beamsplitter. Be sure to measure to the center of the object scene and from the center of the object scene to the center of the plate holder. If the sums of both beams are not within 1/2 inch (1.27 cm) of each other, move mirror M5 to adjust the object beam until its total path length is equal to the reference beam total path length. Re-checking Polarization We will continue to assume that the laser's output polarization is horizontal with the laser being in an upright position as mentioned in Creating a Single-Beam Transmission Hologram section. If you have set up the optical arrangement properly by following Figure 20a and used the retro-reflection technique so that all the beams are at the same height above the table, you will find that the polarization orientation of the reference beam at the plate holder and the polarization orientation of the object beam at the object scene are both horizontal. Here is how you test this: Place your 4 inch x 5 inch (10.16 cm x 12.7 cm) white mounting board screen in the plate holder. Block the object beam using a piece of black mounting board so only the reference beam is throwing light on the white screen in the plate holder. You can lean the board against any of the mirrors in the object beam path to block the beam. Place one of the sunglass lenses in the reference beam between mirror M3 and the plate holder, perpendicular to the beam. Rotate the polarizer so that the reference beam light intensity is maximized on the screen. If the beam is horizontally polarized, then the polarizing lens should be oriented vertically to its normal wearing position on a person's face of horizontal and the intensity should be maximized. Remember that polarized sunglasses have their polarizing lines running vertically when worn on your face. Next, block the reference beam and keeping the polarizing lens oriented vertically, place the polarizer in the object beam between mirror M7 and the object scene. Rotate the polarizing lens back and forth around its vertical orientation and you should find that the object beam is horizontally polarized and the object scene is brightest when the lens is vertically orientated. If there is a difference of only 5 degrees between the two beams, you're in the ball park. I doubt there will be very little difference between the two beams. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

48 Inserting the Diverging Lenses Now that you have checked the path lengths of both beams and their polarization, place diverging lens DL1 in the reference beam at the focal length of the parabolic mirror and check for uniform lighting across the plate holder with the white screen in place. If the illumination of the screen is off center, you can adjust DL1 right or left, and/or up or down to center the illumination. Next, place the diverging lens DL2 in the object beam between mirrors M6 and M7 to uniformly illuminate the object scene. Again, by moving DL2 right or left, and/or up or down, you can adjust the centering of the light on the object scene. Re-check that the reference beam is still centered since you just added weight to the table top with DL2's table mount. Measuring and Adjusting the Beam Intensity Ratio You are just about ready to make your exposure. You need only take a beam intensity ratio reading with the VOM and solar cell, mask certain parts of the system to keep extraneous light from hitting the plate area, and take an exposure reading. Block the object beam at the beamsplitter and measure the reference beam intensity at the plate holder with the VOM and solar cell and write down the voltage reading. The white screen should be removed and the solar cell should be placed at the center of the plate holder, straddling it, and fully facing the incident, diverged reference beam. To get the true voltage reading, you need to take the cosine of 56 and multiple that number (0.5592) by the voltage reading. Next, unblock the object beam, block the reference beam, and measure the reflected light from the object scene at the plate holder. Write down the voltage reading. Again, the solar cell should be centered in the plate holder where the plate will sit and fully facing the object scene. No cosine function is used here since the object scene and the plate holder are parallel to each other. The reference beam true voltage reading should be 1.5 to 2 times greater than the object beam voltage reading. If this is not the case, adjust your circular beamsplitter or use the glass plate technique for your 50/50 fixed beam ratio beamsplitter, to get this ratio. The intensity of the reference beam must always be greater than the intensity of any reflected point from the object scene to the plate. Masking Extraneous Laser Light Once the beam ratios are set, you need to do some masking to keep extraneous laser light from hitting the recording plate shown in Figure 20a. Cut a piece of black mounting board 4 inches (10.16 cm) wide and 12 inches (30.48 cm) tall. Without touching DL1's mount, place a 4 inch side of the mask on the table surface close to the back side of DL1's mount and lean the mask towards DL1 with the 4 inch (10.16 cm) width of the mask centered on the lens. This should be done on M2's side of DL1. Circle the laser's beam dot with a pencil. Don't press too hard or you might touch the lens mount with the mask and move the mount. Remove the mask and with a single-edge razor blade, cut a 1/4 inch (6.35 mm) square hole in the mask around the pencil mark. This square should be large enough so that the laser beam does not touch the edges of the square as it passes through the hole. Now place the mask back on the table and gently lean the mask against the lens so the laser beam passes through the hole. This will prevent all of the extraneous noise from the laser cavity, that travels along with the beam, from passing through to the plate and will result in a cleaner, noise-free hologram. Do the exact same thing to diverging lens DL2, placing the mask on M6's side of DL2. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

49 Using five more masking pieces of the same size and without holes, place one mask between mirror M6 and the plate holder, close to M6, so the plate can't see the laser beam reflected off the surface of M6. Place a second mask between the beamsplitter and the plate holder, close to the beamsplitter, so the plate can't see the laser beam passing through, and reflected off, the surfaces of the beamsplitter. Place a third mask between DL2 and the plate holder, close to DL2, so the plate can't see the laser beam passing through DL2. Place a fourth mask between the parabolic mirror PM and the plate holder, close to PM, so the plate can't see the laser beam reflected off the surface of PM. Place the fifth mask between M4 and the plate holder, close to M4, so the plate can't see the laser beam reflected off the surface of M4. These masks can be held in place using large binder clips as shown in Figure 20c. I don't recommend using table mounts to lean the masks against. The weight of an additional mount on the table can through your beam alignments off which means more work realigning the beams. Figure 20c: Large binder clips holding masks on table surface Now look through the plate holder toward the object scene. You should be able to scan the area around the object scene and see no other light sources. Also look from the object scene side towards and through the plate holder from various angles to see if you can see any reflections off mirrors or other extraneous light reflections. Put the white screen back in the plate holder and check both sides of the card for extraneous light. If both sides of your white screen are not white, you can just flip the screen around depending on which side of the plate holder you're checking. By doing all this masking, only the image of the object scene will be visible in the hologram. When doing this final check for extraneous light, you can splay your fingers and move your hands in front of the screen on both sides and if there is some extraneous light, you can usually see it as the shadow of your fingers pass over the screen. You then get to play detective and figure out where it's coming from. Bottom line: you don't want any laser light hitting the plate except for the laser light being reflected from the object scene and the reference beam. Exposure, Processing, and Drying Re-check that the reference beam uniformly illuminates the white screen in the plate holder and that the object scene is uniformly illuminated. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

50 Remove the white screen from the plate holder. Block the object beam temporarily and take an exposure reading with the VOM and solar cell of the reference beam only. The solar cell should squarely face the incident reference beam and should be at the center of the plate holder. Do not multiply your voltage reading by the cosine of 56 (see Exposure Calculation Technique under main Exposure link). Use your previous meter readings during other setups to estimate your exposure time. Unblock the object beam. You can now place the shutter in position and make an exposure. Remember to use film first and once you have the correct exposure density, make the final hologram with a plate. You need a plate for the multi-beam white light reflection display hologram setup. Make sure the plate's emulsion faces the object scene. Note: Once you get the correct exposure density using film, finish processing the film and, once it's dry, place it back in the plate holder, block the object beam and object scene, and use the original reference beam to reconstruct the image and view it. Look for any extraneous light showing up around the image area, or on the hologram itself, that was not masked out. Do this before making the final plate hologram. Once the plate is made and you have an image, rotate the plate 180 degrees horizontally to project a pseudoscopic real image out into space on the opposite side of the plate from the original object scene. If the reference beam was collimated correctly, the real image should be the same size as the original object scene. Using Two Object Beams If you want to use two object beams to illuminate more areas of the object scene, you can setup the arrangement shown in Figure 20d. For this arrangement, you will need one additional beamsplitter, one additional diverging lens, and one additional 5 inch x 7 inch x 1/4 inch (12.7 x 17.8 cm x 0.6 cm) mirror. These additional optics are shown in Figure 20d as BS2, DL3, and M8. Beamsplitter BS2 is aligned with the center of the plate holder and object scene as shown in Figure 20d. This makes it easier to have the path lengths of the two object beams equal. The path lengths of both object beams should be the same and the beamsplitter should be another 50/50 fixed beam ratio beamsplitter. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

51 Figure 20d: Multi-beam transmission hologram setup with two object beams. When measuring the total path length of the object beam to equal the reference beam's total path length, just use one of the paths after beamsplitter BS2 in your calculation. You will also need two additional masks. One between DL3 and the plate holder positioned close to DL3 and one between BS2 and the plate holder positioned close to BS2. Creating A Multi-Beam White Light Reflection Display Hologram You are now going to use the real image of your final transmission hologram plate from your multi-beam transmission setup as the object scene in this multi-beam white light reflection display hologram setup. You're also going to use your multi-beam transmission setup for this reflection hologram setup, with a few changes. Figure 21a illustrates the setup if you want to hang your hologram on a wall and use overhead ceiling lighting. Later, in Figure 21d, I'll show you the setup if you want to hang your hologram on a wall and use underneath table based lighting. Figure 21b shows a close-up of H1, H2, and the real image positions. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

52 Figure 21a: Multi-beam white light reflection display hologram setup for overhead reconstruction. Figure 21b: Close-up of H1, H2, and real image. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

53 There are a number of things that have changed in this setup from the multi-beam transmission setup, which we'll do in a moment: The original object scene has been completely removed. The labeling of the plate holder has been changed to H1 instead of PH. In holography lingo, the transmission hologram projecting the real image is called H1 and a second plate holder has been inserted to hold the reflection hologram recording plate, which is called H2. The object scene OS is now the projected real image from H1 and the H2 plate holder is shown straddling the real image. I've switched the reference beam and object beam labeling. The transmission hologram's setup reference beam is now called the object beam O because it is now reconstructing the real image as the object scene for H2. The transmission hologram setup object beam is now called the reference beam R. Three additional mirrors (M8-M10) have been added to the new reference beam. A second plate holder has been added to hold the recording plate for H2, the reflection hologram you're now going to make. Note: It is important in this reflection hologram setup that you do not move any of the components, at this time, in the new object beam path including the laser, mirror M1, and beamsplitter BS which are outside the object beam path. This will insure that the incident beam on transmission hologram H1 is exactly the same as it was when you made it, and this in turn, will guarantee that the projected real image is distortion free. New Object Beam Changes The first change is to remove the object scene OS from the optical table. The loss of weight on the optical table by removing the object scene OS and its mounts may cause the reconstructing object beam O to slightly shift its position at H1. We'll tweak this later. The second change is to insert H1 in its plate holder and orientate the transmission hologram H1 so its projecting its real image. You're now ready to make changes to the new reference beam. Setting Up the Components in the New Reference Beam This section should be read through completely before implementing because you'll probably have to build and paint some more components. Mirrors M4 and M5 can stay where they were originally placed. Mirror M6 has been moved forward so it's reflected beam does not strike diverging lens DL2. You can now insert plate holder H2 into the setup in the position shown in Figure 21a. You should place another white screen in plate holder H2 so you can see the real image projected on the screen. Move the plate holder forward and backward relative to H1 to place the center of the real image straddling the screen. Make sure H2 seats squarely facing H1. Now, looking at Figure 21a, imagine a straight line running through mirror M8, mirror M7, plate holder H2, and plate holder H1. The center of all of these components should be aligned along this line and, of course, at a height of 9 inches (22.86 cm). Mirrors M8, M9, and M10 should all be 5 inch x 7 inch x 1/4 inch (12.7 cm x cm x 0.6 cm) in size. Mirror M7 can be changed to a smaller 1 inch x 1 inch x 1/8 inch (2.54 cm x 2.54 cm x 0.3 cm) mirror size. Place mirror M10 low on the table so it can reflect the beam from mirror M9 upward to the center of plate holder H2 at an angle of 56 degrees as shown in Figure 21b. Make sure it is centered along the imaginary line. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

54 You're next going to insert mirror M9 about 20 inches (50.8 cm) above the table surface, as shown in Figure 21a, and centered along the imaginary line. Because mirror M9 is so high above the table, you need to put together a very sturdy mounting structure. Use Figure 21c below to follow along. Take two table mounts and drop a connector on each pole. Now take an 11 inch (27.94 cm) long aluminum rod, 0.5 inch (1.27 cm) diameter, and run it through the connectors. This is a stability bar to help keep the whole structure rigid. Tighten each connector to its pole about 2.5 inches (6.35 cm) below the top of the table mount's pole. Keep the stability bar loose at this time in its connectors for later adjustments you'll make to the distance between the two table mounts. Following Figure 21c again, add two more connectors to the top of each table mount's pole and tighten them in place. Take notice of the orientation of all the connectors in relation to the poles and rods (poles and rods are the same thing). Now take two 12 inch (30.48 cm) long, 0.5 inch (1.27 cm) diameter aluminum extension rods and insert one vertically in the two open connector's holes on one table mount and do the same with the other extension rod on the other table mount. Tighten the thumb screws on the connectors for each extension rod. Now put another connector on each extension rod for mounting the 5 inch x 7 inch (12.7 x cm) mirror M9 to the two extension rods. The 7 inch (17.78 cm) width of the mirror should span the distance between the two extension rods. Two 1/4-20 short rods are attached to each 5 inch (12.7 cm) side of the acrylic plastic mirror mount. The plastic mount is slightly larger, 5.25 inches x 7.25 inches x 0.5 inches thick (13.34 x x 1.27 cm), than the mirror and the mirror is attached to the plastic with epoxy. In Figure 21c, the distance between the extension rods is inches (21.9 cm). Once you have the mirror locked down, you can lock down the stability bar. Finally, make sure you have painted any new mounting structures flat black. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

55 Figure 21c: Table mounting of mirror M9. You will need to tilt mirror M8 upward to direct the beam to M9. You will now probably need to move mirror M9 along the imaginary line while also adjusting the tilt angle of mirror M8 and mirror M10 to get the desired 56 degree angle. The closer M9 is to M10, the steeper the angle will be reflected from M10 to the plate holder. Keep the white screen in plate holder H2 while you make these adjustments so you can see the position of the laser beam dot which should be centered on the screen. Next, check the polarization between mirror M3 and H1, and also check it between mirror M10 and the screen. They should both be horizontal. Although you would think that the new reference beam should be vertical because of its incidence on the H2 plate is from underneath instead of from the side, its horizontal polarization is needed to match the real image's horizontal polarization to achieve maximum brightness. The 56 degree incident angle will eliminate any internal reflections in the plate or sandwiched film. Now measure the path lengths. The new object beam distance should include the distance from H1 to H2. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

56 The path length of the new reference beam path should be the only path adjusted to match the distance of the new object beam path. With the insertion of mirrors M8-M10, the path length of the new reference beam has substantially increased. Try to use mirror M5 to make this adjustment. You may have to move M4 closer toward M5. Keep M7 where it is. With all these mirrors being adjusted, you may have to realign the beam from M7 through M10. Once the path length is established for the new reference beam, insert diverging lens DL2 into the setup between mirror M7 and mirror M8. Move the lens between these two mirrors until you achieve uniform illumination on the screen in H2. You can use a 10x or 20x microscope objective or a plano- or doubleconcave lens with a -15 mm or -9 mm focal length. Now check the beam ratios. They should be in the range of 2 to 1 (2/1) and 1.5 to 1 (1.5/1). Use the cosine function here on the new reference beam. Take an exposure reading measuring only the reference beam. The solar cell should face the incident reference beam square on (perpendicular to the beam). Determine your exposure time and double it. You want to achieve an exposure density of 2 for a reflection hologram. Do not use the cosine function. Finally, you need to mask certain parts of the table to eliminate extraneous light at H2. o Place a mask between the parabolic mirror PM and H2, close to PM, so H2 can't see the laser beam reflected off the surface of PM. o Place masks upstream of both diverging lenses DL1 and DL2, close to each lens as you did for the multi-beam transmission hologram, to eliminate extraneous laser cavity noise. o Place a mask between mirror M7 and H2, close to M7, so H2 can't see any reflections off the back side of M7 or beam dots on DL2 or mirror M8. o You may also need additional masks to prevent H2 from seeing M1, BS, M3, M4, and M6. o Finally, look through the plate holder H2 toward H1. You should be able to scan the area around H1 and see no other light sources. Also look from the H1 side of H2 towards and through H2 from various angles to see if you can see any reflections off mirrors or other extraneous light reflections. o Put the white screen back in the plate holder and check both sides of the card for extraneous light. Check the reference beam illumination again at H2 for uniform illumination and check that H1 is uniformly illuminated. You may have to move DL1 left or right and/or up or down to uniformly cover H1. Put your shutter in place. Go ahead with the recording procedure and processing. Check the density of your first recording and make exposure time corrections if needed. Make sure that when you put the plate in the H2 holder, the emulsion is facing H1. This means that when the hologram is illuminated as it hangs on a wall from overhead, the emulsion will face the viewer. This also allows the glass side of the bleached plate to be painted flat black for a much brighter image. Use the painting procedure described previously. If you decide to place the image in H2 to be projected in front of the plate instead of straddling the plate, move H2 away from H1. If you want the image to be further behind the plate, move H2 closer to H1. Remember, though, that you can only move H2 so close to H1 without the reconstructing light for H1 hitting H2, which you don't want. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

57 If you want your final white light reflection display hologram to be illuminated from underneath the plate with the illuminating light source on a table top, use the setup shown in Figure 21d. Figure 21d: Multi-beam white light reflection display hologram setup for underneath reconstruction. A reflection hologram produces a monochromatic colored image. Since this hologram will be viewed with a white light source described in the next section, the color reconstructed in the image will depend on two factors: Whether the hologram is air dried or dried with heat after processing and after painting. If the hologram is air dried, the fringe shrinkage will be minimal and your image should be gold in color. Drying the hologram with heat shrinks the emulsion, which in turn, shrinks the interference fringes in the emulsion and causes the color of the image to shift toward green and blue. The angle of the reconstruction illumination. The greater the reconstruction angle is away from the plate's normal causes the image color to shift towards the blue end of the spectrum. You should be reconstructing the image at a 56 degree angle from normal to get a gold colored image. Lighting Techniques I produce white light reflection display holograms because the images are monochromatic (gold in color) and lend themselves to a more realistic representation of the original object or scene. Because of the diffractive nature of holograms (diffraction lenses), keeping the image close to the plate plane enhances the image's monochromatic effect (single color reconstruction) and resolution (sharpness). As the image is moved away from the plane of H2, factors come into play that decrease monochromaticity and resolution. This can cause the image to no longer be gold in color and start showing multiple rainbow colors. These multiple colors are actually multiple colored images of the object scene being created as the white light starts to spread into a rainbow the further the reconstructing light moves past the plate. The actual term for this phenomenon is called white light dispersion. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

58 My normal method of illuminating display reflection holograms is by hanging the hologram on a wall and using track lighting on the ceiling to illuminate the hologram (overhead lighting or overhead reconstruction) as shown in Figure 22a. Figure 21a would be the appropriate optical setup for this. Figure 21c would be the appropriate optical setup to illuminate the hologram with the light source on a desktop (underneath lighting or underneath reconstruction) as shown in Figure 22b. The reconstructing angle for both types will be the recording angle which is 56 degrees. Figure 22a: Lighting arrangement showing ceiling illumination (Figure 21a optical setup). Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

59 Figure 22b: Lighting arrangement showing desktop illumination (Figure 21d optical setup). Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

60 In the following two photographs, both the skull and the human brain are projected 3 inches (7.62 cm) out in front of the plate. You'll notice that you can project fairly far out in front of the plate without getting any dispersion. Figure 22c: Close-up of an 8" x 10" hologram of a real human skull. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/

61 Figure 22d: Close-up of an 8" x 10" hologram of a sagittal section of a real human brain. I use Halo or Hampton Bay brand halogen lamps. Specifically, I use the L2710 MBX lamp holder or equivalent. The lamps that plug into the holder should be at least 65 watts and have a narrow beam designation [ (Sylvania brand 65MR16Q/NSP(FPA) ]. All lamps should have a clear, glass envelope, not frosted. You can see these lamps and their holders in Figures 22a and 22b. As a final note, don't hesitate to contact me via if you have any questions, comments, or suggestions regarding anything on this web site. I built it as a teaching tool and your feedback is welcome. Copyrighted 1996 Author: Stephen William Michael Revised 5/24/