Jeff Blyth
jeff@biotech.cam.ac.uk

About the author
After graduating in 1973 in Applied Chemistry he worked in a company with dichromated gelatin, unrelated to holography. In ’77, he was amazed to see a holographic pendant made using the very material he was researching. His life ‘changed for ever’. He subsequently worked on photopolymer materials for Ilford, which became the subject for an MPhil at Wolverhampton Polytechnic. Since ’91 he has been involved in ‘blue sky’ research at the Institute of Biotechnology in Cambridge, UK. Jeff is the recipient of the Royal Photographic Society’s Saxby award for 2003
(http://www.holography.co.uk/events/saxbyaward/jeffblyth/jeff.htm).

Until recently, I always thought that if a small object reflected laser light well enough, and was kept completely still during any holographic exposure, then you could always make a reflection hologram of it which would reproduce monochromatically the features of that object. Well it turns out I have been deluded for a long time…

My object in question is in the centre of the photo here. It is a well made glass “corner-cube”¹

Anyone who looks at this actual object will see a fascinating sight of a single eye looking back at them from the central section of the cube corner (CC). I also find it interesting to ask people which eye they think it is. Some decide it is their right, others their left, and only a few say it is both eyes equally. It does nicely show that most of us seem to have a dominant eye independently of whether we are right- or left-handed. The fact is that the eye you see is the eye that is looking at it so it can be both eyes simultaneously.

For an experiment I positioned a piece of pre-swollen BB640 holographic plate on the beaker and shot a single beam “in-line” hologram with a 633nm HeNe laser. Now you can see my camera lens in the photo of the CC but not in the hologram of the CC. Therein lies the heart of the problem – you just cannot reproduce in the hologram that same “eye effect” that follows you around the room from a true retro-reflector.

I had a serious use in mind for getting a holographic corner cube to act as a direction tolerant sensor but it was not to be or at least not entirely so. However, it strangely does do a quasi retro-reflection of a torch beam in 3 positions, 120^∘ apart, and each reflection is triangular and is uniformly orange whereas when the light source is almost directly overhead, the result is a green replay, as you can see in the photo. Mini corner cube arrays are familiar to all of us as bicycle reflectors on mud guards and pedals. Even those bright fluorescent sash bands that cyclists wear are seen under a microscope to be made of micro corner cube arrays. If you make a hologram of these arrays, just as with the single CC, this hologram can also replay at specific wide angles but again at a longer wavelength than that seen along the normal. I do not have an explanation for this effect – would anyone like to volunteer one please?

The retro-reflectivity of CCs is caused by a reflection from each wall of the CC in turn. But a flat hologram inspite of all its wonderful properties just cannot cause light to carry out three internal reflections. So that was holographic problem number 1. Now, what happens if you actually make a real holographic CC out of three triangular pieces of flat in-line reflection hologram? Well, I tried doing this using the corner of a common plastic (slide holder) box as a template. It was sprayed black and a side of the box was partially removed as you see in photo. It is then that the inherent holographic problem number 2 crops up…

As you move the light source away from the optimum position lighting up any bright reflection hologram, you get some wavelength shift just before it fails to replay at all. Saxby refers to this as the “Venetian blind effect”. (As you view through a venetian blind, the distance between the slats seem to shorten as you view them more obliquely. This is analogous to what is happening to the holographic fringes). So in my three-dimensional holographic CC you can hardly have three consecutive light bounces because the angle change will cause a wavelength shift each time which will simply not match the fringe spacing on each consecutive face.

I then did the simplest experiment in a dark room with a torch beam shining in my eye. I could just make out the light from my pupil only near the apex of the CC in the box. (The holographic grating material had been particularly efficient at acting as a normal diffracting mirror before it was cut into triangles). I could prove it was a holographic reflection, rather than just a three-fold specular reflection off the smooth surfaces, by allowing breath to condense on the system which was glued down with the emulsion outwards. The feeble spot momentarily changed from green to yellow and slightly brightened up, perhaps because it became more broadband, just enough to increase the tolerance angle for the light between each reflection to reconstruct, but it was still very feeble and restricted to about the first couple of millimetres from the
apex.

So I think I can now put the “narrow-band holographic retro-reflector” in that special box marked “holographic howlers”, wherein lie those wretched Hollywood perennials of giant three-dimensional real images of people reconstructed in mid-air without any holographic plate producing it in sight and also, dare I say it, full 3D colour holograms looming out of a TV, etc, etc.

Footnote

¹The £180 corner cube was purchased from Mr. Eric Frisk (opticalworks@btconnect.com) or cube corner, and it is sitting in a plastic beaker for support.

John Pecora
holograms3d@yahoo.com

About the author
John Pecora is a computer specialist and a certified Microsoft Systems Engineer. He made his first holograms using a sandbox holography kit in 1980. He has worked for holographic companies making photoresist holograms for embossing. He is now an amateur holographer fabricating his own DCG emulsions. He has always liked reverse engineering, and enjoys taking an idea from concept to final product.

Here are some tips for saving money on ‘lab’ equipment. It is surprising how many everyday objects can be used effectively in holography. These are just suggestions. Please remember that it is your responsibility to pay attention to safety, and use common sense.

• Heating pads used with three or more settings can be used as adjustable heaters for processing trays. Simply put the heating pad under the tray and turn the pad on to the desired setting.
• Black foam board can be used for blocking stray light. The type that is black throughout is best as the edges stay black even when they are cut. This material can also be used for making an iris.
• A shutter can be made from most old 8 mm movie cameras. They have a low voltage electric shutter. Remove this unit and set up a circuit with the original voltage of the camera, and a switch.
• A thick piece of glass, 1/4′′ or thicker, can be used as a beam splitter. Using the thick piece of glass allows a small piece of electric tape to be placed over the glass to block the secondary reflection off the back.
• Sandwich boxes can be use as processing trays and also as storage for the chemistry without having to pour the liquids back into bottles after each session. They come in many sizes and shapes with airtight lids. Store sealed containers with chemicals in a dark, dry, cool place when not being used.
• Rubber inner tubes can be used as the dampening mechanism between a holographic table and the support legs.
• A slab of granite can be used as a holographic table.
• Most old overhead projectors contain large front surface mirrors and large Fresnel lenses. They can be purchased at yard sales and flee markets for just a few dollars.
• Most photocopiers and fax machines contain front surface mirrors.
• New Jefferson Nickels have a weight of 5 grams and new Lincoln Pennies have a weight of 2.5 grams. Standard paper clips have a weight of 1 gram. To verify the weight of the paper clips put a nickel on one side of the balance and find 5 paper clips of the same size that equals the nickel. These can be used on a balance for measuring out chemicals.
• A hair dryer can be used to dry a piece of holographic film or plate after processing. Drying intensity and heat is variable with very inexpensive dryers.
• Polarizers can be found in polarizing sun glasses. These can be used to slightly modify the intensity of throughput laser light by inserting it into the beam path and rotating. They can also be used to compare the polarization of light at different locations in an optical set-up.
• Two pieces of window pane glass and binder clips can be used to sandwich a piece of holographic film. This will hold the film rigid and flat.
• A microwave can be used to heat the deionized or distilled water needed for mixing up processing chemistry. But please be careful to keep chemical-contaminated containers separate and secure. One method is to heat the water in a clean container in the microwave and then pour it into the chemical container for mixing, always keeping the clean container free of any chemicals.
• Two-part, fast-hardening epoxy is great for securing two pieces of metal without the need for drilling and tapping. This also allows the disassembly with just a small sharp blow to one of the pieces.
• A pinhole can be made by sandwiching 5 or 10 pieces of aluminum foil together and poking with a pin while the pile is on a hard piece of rubber. Each piece of foil will have a slightly different size of pinhole.
• Automobile windshield wiper blades can be used as a squeegee. If you epoxy two blades to a pair of scissors then, when the scissors are closed 3/4 of the way, you can squeegee both sides of the film at the same time. For plates this is not necessary as you can do one side at a time with a single blade.
• Clothes pegs on a line can be used to hang up films to dry. After clamping the film at two corners with the pegs, clamp two more at the bottom corners to keep the film straight while drying.
• Dishwasher drying agent can be in place of Photoflo^TM in the final rinsing bath. Use an agent that does not have fragrance and, preferably, that is clear.
• Sodium carbonate can be purchased cheaply as a chemical for increasing the pH of swimming pools and spas.
• Sodium bisulfate can be purchased cheaply as a chemical for decreasing the pH of swimming pools and spas.
• Sulfuric acid can be purchased as car battery acid. Most formulas call for concentrations that are lower than that sold as auto battery acid.
• Black Sanford Sharpie markers, which come in different sizes, are ideal for blackening optics, mounts and anything small you want to reduce reflections on. They are permanent markers that write on almost anything.
• Paper Mate^TM liquid paper correction is great for painting objects for holography. It dries to a flat white and diffuses the light very well.

Oliver Bimber
bimber@ieee.org

About the author
Oliver Bimber is a Junior Professor for Augmented Reality at the Bauhaus University Weimar in Germany. He holds a PhD in Computer Science from Technical University of Darmstadt. Contact him at bimber@ieee.org. More information is available at http://www.HoloGraphics.de

Just like computer graphics, holograms are being applied as tools to solve individual research, engineering, and presentation problems within several domains. Up until today, however, these tools have been applied separately. The overall goal of our project is to combine both technologies to create a powerful tool for science, industry and education. We are currently investigating the possibility of integrating computer generated graphics and holograms.

Our goal is to combine the advantages of conventional holograms (i.e. extremely high visual quality and realism, support for all depth queues and for multiple observers at no computational cost, space efficiency, etc.) with the advantages of today’s computer graphics capabilities (i.e. interactivity, real-time rendering, simulation and animation, stereoscopic and autostereoscopic presentation, etc.).

Several engineering and computer science topics will be addressed throughout the project: The potentials of different hologram types with respect to the project’s goal have to be investigated. New three-dimensional displays that combine computer graphics and holography will be engineered. New real-time rendering algorithms, registration methods, and human–computer interaction techniques that are adequate for the proposed metaphor will be developed.

The outcome will be a three-dimensional display concept whose application is envisioned in areas such as scientific visualization (e.g., paleontology, pathology, density, medicine, biomedicine, orthopedics or archeology), industrial simulation (e.g., design, manufacturing and quality assurance), and education (e.g., medical training or public museums).

Here are some of our initial results.

Using digital light to reconstruct the holographic image

The two basic hologram types—transmission and reflection—are both reconstructed by illuminating them with spatially coherent light (i.e. using a point-source of light). These two types have generated a number of variations. Although some holograms can be reconstructed only with laser light, others can be viewed under white light.

Rainbow holograms, one of the most popular types of white-light transmission hologram, diffract each wavelength of the light through a different angle. This lets viewers observe the recorded scene from different horizontal viewing positions but also makes the scene appear in different colors when observed from different vertical points of view. In contrast to rainbow holograms, white-light reflection holograms can provide full parallax and display the recorded scene in a consistent color (monochrome or multi-color) for different viewing positions.

Conventional video projectors represent point sources that are well suited for viewing white-light reflection or transmission holograms. Today’s high-intensity discharge lamps of projectors can produce a very bright light. The main advantage for using video projectors is that the reference wave used to reconstruct the hologram can be digitized. Thus it is possible to control the amplitude and wavelength of each discrete portion of the wavefront over time.

Figure 1: The projected reference waves and the resulting holographic images.

Figure 1 shows the projected reference wave in different states, and the resulting holographic image of a monochrome white-light reflection hologram. A uniform reference wave reconstructs the entire hologram uniformly. Selectively emitting light in different directions allows us to create an incomplete reference wave that reconstructs the hologram only partially. Local amplitude variations in the reference wave result in proportional amplitude variations in the reconstructed object wave. Variations in wavelength do not lead to useful effects in most cases due to the wavelength dependency of holograms. But this is still a matter for further investigations.

Partially reconstructing object waves

It is possible to reconstruct the object wave of a hologram only partially, leaving gaps where graphical elements can be inserted.

Both reflection holograms (without an opaque backing layer) and transmission holograms remain transparent if not illuminated. Thus, they can serve as optical combiners—leading to very compact displays.

Real-time computer graphics can be integrated into the hologram from one side, while illuminating it partially from the other side [1]. Thereby, rendering and illumination are view-dependent and have to be synchronized.

If autostereoscopic displays are used to render 3D graphics registered to the hologram, both holographic and graphical content appear three-dimensional within the same space. If depth information of both is known, correct occlusion effects between hologram and graphics can be generated.

Figure 2: Rainbow hologram of a dinosaur skull combined with graphical representations of soft tissue and bones.

Figure 2 shows a rainbow hologram of a dinosaur skull combined with graphical representations of soft tissue and bones. If the holographic plate is illuminated with a uniform light, the entire hologram is reconstructed. If the plate is illuminated only at the portions not occluded by graphical elements, the synthetic objects can be integrated by displaying them on the screen behind the plate.

Light interaction

The reconstructed object wave’s amplitude is proportional to the reference wave’s intensity. In addition to using an incomplete reference wave for reconstructing a fraction of the hologram, intensity variations of the projected light permit local modification of the recorded object wave’s amplitude.

Practically, this means that to create the illumination image which is sent out by the projector, graphical shading and shadowing techniques are used to reconstruct the hologram instead of illuminating it with a uniform intensity. To do this, the real shading effects on the captured scenery caused by the real light sources used for illumination during hologram recording, as well as the physical lighting effects caused by the video projector on the holographic plate, must both be neutralized. Next, the influence of a synthetic illumination must be simulated [1].

Using conventional graphics hardware, it becomes possible not only to create consistent shading effects, but also to cast synthetic shadows correctly from all holographic and graphical elements onto all other elements.

Figure 3: A rainbow hologram with 3D graphical elements and synthetic shading and shadow effects.

The figures show the same rainbow hologram as above with 3D graphical elements and synthetic shading effects. Shadows are cast correctly from the hologram onto the graphics and vice versa. A virtual point-source of light was first located at the top-left corner, and then moved to the top-right corner, in front of the display.

Proof-of-concept prototypes

The desktop prototypes shown in figure 4 consist entirely of off-the-shelf components, including either an autostereoscopic lenticular-lens sheet display with an integrated head-finder for wireless user tracking, or a conventional CRT screen with active stereo glasses, wireless infrared tracking, and a touch screen for interaction.

Both prototypes use digital light projectors. A single PC with a dual-output graphics card renders the graphical content on the screen and illuminates the holographic plate on the video projector.

Figure 4: An autostereoscopic (left) and a stereoscopic (right) proof-of-concept prototype.

In both cases, the screen additionally holds further front layers—glass protection, holographic emulsion, and optional mirror beam splitter (used for transmission holograms only).

Interaction with the graphical content is supported with a mouse or a transparent touch-screen mounted in front of the holographic plate.

Experiments with a digital multiplex hologram

Digital holography uses holographic printers to expose the photosensitive emulsion with computer generated or captured images.

This results in conventional holograms with digital content rather than real scenery. Pre-processed 2D and 3D graphics or digital photographs and movies can be printed. This allows, for instance, the holographic recording of completely synthetic objects, real outdoor scenes, and objects in motion—which is difficult and sometimes impossible to achieve with optical holography.

Like optical holograms, digital holograms can be multiplexed. This allows us to divide the viewing space and to assign individual portions to different contents.

The content for digital holograms can easily be created by non-experts, and the printing process is inexpensive. Usually a 3D graphical scene, a series of digital photographs or a short movie of a real object is sufficient for producing digital holograms. However, these digital holograms lack in the quality (resolution, color appearance, sharpness, etc.) of conventional optical holograms.

Figure 5: A multiplexed digital reflection hologram of a car headlight with integrated CAD data.

Figure 5 shows a digital color white-light reflection hologram of a car headlight. It was generated by taking 360 perspective photographs from different angles (in 0.5^∘ steps to cover a 110^∘ total viewing zone plus two 35^∘ clipping areas). The perspective photographs were multiplexed into different sub-zones (40^∘ = 80 images for the front view + 2 × 35^∘ = 140 images for the side and rear views +  2 × 12.5^∘ = 50 images to fill the partially visible clipping area outside the 110^∘ total viewing zone + 2 × 22.5^∘ = 90 images to fill the invisible clipping area outside the 110^∘ total viewing zone).

Consequently, three different partial views (front, rear, and side) can be observed by moving within the total viewing zone of 110^∘. After registering the holographic plane and calibrating the projector, interactive graphical elements, such as wire-frame or shaded CAD data can be integrated into the hologram.

Holographic windows

The ability to control the reconstruction of a hologram’s object wave allows integrating them seamlessly into common desktop-window environments.

If the holographic emulsion that is mounted in front of a screen is not illuminated, it remains transparent. In this case the entire screen content is visible and an interaction with software applications on the desktop is possible in a familiar way.

The holographic content (visible or not) is always located at a fixed spatial position within the screen/desktop reference frame. An application that renders the graphical content does not necessarily need to be displayed in full-screen mode (as in the examples above), but can run in a ‘windows’ mode—covering an arbitrary area on the desktop behind the emulsion.

If the position and the dimensions of the graphics window are known, the projector-based illumination can be synchronized to bind the reference wave to the portion of the emulsion that is located directly on top of the underlying window. Thereby, all the techniques that are described above (partial reconstruction and intensity variations) are constrained to the window’s boundaries. The remaining portion of the desktop is not influenced by the illumination, the graphical or the holographic content.

In addition, the graphical content can be rendered in such a way that it remains registered with the holographic content—even if the graphical window is moved or resized.

This simple, but effective technique allows a seamless integration of holograms into common desktop environments. It allows us to temporarily minimize the “holographic window” or to align it over the main focus while working other applications.

Figure 6: A holographic window in different states on a desktop together with other applications.

Figure 6 shows a holographic window in different states on a desktop together with other applications. It displays an optical (monochrome) white-light reflection hologram of a dinosaur skull with integrated graphical 3D soft tissues. A stereoscopic screen was used in this case, because autostereoscopic displays (such as lenticular screens or barrier displays) do not yet allow an undisturbed view on a non-interlaced 2D content.

Outlook

Holograms can store a massive amount of information on a thin holographic emulsion. This technology can record and reconstruct a 3D scene with almost no loss in quality. Moore’s law—which asserts that computing power doubles every 18 months—must be applied many times for graphical or electro-holographic rendering techniques and displays in order to reach this quality at interactive frame rates. A combination of interactive computer graphics and high-quality holograms represents an alternative that can be realized today with off-the-shelf consumer hardware. We believe that this concept can be beneficial for many applications.

Archaeologists, for example, already use holograms to archive and investigate ancient artifacts. Scientists can use hologram copies to perform their research without having access to the original artifacts or settling for inaccurate replicas. They can combine these holograms with interactive computer graphics to integrate real-time simulation data or perform experiments that require direct user interaction, such as packing reconstructed soft tissue into a fossilized dinosaur skull hologram. In addition, specialized interaction devices can simulate haptic feedback of holographic and graphical content while scientists are performing these interactive tasks. An entire collection of artifacts will fit into a single album of holographic recordings, while a light-box-like display such as that used for viewing x-rays can be used for visualization and interaction.

The same applies to the biomedical domain that already uses digital volumetric holograms produced from CT or MRI data of inner organs.

In the automotive industry, for instance, complex computer models of cars and components often lack realism or interactivity. Instead of attempting to achieve high visual quality and interactive frame rates for the entire model, designers could decompose the model into sets of interactive and static elements. The system could record physical counterparts of static elements in a hologram with maximum realism, and release computational resources to render the interactive elements with a higher quality and increased frame rate. Multiplexing the holographic content also lets users observe and interact with the entire model from multiple perspectives. Beside display holograms, holographic interferograms used for non-destructive measurement and testing are yet another example of industrial applications. Analogue interferograms that indicate motion, vibration, or deformations of objects can be combined with digital simulation data.

Augmenting holograms in museums with animated multimedia content lets exhibitors communicate information about the artifact with more excitement and effectiveness than text labels offer. Such displays can also respond to user interaction. Because wall-mounted variations require little space, museums can display a larger number of artifacts.

Figure 7: Illustrations of envisioned future applications: Museum displays, and scientific visualization and simulation.

Figure 7 shows two illustrations of envisioned future applications: A wall-mounted display in a museum environment, with a ceiling-mounted video projector replacing conventional spotlights, and a desktop display that can be used in a light-box fashion. A special input device allowing interaction, including haptic feedback of holographic and graphical content.

The technical and scientific progress that is planned to be made during this project can be organized into six linked goals:

Holograms

Investigation of different hologram types and individual solutions that combine them with computer graphics. Optimization of optical properties during the recording process to achieve the best possible effects.

Displays

Experiments with different optical setups that support the integration of computer graphics into the different hologram types. Experiments with different stereoscopic and autostereoscopic techniques. Investigation of different display form factors that serve a variety of applications.

Calibration and registration

Development of fully- or semi-automated methods that calibrate the display optics, extract auxiliary information (such as depth) recorded in the hologram and register holographic and graphical content.

Rendering

Development of effective rendering and illumination algorithms that support different hologram types, special effects, and a realistic and consistent presentation of holographic and graphical content at interactive frame rates.

Interaction

Investigation of the potentials and limitations of existing interaction techniques and devices in combination with interactive holograms. Development of new interaction forms that are suited for the different display approaches and potential application areas.

Demonstration and evaluation

Implementation of demonstrators that address different application areas. The effectiveness of the proposed concept is evaluated by presenting the demonstrators to domain experts.

Acknowledgements

This project is supported by the Deutsche Forschungsgemeinschaft (DFG). The experiments shown in figure 5 were supported by DaimlerChrysler AG Research Technology. The content shown in figures 2, 3 and 6 was provided by Ohio University’s department of Biomedical Sciences. The HoloGraphics project is supported by the Deutsche Forschungsgemeinschaft (DFG).

References

[1]  O. Bimber January 2004 Combining holograms with interactive computer graphics IEEE Computer 85–91