Manuel Ulibarreña
m.ulibarrena@umh.es

About the author
Manuel received his PhD from the Universidad Miguel Hernandez in Elche, Spain, in 2003. For his project he studied the characteristics of the ultrafine BB640 emulsion in relation to holography. He is currently associate professor of non-destructive testing in the same university. He has been involved in holographic research since 1985, and his current area of interest remains silver halide holographic recording materials.

All introductory text books about holography make a comparison with photography, saying that photography is a technique that is only capable for recording the intensity of the light scattered by a real object, while holography records both intensity and the phase of that light. What they fail to mention is that when one looks at a display hologram, the image is in most cases monochromatic, so in this case this advantage of holography over photography is completely true only when comparing black and white photography with monochromatic holography.

Today almost all photography is in full color. In order to make the advantage mentioned true for color photography, the holograms must be recorded in full color. This has been obtained in rainbow holography by multiplexing three rainbow holograms with different slit locations, so that an image color is obtained with a single laser and monochromatic recording material. But the color of image depends strongly on the relative position of the slit, and in any case this color is synthesized from three or more monochromatic holograms, with the object illuminated with a monochromatic source. So that although we can get good color, the color cannot be considered as “real”. True color holograms can only be obtained from the light scattered by an object illuminated by several different wavelengths. Two major practical problems are found when trying to make such a hologram.

The first one is related with the lasers. While red lasers (He–Ne or semiconductor lasers) are common in many holographic laboratories (they are cheap and easy to maintain) green and blue lasers are more difficult to find since they are more expensive (e.g. argon ion, frequency-doubled Nd–YAG) or have limited holographic performance (e.g. He–Cd).

The other problem is the recording material. Traditionally, the materials used for recording color holograms have been panchromatic silver halide emulsions, multiple layer dichromated gelatin and panchromatic photopolymers. For dichromated gelatin, with spectral sensitivity in the blue–green zone of the visual spectrum, the red gratings had to be recorded with very special procedures. The blue and the green holograms were recorded individually, not multiplexed, and the final multiple-band hologram had to be built up by laminating the three individual gelatin layers one over the other with complex and costly techniques [1]. In the late 80’s and early 90’s, commercial panchromatic photopolymers [2] emerged as an alternative to DCG [3]. Silver halide-sensitized gelatin processes with panchromatic emulsion PFG03-C have also been used, with high diffraction efficiencies for single wavelength recordings [4].

Silver halide holographic emulsions exhibit a better sensitivity than all the recording materials mentioned above. Nevertheless, although they have been used for recording multiplexed reflection holograms with different laser lines [5, 6], their usage has been limited by their relatively low index modulation capacity, as well as by their spectral sensitivities, since most of them are sensitized to a single spectral band only. Besides, the material is composed of ultra fine silver halide grains, with an intrinsic absorption band around 400 nm. Therefore, blue recordings do not work properly, since they have low diffraction efficiencies due to high levels of absorption and scattering. This led to the use of techniques involving recordings in more than one plate [7] or more than one recording material [8]. The use of monochromatic emulsions for multiplexing reflection gratings with different swelling factors between recordings has been also reported [9], although in this case we are again working with synthetic color. In all these configurations, emulsions were useful for display and artistic holography. In the mid 90’s, new ultra fine grain panchromatic emulsions, especially Slavich PFG-03C, with a mean grain size of 10 nm—smaller than that of the emulsions previously available—boosted advances in these two fields [10]. More recently, results obtained with a non-commercial ultra fine grain panchromatic emulsion have been reported, with diffraction efficiencies for single exposure diffraction gratings higher than 50% [11].

There are several aspects that have to be considered when working with multiplexed reflection holograms with different wavelengths in silver halide materials. The first is the high scattering, mentioned above, that occurs in the blue part of the spectrum. This scattering can be reduced by working with ultra fine grain emulsions. In this study we used the new panchromatic ultra fine grain emulsion BBVPan, based on the existing family of BB emulsions, currently manufactured by Colourholographics Ltd, with a mean grain size of 20 nm. The second aspect is related to shrinkage or swelling of the emulsion after the plate is processed, since in reflection holography this is directly related to the wavelength of reconstruction, and this affects the final replay spectrum and color rendition of the grating. The last aspect is the effect of multiple exposures on a single emulsion, since this is associated with a reduction in the diffraction efficiency. This reduction has been historically evaluated as inversely proportional to the square of the total number of recordings [12].

Experiments

We recorded color reflection holograms using the new panchromatic ultra fine grain emulsion BBVPan, batch no. 174. In all our works with BB emulsions we have presensitized them in order to reduce the exposure times. Previous work in reflection holography with BB640 emulsions showed a response in the 604 nm range, instead of the expected 633 nm of the He–Ne laser, an effect caused by the presensitizing bath composed of a 3% triethanolamine (TEA)–water solution. TEA is an electron donor that increase the speed of photographic emulsions and photopolymers. But it is also known to be a swelling agent used to reduce the replay wavelength of the holograms, and has this additional effect when used in presensitization. We found that an additional water bath following the first TEA solution bath reduces the swelling effect. The soaking time in this second bath is important and has to be adjusted to obtain the proper wavelength at reconstruction.

In all the tests reported here, plates were presensitized by soaking for 2 min in a 3% TEA water solution, 7 min in a deionized water bath, drying with a photographic roll and warm air, and leaving in the exposure room for half an hour in normal laboratory conditions (20^∘ and 60% RH) [13].

For the above characterization study, plates were exposed to single collimated beams in a Denisyuk configuration [14] using a blue He–Cd laser (wavelength 442 nm), a green frequency-doubled Nd–YAG laser (wavelength 532 nm), and a red He–Ne laser (wavelength 632.8 nm). The recording setup consists of an optical sandwich composed of a first surface mirror that reflects the incident beam back into the emulsion. The emulsion side of the plate is in contact with the mirror via an index matching fluid, and the glass side is in contact with an anti-reflection coated glass plate via another thin layer of index matching fluid to prevent internal reflections. The setup schematic is shown in figure 1.

Figure 1: Recording setup used for the characterization study of the BBPan emulsions.

With this configuration spatial frequencies of 7145 l/mm (blue), 5936 l/mm (green) and 4990 l/mm (red) were recorded (considering a refractive index of 1.579 for the unexposed emulsion). The sandwich was mounted on a computer controlled motorized holder which enabled us to record 9 gratings with different exposure energies on a 2” × 2.5” plate. The size of the plate was obtained by cutting each of the 4” × 5” plate into 4 pieces, since at the time we performed our study we had only 5 of these plates available.

The diffuse object color hologram study was performed with a holographic setup in a Denisyuk configuration shown in figure 2. The folding mirrors were sequentially placed in order to multiplex the three holograms, starting with the blue, then the green and finally the red laser. Plate size for this study was 2” × 2.5”, except for the last one that was 4” × 2.5”.

Figure 2: Recording setup used for the diffuse object color hologram study of the BBPan emulsions.

Exposed plates were developed with AAC developer (Ascorbic Acid 18 g/l + Sodium Carbonate 60 g/l) [15]. After washing they were bleached with fixation-free rehalogenating bleach R-10 (Potassium Dichromate 2 g/l + Sulphuric Acid 10 cc/l + Potassium Bromide 35 g/l). After bleaching, the plates were washed and soaked in deionized water with a few drops of Photoflo and Acetic Acid to prevent printout, and dried in the normal laboratory conditions mentioned above.

After drying, the plates recorded in both the characterization setup and the diffuse object setup were analysed using a fibre fed spectroradiometer. With this device we measured the zero order of the grating with a replay angle of 0^∘, matching the recording geometry. A short arc xenon lamp was used as the light source, collimated and polarized perpendicular to the plane of incidence to match the recording conditions. Light was collected by an optical fibre that fed the spectrophotometer and data were transferred to a computer for storage and analysis. Reflection losses were experimentally evaluated and found to have a value of 6.7%. The schematic of this setup is shown in figure 3.

Figure 3: Analysis setup.

Two different studies were performed: a preliminary spectral sensitivity characterization of the plates, followed by a study of multiplexed gratings on a single plate with plane gratings and a diffusing object.

Characterization of the Plates

The plates were first tested for single wavelength recordings with each of the laser beams used in order to check their spectral sensitivity and the response of the material when recording holographic reflection gratings. Three sets of tests, one for each wavelength, were carried out, including presensitizing, exposing, processing and analysis as explained above. Exposure energies ranged from 30 to 2400 J/cm² for the He–Cd laser and from 150 to 2400 J/cm² for the frequency-doubled Nd–YAG and He–Ne lasers.

Multiplexed Holograms

In this case, the exposure of the reflection gratings was made sequentially on the same area of the plate, starting with the blue wavelength, followed by green and then red. We obtained a set of multiplexed reflection gratings with different exposure energy combinations for each wavelength, ranging from 120 to 225 J/cm² for the He–Cd laser, 150 to 250 J/cm² for the frequency-doubled Nd–YAG laser and 800 to 1200 J/cm² for the He–Ne laser.

Results

Single sensitivities of BBVPan plates for each of the three recording wavelengths are presented in figure 4, and the most relevant results are summarized in table 1. From these results it is clear that the exposure energy for maximum Diffraction Efficiency (DE) of this emulsion is the highest for the blue wavelength ( 320 J/cm²), followed by the green (1200 J/cm²) and with the lowest sensitivity for the red (2400 J/cm²). Replay wavelengths match very closely with those used at recording, with an error of less than 2%. This wavelength shift can be modified by changing the soaking bath times in the presensitizing process.

Figure 4: Dependence of the diffraction efficiency of BBVPan plates on exposure energy for each of the recording wavelengths.

Figure 5: Transmission spectra corresponding to the three single wavelength holographic reflection gratings with maximum DE recorded on three BBVPan plates.

One of the key characteristics of this new material is its even response to all the wavelengths we used. Former western emulsions suffered from high absorption in the blue region of the spectrum. This limited their use for recording multiplexed color reflection holograms on a single plate. With the experimental setup described above, we worked not only with three well-separated wavelengths, but with the highest spatial frequencies holographically achievable in each case. The results obtained show that the maximum diffraction efficiency with each of these spatial frequencies is almost constant, with a small drop in the case of the blue wavelength due to the proximity of the absorption band of the silver halide grains, located at about 400 nm, and the absorption band of the supporting glass plate and the gelatin emulsion. For the red wavelength there is another small drop in diffraction efficiency, but this time due only to the low sensitivity of the plate in this region of the spectrum. Nevertheless, all maximum DE values for single wavelength are well above 70%.

The transmission spectra of the recordings corresponding to the three diffraction efficiency maxima are shown in figure 5. The blue band is affected by the absorption of the ultra fine grain emulsion referred to above, thus reducing the diffraction efficiency of this recording, although the zero order is of the same magnitude as that obtained with the green wavelength.

With all the information obtained after this preliminary study, we multiplexed three reflection gratings, each with a different wavelength, onto a single plate, following the procedure described in section 2. The best result was obtained with a sequence of exposures with energies of 150 (442 nm) + 250 (532 nm) + 1200 (632.8 nm) J/cm², at which the diffraction efficiencies of each band are balanced, as shown in table 2, with the corresponding spectral transmission curve shown in figure 6. The DE for all the recordings is higher than 52%, which indicates a high index modulation capacity for this material. Other exposure energy sets were tested, and small changes in one of the exposures were seen to substantially affect the DE of all three bands.

In order to check that the effect of crosstalk between diffraction bands was negligible, we applied a model based on Kogelnik’s theory with three bands [16]. With this model we obtained the index modulation and effective thickness of the multiple band recordings with great accuracy. Experimental data were fitted and a good match was obtained, as can be seen in figure 6, in which the dashed curve corresponds to the theoretical approach. The best result was obtained for an effective thickness d of 7.3 m. Approaches with three different values of the index modulation n₁, one for each wavelength, were tried, but the best result was obtained when each one had the same value, namely 0.027.

Figure 6: Transmission spectrum of the multiplexed holographic reflection grating recorded on a single BBVPan plate. The dashed line shows the result obtained with the theoretical simulation.

After looking at the results obtained in this study with reflection diffraction gratings, several of the points raised in the introduction may now be discussed. The high diffraction efficiencies obtained with the multiplexed holograms contradicts what has been said about a reduction in diffraction efficiency of multiplexed holograms (although those studies were done with angular multiplexing). In fact, results obtained with this material show that its modulation capacity is greater than that needed to record a unique holographic grating, since if we consider such a case, we can obtain a maximum index modulation of 0.054, while if we use the sum of the three individual index modulations as the storage capacity of the material, a value of 0.081 is obtained. This is much higher than any other reported value for a silver halide emulsion. Therefore, the total DE is not reduced by multiplexing several gratings, but is increased to another value that corresponds to the real storage capacity of this emulsion.

After completing this study with gratings, and considering the sensitivity values obtained, we continued with a study with diffuse object holograms. We chose an object containing tones which were hard to reproduce, such as skin color, yellow-orange, and blue. The best result, one with high brightness and good color reproduction, was obtained with exposure energies of: 225 J/cm² (He–Cd), 350 J/cm² (Nd:YAG) and 1.5 mJ/cm² (He–Ne), values which are not very far form the optimal values obtained for diffraction gratings. A color reproduction of the final hologram, replayed with an halogen lamp, and with the real object under the same illumination, is shown in figure 7.

Figure 7: Color reproduction of the final diffuse object hologram compared with the real object under the same reconstruction/illumination source.

Replay wavelengths with an angle of reconstruction of 45^∘ are 445 nm, 538 nm and 639 nm, which are very close to the recording wavelengths. A sample of the transmittance spectrum obtained with the analysis device with the probe beam incident at an arbitrary location of the hologram is shown in figure 8. Estimated diffraction efficiencies for each of the bands are around 20% for the blue and green bands and around 40% for the red, which are not bad values for a diffuse object. It must be noted that the transmittance spectrum depends on the area of the hologram sampled.

Figure 8: Transmission spectrum of the diffuse object hologram recorded on a single BBVPan plate.

Conclusions

We tested the new BBVPan panchromatic holographic emulsions for reflection holography. The material was first evaluated using single recordings with three different wavelengths, and a DE higher than 72% was reached in all cases. After characterization, the plates were used to record a three-band holographic reflection grating with spatial frequencies from 5000 l/mm to more than 7000 l/mm, all with high diffraction efficiencies, namely over 52% for each of the three bands. The study with diffuse objects rendered good color images with good diffraction efficiencies. With the properties mentioned, this material can be used to manufacture holographic combiners for projection display systems, as well as for color display holograms.

Acknowledgements

I am grateful Michael Medora of Colourholographics Ltd for providing the holographic plates used in this work.

References

[1]      J. R. Magariños and D. J. Coleman 1985 Holographic mirrors Opt. Eng. 24 (5) 769–780

[2]      T. H. Jeong and E. Wesley 1989 True color holography on du Pont photopolymer material Holosphere 16 (4) 20

[3]      T. J. Trout, W. J. Gambogi and S. H. Stevenson 1995 Photopolymer materials for color holography Proc. Int. Conf. on Applications of Optical Holography SPIE 94–105

[4]      J. M. Kim, B. S. Choi, Y. S. Choi, J. M. Kim, H. I. Bjelkhagen and N. J. Phillips 2002 Holographic optical elements recorded in silver halide sensitized gelatin emulsions. part ii. Reflection holographic optical elements Appl. Opt. 41 (8) 1522–1533

[5]      L. H. Lin, K. S. Pennington, G. W. Stroke and A. E. Labeyrie 1966 Multicolor holographic image reconstruction with white-light  illumination Bell Syst. Tech. J. 45 659–661

[6]      J. Upatnieks, J. Marks and R. Fedorowicz 1966 Color holograms for white light reconstruction Appl. Phys. Lett. 8 (11) 286–287

[7]      P. Hariharan, W. H. Steel and Z. S. Hegedus 1977 Multicolor holographic imaging with a white-light source Opt. Lett. 1 8–9

[8]      T. Kubota 1986 Recording of high quality color holograms Appl. Opt. 25 4141–4145

[9]      P. M. Pombo, R. M. Oliveira and Jo an L. Pinto 2002 Color control in reflection holograms recorded in Slavich PFG-01 emulsions Practical Holography XVI and Holographic Materials VIII ed T. John Trout   and SPIE 399–404

[10]      H. I. Bjelkhagen, T. H. Jeong and D. Vukicevic 1996 Color reflection holograms recorded in a panchromatic  ultrahigh-resolution single-layer silver halide emulsion J. Imaging Sci. Technol. 40 (2) 134–146

[11]      C. Wang, D. Pu, T. Zhu, J. Wu and M. Tang 2002 Panchromatic ultra-fine-grain silver halide emulsions and their  properties in reflection holography Holography, Diffractive Optics and Applications ed Y. Sheng, D. Hsu and J. Chen   SPIE 121–125

[12]    M. K. Shevtsov 1985 Diffraction efficiency of phase holograms for exposure superposition Sov. J. Opt. Technol. 52 1–3

[13]    M.  Medora 2003 Colourholographics Ltd, Braxted Park, Gt. Braxted, Witham, Essex,  CM8 3XB England (Personal communication)

[14]    Y. N. Denisyuk 2001 Photographic Reconstruction of the Optical Properties of an Object in its Own Scattered Radiation Field, volume MS 171 of Milestone Series SPIE 22–24

[15]      M. Ulibarrena, M. J. Méndez, L. Carretero, R. Madrigal and A. Fimia July 2002 Comparison of direct, rehalogenating and solvent bleaching processes with BB640 plates Appl. Opt. 41 (20) 4120–4123

[16]      M. Ulibarrena, L. Carretero, R. F. Madrigal, S. Blaya and A. Fimia 2003 Multiple band holographic reflection gratings recorded in new ultra-fine grain emulsion BBVPan Opt. Express 11 (25) 3385–3392

For someone who has been occupied with holography for almost 30 years in a variety of ways, it is gratifying to see its nearly 60 year history written and between hard covers. Of course there have been numerous potted histories before, in catalogues, text books and on websites, but none has attempted such a thorough or well-researched treatment previously. A historian of science and technology, and an optical physicist, Sean Johnston was clearly the man for the job.

At the beginning of this book I counted 81 acknowledgements. Johnston personally interviewed 50 assorted researchers, artists and entrepreneurs and consulted another 55 sources. The bibliography occupies 41 pages at the rear of the book and must represent untold hours of reading. The index stretches to 28 pages and is a veritable Who’s Who and What’s What of holography.

Johnston’s timing was fortuitous. Most of the founding fathers of holography, with the exception of Gabor, were still alive while he was doing his research and he was able to question them face to face. By the time of publication, Benton, Leith and Denisyuk had died and a chapter in holography’s history was over – a good moment, perhaps, to reflect on the past and assess the successes and failures of a still developing medium.

In his Preface, Johnston raises the question “Why attempt a history of what some still see as an immature subject?” and answers that “One reason is because notions of maturity carry questionable assumptions. Holography is a young science that illustrates how new subjects come to be. It provides answers to questions such as, How does a scientific subject materialize? How does its content stabilize? How do those who practice it come to recognize themselves as a distinct group? And how do its definitions and products depend on their environments?” He goes on to say that “This visionary subject exemplifies how science, technology and wider culture are woven inextricably in the modern world”.

Later, Johnston observes that “Like the multiple perspectives offered by the hologram… each community had a different vision of its problems and potential and each shaped a history and forecasts to fit”. Depending on your interest in and involvement with holography, different parts of this book may fascinate you more than others but Johnston has explored all its aspects in depth.

Overview of the book

The book is divided into four parts envisaging the development of holography as

1. An intellectual subject
2. A visual medium and scientific technology
3. A social practice
4. An economic and cultural activity

• Creating a Subject—examines the origins of holography in Britain, the Soviet Union and America and argues that it was the particular post-war context of science and industry, combined with a spirit of military exploration in the context of the Cold War, that explains the nurturing and flourishing of the original ideas.
• Creating a medium—focuses on the boom years of 1964–1973 and the frenetic research to extend the technical capabilities of holography.
• Creating an Identity—explores the disparate elements that came to call themselves holographers: Scientist-engineers, fine art or aesthetic holographers and artisanal holographers. The latter coming from ‘a distinct counterculture community to transform the subject during the 1970s with radical methods and goals’.
• Creating a Market—focuses on the commercial sprouting of holography alongside its popular understanding and the co-evolution of both the cultural meaning of holograms and viable commercial products.

My personal involvement with holography begins with the third section so, while I was familiar with many of the names at the beginning of the story, I had not seen their origins and interactions plotted so clearly.

Johnston takes us to an industrial electrical lab in Rugby, England where Dennis Gabor worked for British Thomson-Houston and conducted his original research, to a state scientific institute in Leningrad, USSR where Yuri Denisyuk did his, and to a classified research lab in Willow Run, Michigan, USA where Emmett Leith and Juris Upatnieks made the breakthrough that led to holography as we know it. We meet George W. Stroke, Professor of Electro-Optics at the University of Michigan, an uncongenial figure who constructed the first historical summary of holography and arguably its earliest and most influential packaging. He was influential in the awarding of the Nobel prize to Gabor in 1971 and denying the prize to his Willow Run rivals. (This is possibly the most controversial story to emerge from Johnston’s research).

We see inside the Conductron Corporation who were the first to commercialise holography and meet Kip Siegel the manager whose realisation that what one had to do to make money from holography was “to sell the promise of technology to investors” became the blueprint for many holography entrepreneurs to come. It was there in the late 1960s that mass produced holograms (500,000 laser transmission copies on film for the 1967 Science Year publication) first happened and that an artist, Bruce Nauman in 1968 and Salvador Dali in 1970, had their first chance to use the medium.

It is around this period that we first meet Lloyd Cross, an early worker with pulse lasers, whose subsequent activities as a teacher at the San Francisco School of Holography and manufacturer of holograms at the Multiplex Co would introduce holography to a wider world of artists and entrepreneurs.

Steve Benton makes his first appearance at Polaroid, under the relaxed regime of Edwin Land, and his invention of the rainbow hologram, adopted by artists and eventually commercialised as the ubiquitous embossed hologram, combined with his fostering of a spirit of communication through conferences and collaborations with artists, gets the recognition it deserves. Tung Jeong from Lake Forest College, who taught many workshops and hosted many conferences and Jerry Pethick, who conceived of the sandtable as a way of making holography accessible, are two other names that stand out from the early days.

As holography begins to move further into public consciousness we see the growth of exhibitions and museums, led by Posy Jackson and Jody Burns at the Museum of Holography in New York and later Matthias Lauk in Germany, the subsequent burgeoning of galleries and cottage-industry display holographers and the reluctance of the artworld to accept holography. We encounter Steve McGrew, Ken Haines and Mike Foster who give birth to the hologram embossing industry in the USA and Nick Phillips and Jeff Blyth who kick-start reflection holography in the UK. Testimony is heard from Jim Trolinger, Bill Fagan and others involved in the non-destructive testing applications of holography.

Women feature prominently in the development of art holography so we are introduced to Margaret Benyon, Harriet Casdin-Silver, Anait, Edwina Orr and Eve Ritscher amongst others.

This is not just a tale of relentless progress, however. Johnston recognises that there have been dead-ends and failures too and recounts the falling off in popularity of various aspects of holography.

The insider stories make fascinating reading and brought the book to life for me. As a non-scientist I cannot vouch for the accuracy of all the technical elements of the book, though nothing stood out as inaccurate, but the chapters that dealt with the entrepreneurs and artists in holography seemed to illuminate the whole picture very evenly. Doubtless there will be individuals who feel that their contribution to the story has not received sufficient attention (I was surprised not to see a mention of Walter Clarke as a business man or collector, for example) but I am sure that most would commend Sean Johnston for his thorough and wide ranging history.

Johnston writes well and tells the story persuasively in a way that should appeal to those interested in the sociology of science as well as to holography insiders. There are plenty of amusing passages as the urban myths and popular misconceptions about holography are identified and some of the wideboys and eccentrics attracted to holography make their appearance, and there are moving moments too as when Leith and Denisyuk finally meet in 1989 at the end of the Cold War and share a car journey : “The two, having travelled the same road half a world apart, could do so together at last”.

For the second edition, the misspelled ‘Agfa-Gavaert’ should be amended to ‘Gevaert’ and Pim Giebels would probably rather not be addressed as ‘Goebbels’, but in two readings I did not recoil at any howlers and could not imagine that any of the protagonists would be offended at the way their story has been told.

I think that Johnston has served the holographic community very well and deserves our support. I would hope that at least everyone whose name is in the index buys a copy, and a lot more too. And for all those with an interest in what holography might become, I would suggest that by reviewing the past we are perhaps more able to imagine the future.

There is an omission in the formula at the top of page 471. After the ferric sulphate line, add a new line as follows: ’sodium hydrogen sulfate, crystals…30 g’. Graham offers his apologies to any holographers whose bleach stage took three hours as a result of this omission.

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.

Rebecca Deem
redeem75@yahoo.com

About the author
Rebecca first saw holograms at an art gallery in 1970 while completing an Art supervision degree. In 1988, she received the Shearwater Foundation Art Holography Award. In 1995 with partner Fred Unterseher, she co-founded Zone Holografix Studios, an art and teaching studio with a pulse laser lab. She continues to exhibit artwork, teach and write for electronic and print publications.

The Practical Holography XVIII: Materials and Applications conference, was held in San Jose, CA, on January 19th and 20th as part of the week long International Society for Optical Engineering (SPIE) and IS&T Electronic Imaging conference. Tung H. Jeong and Hans I. Bjelkhagen, chaired the Practical Holography program. Dr. Jeong, with the help of Steve Smith (MIT), received approval for the conference to be dedicated to Dr. Stephen Benton (inventor of the rainbow hologram). There was a tribute to him, on the evening of January 21, resembling the MIT event in November, just after his death (http://www.holographer.org/articles/hg00004/hg00004.html). Jeannie Benton gave the opening remarks, followed by attendees who read their tributes. The written version will be published in the upcoming rainbow cover proceedings. SPIE created a CD-ROM of all 17 of the Benton-edited proceedings of the annual holography conferences he chaired, with an embossed hologram portrait of him produced by Toppan on the jacket.

Overall, there were no new breakthroughs reported. However, there were a number of progress updates and developments that are noteworthy. Two presentations from Japan stood out in the Digital Holography I session:

• Yuri Sakamoto reported on work by a team from Yamaha Corp. They have developed a device that ‘draws’ hologram data onto CD-R disks (based on the Yamaha RD-F1 CD-R/RW drive). The drive produces holograms in a short time, at low cost and at high resolution. It is small and portable, so that all of the processes of desktop ‘hologramming’, from calculation to recording, are possible with just one PC. They evaluated the optical properties of the disk and developed a new method of calculating the hologram data, suitable for a CD-R disk to produce 3D imagery. Sakamoto showed preliminary results of holograms with animated wire-frame images (2 cm²) written on conventional CD-Rs. The results show real promise.
• The other project under way in Japan is the “Real-time color holography system using a high-resolution reflective LCD panel”, by the team at Himeji Institute of Technology. They pointed out that a reflective LCD panel has much higher density and can create brighter image reconstructions than a transmissive one, and is therefore more suitable for red, blue and green (RBG) imaging. The holograms are displayed in real time on the RGB reflective LCD panel, and an RGB LED is used as the reference beam. The hologram is either calculated numerically as a computer generated hologram (CGH), or is recorded as an interference fringe pattern for a real object by a high-resolution complementary metal-oxide-silicon (CMOS) sensor.

In the Materials I session, Aachen University of Technology in Germany reported on the sustained development of materials used in the manufacture of high efficiency HOEs. They continue to use DCG as a critical part of the processing regime for holographic film on glass or plastic substrate. Using electron microscope photographs they illustrated their latest findings of the expanded bandwidth of the HOEs. They are investigating broader usage for solar applications.

SPIE, Bellingham, WA received a 2003 Shearwater Foundation award of $11,000.00 to support the attendance of ten artists to the conference. The first formal art holography session took place on the 19th, chaired by Fred Unterseher. Five artists presented papers including:

The following artists attended and exhibited work:

Steve Smith (MIT) organized the art exhibition including Dr. Benton’s holograms. These remained on view through the Photonics West conference which followed Electronic Imaging.

Michael Page’s presentation on the holographic stereogram light valve printer was well received. He showed his work and the work of his students with many examples of digitally animated subjects and live figures. August Muth showed LARGE format DCGs including a piece titled “Catherine’s Pond” a kidney shaped Koi pond with ten layers of DCG holograms embedded in heavy glass, mounted on a turntable. The imagery of shells, a koi fish and some leaves floated throughout the layers of the 100 lb glass art sculpture. Pearl John showed “LUNCH” a reflection hologram with CO₂ laser-etched writing on the fruit of the subject as well as the frame. Marie Christian Mathieu presented “Soup (e)” which combined interactive computer animations with a reflection hologram placed behind a 15” laptop monitor screen. She appears to swim within the holographic soup bowl that straddles the plane of the computer screen. (See animation at http://www.ontogenetic.org/soupe.html.)

The SPIE Holography Technical Group chaired by Mike Klug met during the evening of January 20th. There was a great deal of discussion about the conference scheduling dates of Electronic Imaging (EI) and Photonics West (PW). Many of those attending Practical Holography want to visit the exhibits featured at the Photonics West conference which doesn’t open until the week after Electronic Imaging. Therefore many are obliged to attend both conferences or choose one over the other. The attendance to Practical Holography appeared to be lower this year due to this conflict, as well as the difficulty some speakers had obtaining visas from Korea, China and the former Soviet Union (FSU) countries. The Photonics West conference is by far the bigger event with an attendance in excess of 14,000 this year and 800 exhibitors, while Electronic Imaging attracted around 1500 participants and a small number of exhibits. Voters apparently hoped to resolve the issue with a vote requesting to remain with EI while both conferences take place concurrently, EI in Santa Clara and Photonics West in nearby San Jose. This would give groups like Practical Holography the option of visiting the vast array of exhibits.

At the close of the meeting, a moment of silence was held for the extraordinary figures in holography who died during 2003. As previously mentioned, the conference was a tribute to Dr. Benton, the inventor of the rainbow hologram who died in November. He was preceded by Dr. Pal Greguss inventor of the acoustic hologram and panoramic annular lens (PAL). Artist and innovator Jerry Pethick, who held the patent for the sand table stability system and who was a founder of the San Francisco School of Holography died in July, followed by Peter Nicholson, artist, inventor and pioneer of pulse laser holography, in December.

To see pictures of the SPIE event, visit: SPIE (spie.org) at
http://electron icimaging.org/program/04/

The proceedings of the conference are dedicated to Steve Benton, with a rainbow colored cover, and an embossed Toppan rainbow hologram of Steve on the inside cover (http://electronicimaging.org/program/04/index.cfm? fuseaction=proceedings).

SPIE also has a CD-ROM of all Benton-edited Proceedings of the annual holography conferences he chaired, with the Toppan hologram on the jacket (http://bookstore.spie.org/index.cfm?fuseaction=DetailCDROM&productid =551655).

Footnote

^*This article was completed in March 2004, but was seemingly lost in ‘cyberspace’ till May. Apologies to all — Editor.

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.

Michael Harrison
www.dragonseye.com/Holography

About the author
Michael was born on an insignificant little blue-green planet orbiting an unregarded yellow sun out in the unfashionable backwaters of the western spiral arm of the galaxy and is a computer programmer currently working in the games industry. He has been interested in holography and has been making holograms since 1984, with a few long breaks for Real Life. He hopes to soon take over the world by combining his experience in 3D graphics with holography.

The release of Graham Saxby’s third edition of Practical Holography shows how well the author understands what people need from a complete book on holography. He takes the reader through what a hologram is, the history of holography, what sort of light sources can be used to make a hologram, what kinds have been made to date and so on through to making and displaying your own images.

The book is written in a clear and concise manner and is augmented by additional tips, definitions, and observations in the margins as well as extensive source references at the end of each chapter.

If you’re new to the field or hobby of holography you should make this one of the first books you buy. Even if you’re an old hand this book will probably show you a few new tricks.

Overview of the book

The first four chapters (What is a hologram, How holography began, Light sources for holography, The basic types of hologram) should be read through by anyone starting out in holography. You don’t necessarily need to understand everything in those chapters right off, but by reading those basics you’ll be better prepared to digest the rest of the book.

The first chapter explains what a hologram is, interference, diffraction, amplitude and phase gratings in a way that most interested readers will have no trouble understanding.

The book continues through the history of holography, the light sources used to make holograms, the basic types of holograms and describes the materials and processing used in making holograms.

It’s in chapter six that Saxby begins explaining how the reader can make their first single-beam hologram using a gas or diode laser. This chapter has complete details on all the equipment needed, how to set it up, shoot it, process the exposed film and view your finished hologram. If you are unfortunate enough to end up with a dim or non-existent image (which is likely to happen the first time) the author takes you through the steps needed to find out what happened and how to fix the problem.

Chapter seven then takes the reader through more advanced single-beam configurations and introduces a few new tools and methods such as using a spatial filter, index matching film and multi-exposure techniques.

The rest of the book shows a similar progression, taking the reader through more complicated steps such as making transfer holograms, building a holography lab, creating master and copy holograms, homemade optical elements and so on.

For those with a mathematical bent, the first three appendices contain information you’ll want to read and digest after going through the first few chapters of the book. These appendices are clearly written and approachable even to those who aren’t particularly adept at mathematics. Saxby also includes an appendix with worksheets for computing the geometries needed for several forms of multicolor holograms.

If you’ve been around the block a time or two (or at least ridden with someone else) you might think that this book would offer little new material. In fact, there is plenty of material for the more experienced holographer. The material ranges from information on fiber optics use, color holography, edge lit holograms and beyond. See the included table of contents from chapter 16 on.

Differences between this and the previous edition

Extensive side notes have been added which expand on and clarify the information given in the main text. These comments could have been left out and the book would not have suffered but by adding them the author gives information that enriches the main text.

The chapter on light sources used for holography has been expanded to include information on diode lasers as well as new information on DPSS (diode-pumped solid-state) and white light laser sources.

The pages devoted to copying holograms have been greatly expanded from one chapter with six pages to two chapters of nearly thirty pages. These cover several techniques for copying both transmission and reflection holograms and close by covering the relatively new technique of edge lighting holograms.

Natural color holography now has a full chapter devoted to it which starts by covering how we perceive color as well as details on how the eye responds to light of differing wavelengths. Details are then given on how individual primary colors are commonly combined to form colors that you won’t find in the natural spectrum. From there the author describes how lasers of differing color may be combined on the table to create a simulated full-color image. While this chapter won’t give you all the details you need for natural color holography it will get you started and there are several references at the end of the chapter that can carry you further.

A chapter has been added covering non-silver processes for making holograms and even includes limited information on coating your own glass plates. This chapter starts out by mentioning the high sensitivity of silver-halide emulsions and discussing the major reasons for its use. Saxby moves on to the details of dichromated gelatin (DCG) use and outlines methods for mixing DCG, coating glass plates, exposure and processing. If you’re interested in extremely bright holograms, this section will definitely whet your appetite for rolling your own plates. From there he moves on to brief discussions of SHSG, photopolymers, photothermoplastics and other processes. He doesn’t go into the same detail in the later sections as he does for DCG but there are plenty of references at the end of the chapter.

Holographic stereograms now have an entire chapter devoted to them. This chapter includes instructions for making several different kinds of stereogram and details for creating good source material, usually photographs. The author even outlines some methods for computer control of a simple holoprinter as well as color control to obtain achromatic and full-color transfers.

A new chapter on the use of holography in biology and medicine includes information on hologram use for dental training, ophthalmology and stereogram use with PET and CAT scan data.

The appendix on processing formulas has been updated and expanded and now includes instruction on creating your own emulsion.

So, the book is perfect is it?

While there are a few typographical errors that the publisher is working to correct in future printings, there really isn’t much that I could find fault with. A few relatively minor complaints are detailed below.

Interferometer testing isn’t introduced until chapter 11 but making your first hologram begins in chapter 6. The reasoning behind this is likely twofold.

1. All the table setups prior to chapter 11 involve using a single beam for the reference and object light and there are less stringent stability requirements for single-beam setups.
2. Setting up an interferometer requires two mirrors, one beam splitter and one lens as well as mounts for all of those elements and Saxby makes an effort to minimize the equipment needed to get going.

Forcing the reader to buy additional optics needed only for the interferometer could be seen as an impediment to bringing people into the field. My only complaint with that reasoning (if that really is what determined where table testing was placed in the book) is that knowing your environment can be critical in understanding your failures when starting out in holography. You will have a few failures in the beginning, especially if you’re not working in a dedicated laser lab. While single-beam setups are less sensitive to vibration problems, they aren’t immune and testing your area with an interferometer can give you invaluable information about what limitations you start out with.

Not all film listed is still available (Kodak no longer makes plates) or available to the general public (most photopolymer material). This is not surprising as this is an area of the field that is in constant change. New materials are appearing as old materials are being refined or disappear completely. You’d be better served by doing an Internet search or checking in the Holography Forum (www.holographyforum.org) for the current state of the art.

My soft cover copy is only three months old and is already coming apart at the binding. This may be a problem with that particular run.

All three editions of Practical Holography have included a hologram of some kind. The first edition actually included two, an embossed hologram on the cover and a silver-halide hologram on the first page. Unfortunately the second and third editions have only included embossed holograms on the cover and while I’m sure that type was selected for the relatively low production cost, they are not the best examples of the art. The depth of field available with embossed holograms is severely limited and while they are visible in almost any light, embossed holograms lack the impressive sense of 3D available from simple reflection holograms on silver-halide or photopolymer.

Summary

I have no trouble at all in recommending the third edition of Practical Holography to everyone interested in holography. There’s something for every holographer in this book.

Table of contents

Kaveh Bazargan
kaveh@holographer.org

About the author
Kaveh’s interest began with seeing the Royal Academy of Arts exhibition of holograms in London in 1976. He was studying physics at Imperial College London, and continued to complete a PhD in ‘display holography’. His passion remains the quest to achieve ultimate realism with holography.

Over the years I have often wanted to be able to simulate the recording and reconstruction process in holograms. I am particularly interested in display holograms, as opposed to HOEs. During my studies I did write little programs for specific cases, but not a general purpose program. There are two parts to such a program: first to calculate the reconstructed image, having fed in the the recording data, and second, to present this in a nice graphical form, preferably three-dimensional.

Recently I discovered POV-ray (http://www.povray.org), a 3D image rendering engine, and realised it was the ideal tool for the job. Please see the the amazing images people have created with it. POV-ray is free, platform-independent (Linux, Unix, Macintosh, Windows), and produces images equal to renderers at any price. The beauty of using POV-ray is that not only is it a superb 3D rendering engine, but it is also an excellent programming language. Never calling myself a real programmer, I slowly put some equations in, got results, and soon got carried away! Slowly I built in more capability. The more I work on it the more possibilities I see to simulate the holographic process. I can now see that we can simulate full color holograms, ‘pseudocolour’ holograms, and rainbow holograms. At the moment it is limited to transmission holograms, but I hope to extend it to reflection holograms too.

The basic equations used are those of Champagne (J. Opt. Soc. Amer. 57 (1967) p. 51). In this document I’ll not go into the equations, but just the functionality of the program. This version is really still being worked on, and there will be bugs, so I do appreciate feedback.

License

HoloPov is released under the GNU LGPL license (see http://www.fsf.org/licenses/licenses.html#LGPL). The code is therefore ‘open’ and you can modify it and use it free of charge, and to redistribute it, within the restrictions of the license. In the spirit of Free Software, I encourage people to test and modify the code to improve and debug it, and help serve the holographic community.

Requirements

You should be running POV-ray on your computer. I have used version 3.5. The files should run with any installation of POV-ray above 3.5. POV-ray runs on most platforms and is device independent.

Examples

To see some examples of the kind of output produced, please see my other article, “White light transmission holograms”

Availability

HoloPov is available for download here.

File structure

There are three files in the package, holopov.pov and holopov.inc, and optics.inc. You will also need to get CIE.inc from http://www.ignorancia.org/zips/lightsys4.zip. holopov.pov is the file that is run through POV-ray. It will #include holopov.inc which will in turn #include several other files. Most of these are standard files distributed with POV-ray, but optics.inc is a library file of mine, and CIE.inc is a third party file which, amongst other things, works out the approximate color for a given wavelength.

The coordinate system

We use the coordinate system of POV-ray, i.e. a left-handed cartesian system. The hologram is always in the x–y plane, centered at the origin. If you imagine your computer screen as being the hologram, with the x-axis pointing to the right, and y pointing up, then the z-axis will be pointing into the screen. The center of the screen is <0,0,0>

I have chosen to have the observer in the z < 0 region, and other points in the z > 0. But I think that any point can have any coordinates. If the formulae have been coded properly, then this should be the case I think. I have not dared test this yet!

Overall concept

The basic idea is that you dial in the recording and the reconstruction parameters, and the program will work out the position of the image. By ‘parameters’, I mean the following:

• position of the object
• position of the recording source
• position of the reconstruction source
• position of the observer
• recording wavelength
• reconstruction wavelength

All these values are entered into the main file, i.e. holopov.pov.

The object

These are the parameters that define the object:

• obj_dist
• obj_angle
• obj_side_angle
• obj_theta
• object_size
• grid_sep

After a lot of thought, I decided that spherical polar coordinates were the best way to enter the data for the object, at least for me.

obj_dist is the distance of the center of the object from the origin, i.e. from the center of the plate. This value is always positive.

obj_angle is the angle of the object in the y–z plane, i.e. above or below the horizon.

obj_side_angle is the angle of the object in the x–z plane, i.e. the angle if we were looking directly from above.

obj_theta is the angle around the y-axis. This the angle one would see if one were looking down the y-axis from above.

To have an object in the observer space, i.e. z < 0, we can use values of obj_theta, obj_angle > 90^∘ or < −90^∘.

object_size needs some explanation. I have defined an object to be not just a point, but a 3-dimensional grid of points. The three coordinates in object_size define the extent of the image in each axis. For example <2,2,2> means a cube, with dimension 2 on each side. grid_sep defines the distance between successive grid points. The smaller the value of grid_sep, the more grid points will be present. If object_size is set to <2,0,2>, the grid will effectively be a sheet, with zero height.

Please note that values corresponding to object distance and angle refer to the average object position, i.e. the center of the object.

Recording geometry

Here are the values to set:

• ref_dist
• ref_angle
• ref_side_angle
• ref_theta
• lambda_r

Again we use spherical polar coordinates. lambda_r is the wavelength used for recording the hologram. Again we should be able to use angles > 90^∘ and < −90^∘ to have, for example, a converging beam. I need to do a bit more thinking on this. For example, how to distinguish between a converging beam and a diverging beam, both referring to the same point in space. I hope that we can have a sign system that caters for all combinations.

Reconstruction geometry

Here are the corresponding values in reconstruction:

• rec_dist
• rec_angle
• rec_side_angle
• rec_theta
• lambda_c

Observer

These values define the observer:

• obs_dist
• obs_angle
• obs_side_angle

I have been using angles of zero, and a negative observer distance to signify an observer on the z-axis, but to be consistent with above.

Camera settings

These are the more or less standard settings used in POV-ray for the camera:

• camera_loc – The camera position. Presently in cartesian coordinated, but might be better to change it to polar in future releases.
• camera_look – Center of the rendered image.
• camera_angle – As in the normal POV-ray definition. To zoom in, we decrease this value.

H1 and H2

I have managed to simulate the effect of having a master hologram, or as it is normally referred to, the H1. The hologram doing the final imaging, I will always refer to as the H2. The dimensions of the two plates are set by:

• h1_width
• h1_height
• h2_width
• h2_height

The main reason for setting the dimensions of the two plates is to take into account the image cut-off, or vignetting, that occurs as the viewer moves around. We also need a value to define the position of the H1 relative to H2. We could of course give the distance between the two, but I have opted to define h1_sep, which is the distance from the center of the object to that of the H1. So as we change the position of the object, all other values being equal, we are also implicitly changing that of the H1.

Options

I have put in a lot of options which have a bearing on how the final image is drawn. These are set as parameters that are tested later in holopov.inc. A value of 0 implies the value is false, any other value sets the value to true. Here are the list of options now available:

• object_points_true – Draws a sphere at each object grid point. The color is a color approximating to the recording wavelength. This is achieved by the Wavelength() macro in CIE.inc (see the file for details). The radius of the sphere is determined by object_sphere_rad in holopov.inc.
• object_grid_true – Joins up the object grid points with cylinders, color as above. The radius of the cylinder is set by grid_cylinder_rad in holopov.inc.
• image_points_true – As with object_points_true, but for image points. Color corresponds to the reconstruction wavelength. Radius is set by image_sphere_rad. I have set the object and image radii separately.
• image_grid_true – As with object_grid_true, but for the image. The radius of the cylinder is set by grid_cylinder_rad.
• ref_beam_true – Draws four lines from the reference point to the four corners of the recording plate. The ‘lines’ are actually cones which taper to zero at the reference point. The maximum radius of the cone is equal to ray_radius. Color approximates to the reference wavelength.
• rec_beam_true – As with ref_beam_true, but for the reconstruction beam. Color as expected.
• obj_beam_true – Draws a cylinder from the center of the plate to the position of the observer. Color that of the recording wavelength, radius set by ray_radius.
• image_beam_true – Joins the center of the image to the observer position. The section from the observer to the plate is a solid cylinder, radius determined by ray_radius, and the section from the plate to the image is drawn as a dotted line. Paramters for this are dotted_space and dotted_ray_rad.
• eye_true – Draws a pair of eyes, a bit evil looking at present, at the observer position, and looking at the center of the image.
• plate_true – Draws the recording plate, the dimensions of which are determined by Plate, in holopov.inc. I have left this in the .inc file, as I have not had to change it much, but it can be moved to the main .pov file.
• vignette_true – This option will draw only those parts of the image which would be visible with the geometry described, i.e. the image will be cut off at the edges of the plate.
• disp_comp_true – Uses ‘dipersion compensation’. This means it disregards the value set for rec_angle, and adjusts it so that the image is reconstructed as close to the object position as possible. In other words, it compensates for lateral chromatic dispersion.

H1 and H2 options

We’ll list the H2 options first, as the the H2 is always present, but H1 is optional:

• h2_visible – This just means that the H2 plate is drawn in the final rendering. Sometimes it might be best not to draw it.
• h2_vignette_true – This will simulate the vignetting effect, or the cut-off, of the h2. In other words, an image point is drawn only when the straight line from that point to the eye intersects with the H2. This works for real and virtual images.
• h2_vignette_visible – This draws a faint volume in space, rather like a distorted pyramid, showing how the vignetting works. It is included for educational purposes.
• h1_visible – As above, but shows the H1. I have only tried this in viewer space, i.e. a real image of the H1 projected, but it should work in the case of a virtual image too.
• h1_vignette_true – As with H2, but this is far more important for holgraphers, as it determines the viewing angle for the image.
• h1_vignette_visible – As with H2.

Camera options

• camera_observer_true – The camera is at the observer position (between the two eyes!). I have added 0.1*y to its value later, so that the camera is not blocked by the image or object beams cylinders, if these are present.
• look_at_image_true – Always keeps the center of the image in the center of the picture.
• orthographic_true – Uses orthographic viewing. Useful if looking down at one of the axes. Actually, we are using a ‘cheat’ orthographic view, because true orthographic is not compatible with screen.inc which we need in order to put the data in the corner of the image. In our case we set a high value for the distance of the camera, and adjust the camera angle to a very small one. If you are interested, see #if (orthographic_true)… in holpov.inc.

Multiple images

Quite often we want to superimpose in the same picture, the result of varying one of the values, for example, observer position, reconstruction wavelength, etc. I have defined a variable called repeat_image. This can be set to a value which determines which parameter is to be varied. For example, we can say #declare repeat_image = Lambda_c to vary the reconstruction wavelength. Note the upper case first letter to distinguish this ‘choice’ from the actual value, which is lambda_c. (I have previously set Lambda_c to a number, so this is just a trick to make it easier to remember what is to be varied.)

In holopov.pov I have inserted and commented out all the possible variables, so these just need to be uncommented. If nothing is to be varied, then we choose None.

The value repeat_amplitude determines the ‘amplitude’ of the variation around the central value set previously. So, for example, if lambda_c has been set to 550, and repeat_amplitude is set to 150, then the range for repetition will be from 400 to 700nm. The number of steps is determined by number_of_steps. A value of 2 will result in two variations, one for 400 and one for 700nm.

Animation

In addition to multiple images in the same output, we can animate the scene by varying any of these parameters. Using multiple images and animations are independent, so it is possible to have a multiple image that is animated too. For instance, we can show the effect of using different wavelengths to reconstruct in the same scene, using multiple images, then animate that whole scene by moving the observer in an arc, by varying obs_theta.

The equivalent of repeat_amplitude in animation is the animate variable, and it is set in the same way as in multiple images.

There is one more parameter that determines how the frames are distributed through time: if sinusoidal_true is set to zero, then the frames are evenly distributed through time. Otherwise, the distribution is sinusoidal. In other words, the variation is fast near the central value, and slows down towards the extreme values. When the final animation is ‘looped back and forth’, this gives a more natural, smooth motion.

Running the animation

Please note that a command for running the animation must be given, otherwise the animation settings will be ignored. The precise method for running the animation depends on your installation. You should either choose animation through the user interface, or use a command line if you are using Linux or the command line version on Mac OS X (which is what I use). In any case, the most important point is that the clock variable must go from -1 to 1. Other options, such as total number of frames, resolution, etc, are set according to your preference.

Of course POV-ray can only produce the individual frames for an animation. You will need to use a utility to convert these to a true movie file. I use QuickTime Pro, and it works pretty well. The best way to look at the animation is to loop the animation back and forth continuously. In QT Pro, choose ‘loop back and forth’ from the Movie menu.

Other comments

Data output

The reason for including screen.inc is to allow data to be output to the file. This works very nicely. Presently only a few values are output, and there is no choice as to what to print. I need to spruce this up a bit.

The maths

This was a bit more complex than I thought. The basic equations are those of Champagne of course, but there is a basic complication in display holography which is not normally mentioned. In a conventional optical system, the ‘pupil’ is normally given, and the principal ray is that which goes through the center of the pupil. So we can apply the equations and get the answer. In a display hologram, we don’t know where the pupil will be. In other words, the precise spot on the hologram that the observer looks through is not fixed. If the wavelength is changed, the image will shift, and the observer will be looking through a different part of the hologram in order to see the same image point. The same happens of course if the observer moves. That point is the center of our optical system.

So before using the elegant equations of Champagne, I had to use an iterative method (Newton’s approximation) to find the principal ray. Only then could I apply the equations. This process has to be repeated for each point in a grid.

I will detail the mathematical background in another article soon.

Divisions by zero

I have caught a few cases where division by zero occurs, e.g. when obj_dist = 0. In this case I use the simple trick of giving it a very small distance. (I think I am showing that I am not a real programmer!) I appreciate any more cases of such errors, as well as suggestions of better ways to avoid them.

Lighting

This is set as a simple set of two lights, which can be changed.

Things to do

Short term

• Check angles for points in −z region
• Group items better as macros in holopov.inc
• Improve data display
• Provide option for a solid looking cube, rather like ball and sticks, as now.
• Provide a user-friendly GUI. It is important that this is portable, and
  more or less device independent. I am presently trying out Revolution
  (http://www.runrev.com)

Longer term

• Show a ‘real’ object and its image, not just grid points or cubes
• Incorporate conventional optics (like a field lens) to modify image

Michael Dalton
mdalton@mac.com

About the author
Michael is a physicist who became interested in holography around 1983. He was co-founder and Technical Business Director of Voxel—a company which developed and patented the concept of multiple exposure Digital Holograms (Voxgrams) for medical applications. He was awarded US patent number 6123733 for the simulation of Voxgrams on desktop computers. He has a strong interest in visual perception.

In a period where the term “Holographic” can be applied to just about anything, even a lipstick (L’Oreal’s Glam Shine), I thought it an good time to go back to first principles and to reflect on what is distinctive about a holographic image and in what when it is an appropriate solution to a visual problem. If we consider the relevant features of the viewers visual system we may be able to construct a better holographic image or three-dimensional imaging system.

A viewer’s visual perception of the world is based on a complex relationship between the eye and the brain. The eye consists of a physical optical system e.g. a lens, and light sensors. The brain contains the neurological system used to interpret the information passed from the eye through the visual cortex to the brain. The parts of the visual system of particular interest to us are the 3D depth perception mechanisms, or depth cues. Whilst depth perception is a complex sum of all of these depth cues, it can be divided into two broad categories: physiological and psychological.

Physiological depth cues

These are considered the strongest depth cues because they are dependent on physical changes in the eye’s optical system. The first three (accommodation, motion parallax, and convergence) can be considered monocular depth cues, since they can be used by those with sight in only one eye.

• Accommodation – This depth cue is dependent on the control of the shape of the eyes by the ciliary muscles to bring the image formed on the retina into a sharp focus.
• Motion parallax – Of two objects moving at the same speed, that which is further from the viewer projects an image which moves across the eye’s retina more slowly than the one which is closer. Therefore the further objects will appear to move more slowly than those which are closer. Correspondingly, if the viewer is moving and the objects are stationary, then the objects which are closer will appear to move faster than those at a greater distance.
• Convergence – To bring an object into focus in the most sensitive portion of the eye, the fovea, objects which are closer require the eyeballs to twist towards each other more (larger convergence angle), than those at a greater distance (smaller convergence angle).
• Binocular disparity – This is a complex binocular depth cue, which assumes that the image projected onto the area of the back of each eye ball, is focused on the same object, and hence is intimately related to the convergence cue. It relies upon the correlation, and hence disparity or lack of correlation, between the two images perceived by the brain. Points that are farther away will appear to have a greater disparity or separation between each perceived image, than those closer to each other. The brain interprets this disparity as being closer, or further to the viewer than the point of convergence.

Psychological depth cues

These depth cues are not derived from physical changes in the viewers eyes, but rather from a higher level interpretation of the images passed from the eyes to the visual cortex in the brain. These depth cues result from our brain’s previous knowledge of how 3D objects should appear in the real world, but, as the renaissance artists in the 15th and 16th centuries realized, can also be triggered by two-dimensional image representations, such as canvases, to create the illusion of depth. They are characterized by being relative measures of distance; they require comparison with other objects in a scene before they can used to infer their relative distances from the observer. These depth cues, when separated from their natural (physiological) counterparts in the 2D world, can become ambiguous as many illusionists and artists, such as Escher, have shown to great effect.

• Stereopsis – One of the strongest psychological depth cues, for those with binocular vision is stereopsis, which can be considered as part of the binocular disparity cue, but which can be triggered by pairs of two-dimensional images or stereo displays such as anaglyphs.
• Occlusion – An object which is nearer to the viewer can partially or completely obscure those further away.
• Linear perspective – As an object moves away from the eye the image projected onto the back of the eye decreases in size and so we perceive them as becoming smaller.
• Aerial perspective – The further an object is, the more atmosphere there is between us and the object. The atmosphere causes light to be scattered, and has a tendency to scatter blue light more. Thus as objects recede in distance their contrast will decrease and they will appear bluer. This is most apparent when looking at distant objects such as mountain ranges, but the effect can also be seen in a smoke-filled room.
• Size – An object that is closer to us will appear larger than when it is moved farther away. If we recognise the object and can determine its relative size in the scene, we can judge the relative depths of objects in the scene.
• Shading/shadows – The shading of an object, the way that light falls on and is reflected by it, can give clues to its orientation, which can be used to relate it to other objects in a scene. When used with other psychological depth cues it can help to sort the relative depths of objects in scene. If an object casts a shadow onto another, then we can determine that it is closer to the light source – an interpretation which can also aid in sorting the relative distances of objects.

Ranges of depth cues

Now let us consider over what distance ranges these depth cues are most effective. In this way we can determine which depth cues are most relevant in particular situations.

Near range depth cues (15 cm–1.5 m)

These depth cues are most important when dealing with objects at arm’s length, such as a surgeon operating with a scalpel or a wine grower picking a grape from a vine. They are implicitly used in everyday tasks which require visual feedback to the brain’s motor control system, such as when we pick up an object. Of the psyhcological cues, occlusion (think of threading a needle) and stereopsis are most useful at this near range.

Medium range depth cues (1.5–150 m)

Here recognising an object (is it a tiger or a cat?) and the rate that it is moving (is that car going to knock me down?) are important visual questions. The importance of the physiological depth cues starts to diminish with increasing depth and we become more reliant on the psychological depth cues.

Long range depth cues (150 m–15 km)

Here the physiological depth cues dominate and at great distances the occlusion and aerial perspective are the only cues which have much effect.

When we are considering the creation of an image or visual display system, by looking at these depth cues, we can construct a display which works to complement our visual perceptive system. To underline the importance of taking these into account, let us first the effect of situations which cause conflicting inputs to our perception system:

Travel sickness – This is a condition caused by the conflict between sensory inputs to the brain. If I am reading while I am being rocked about by the motion of the car or boat in which I am travelling, my visual system tells me that the book I am reading is stationary, but my inner ear tells me I am moving. This conflict between the visual system and the detection of motion from the inner ear can lead to feelings of nausea. By looking out the window instead of reading, the two perception systems both report the same information to the brain and the feeling of nausea will recede.

The complement to this type of conflict can be found in the works of the OpArt or Kinetic artists. Here the viewer is stationary but may see motion in the visual field caused by the interaction of high contrast edges at various angles and frequencies. A more serious occurrence of this type of perceptive conflict may be found in people with ‘scotopic sensitivity syndrome’ where high-contrast repeated patterns such as window blinds or even text on a page can cause feelings of nausea. Once solution being investigated by researchers is to find a particular coloured filter which minimises the effect on the viewer.

Holographic systems

First let us consider the “gold standard” of holography – A full-aperture hologram made at a single wavelength and replayed with the same reference beam.

A full-aperture hologram of an object can trigger all the depth cues that the real object would. There is nothing to suggest that the shape of the object is not in fact “real” other than it’s monochromatic. Just as in the real world, the physiological and psychological depth cues work together and do not conflict each other. Any distortions in the reconstruction system tend to be minimized near the plane of the holographic film, so objects tend to be placed as close as possible to the film plane. Because of this proximity to the viewer, the physiological cues are the most important.

Therefore full aperture holograms are best suited to those displays where the viewer is going to come close to the display and be within a distance that they could interact with the object. Such examples would be in museums or exhibitions. Placing the hologram far from the viewer reduces the effect of the physiological cues and hence reduces its three-dimensional impact on the viewer – one may just as effectively use a video display or stereo display system (e.g. lenticular) to get the viewer’s attention. Unfortunately, the type of objects that can be used in this form of full-aperture holography is restricted by the size of holographic film, and by the need for the object to be motionless for the duration of the holographic exposure.

However there are circumstances where it is not practical to make a full-aperture hologram of the object, due to its size or susceptibility to motion. Here we may resort to stereo displays, which use pairs of two-dimensional images to trigger only the binocular disparity from the physiological and the psychological depth cues. Such examples can be found in lenticular displays, anaglyphs and stereograms. Stereograms use holography to create a series of virtual slits through which the viewer can see stereo pairs of images, without the need for glasses. Since the 2D images used in the recording of these stereograms can generated from any photographic or other scanning technique, or from computer generated simulations or designs, any size of object can be used to create them.

There is, however, a problem with stereo display systems. When used to display near-range objects, the physiological depth cues of accommodation, and convergence both report to the visual system that they are looking at two flat images at a single depth. However the other depth cues, especially binocular disparity (and hence stereopsis) are all reporting differing depths, and this conflict can lead to nausea if the viewer is exposed to such displays for an extended length of time. Therefore, I would not consider using such a system for a surgical applications for example, where the surgeon may be operating for periods of 10 or more hours on a patient at arms length!

This conflict between physiological and psychological depth cues for stereo displays can be tolerated at near ranges for short periods of time. If the display has to be viewed for longer periods of time, e.g. movies, then we can limit objects appearing in the near range, where the physiological depth cues will report conflict, and instead keep them at the medium and long ranges where the physiological depth cues are weak. One other technique for reducing this conflict is to restrict the range of depths in the scene so that the disparity between the depth cues is minimized.

Conclusion

I have briefly explored the relationship between the physiological, and psychological depth cues and how these consideration can be relevant when creating holographic images of scenes which consist of near, medium and far ranges. In a future article I will explore how an understanding of other aspects of the viewer’s perception system, can be useful when constructing a holographic image or another 3D imaging system.

Following just a few simple rules anyone can shoot a parallax sequence on film or video that can be directly converted into a hologram image. The first consideration is that the motion must be going in the correct direction in order to yield positive stereoscopic parallax in the hologram. The reason for this is very simple: one’s left eye must be presented with the left image and the right eye must see an image in the sequence that is to the right of the first image.

Direction of stereoscopic parallax in a hologram

This is true throughout the parallax sequence, thus if image #12 is seen by the left eye then the right eye must be observing an image that is to the right, such as image #20. If the order is reversed, then the left eye will be seeing a frame that should be seen by the right eye (and vice versa) and the three-dimensional image will be pseudoscopic, i.e. spatially inside-out, with the background seeming to be in front of the foreground subject.

The camera motion must be in the correct direction to make a positive stereoscopic parallax

Luckily with the aid of a computer and video-editing and image manipulation software is it possible to re-arrange the order of the sequence, in the event that the parallax is going in the “wrong” direction, but it is a lot easier to film it correctly in the first place, so that the hologram image is orthoscopic.

This is achieved by having anti-clockwise camera motion when orbiting the scene, or left-to-right camera motion when making a lateral pass by the subject. Alternatively, orthoscopic parallax is produced by having the subject move right-to-left or rotate clock-wise on a turntable in front of a fixed camera [33]. Footage that dates back to a previous century can now be digitally manipulated to extract the temporal parallax information that it contains and to translate it to stereoscopic parallax to produce a three-dimensional image.

Where as the subject moves laterally past the camera or rotates the footage will have some parallax that can often used to create a three dimensional image. To create a three-dimensional portrait just a few frames of temporal parallax are required. Where elements in the scene do not move, such as the background, they can be digitally removed and a new background with parallax inserted.

Once a sequence of images from movie film or video has been digitized it can be manipulated in several ways: to eliminate parts of the image, add new elements, modify size or shape discrepancies, create new intermediate frames between key frames, or even create synthetic 3D from 2D images.

As well as live-action films, temporal parallax is now common in animated feature films like “Toy Story”, “Shrek”, “Monsters Inc”, “Final Fantasy” and others where the animation is made using computer generated models rather than flat artwork.

It is possible to have absolute control over the camera angles within the virtual environment, so adding temporal parallax to increase the realism is now commonplace. Likewise, in computer games temporal parallax has been found to increase the realism of games like “Tomb Raider”. Any animated character that exists as a computer models can be quickly and simply down-loaded to make a three-dimensional hologram image.

The rules governing the use of computer models are the same as for deriving parallax from video or film, in that the model must be correctly moved with respect to the virtual camera.

Computer-generated models are now part of the manufacturing process of a wide variety of commercial products, as well as architectural, aeronautical, pharmaceutical and medical imaging.

From scanning electron micrographs of tiny particles of matter to the NASA images of alien worlds brought to us from outer space, parallax is everywhere and this revolution in digital imaging now permits the data to flow directly from the client’s computer to be be digitally written into a full-colour three-dimensional hologram.

Postscript

My sincere thanks to all the holographers working on dot-matrix and direct-write 3D imaging systems who helped me to compile the final sections of this paper by sending text, images and sample holograms. Your assistance was very much appreciated.

References

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[2] Umberto  Eco Ibid

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