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

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).

It is always impressive to see how a green reflection hologram temporarily changes to red when you let breath-moisture condense on the gelatin surface. (This effect is due to the gelatin swelling and the inter-fringe distance in the gelatin film increasing so that it reflects red instead of green light.) Apart from the obvious use of this effect in measuring the relative humidity of gases or liquids [1] our team at the Institute of Biotechnology wanted to use this effect to make many other types of measurements.

It was at the end of August 1995 that I tried an experiment which was to cause a revolution in our Institute and beyond! For over three years I had been experimenting on making so called “Lippmann” silver halide emulsions using basically the traditional method of squirting, alternately, solutions of silver nitrate and potassium bromide into a hot solution of gelatin [2]. The precipitated grains of silver bromide (AgBr) are very much finer in Lippmann emulsions than in emulsions used for normal photography. To be able to record holographic fringes satisfactorily the photosensitive grains need to have diameters at least a factor of 10 smaller than the width of a hologram fringe (around 200 nm, i.e. half the laser wavelength divided by the refractive index of the gelatin layer.) Conventional fine-grained photographic emulsions have grain sizes around 1000 nm or more. The experiment which changed everything for us was to take a just a precoated and hardened gelatin layer and to try and produce those vitally small AgBr grains inside the layer just by using a silver ion diffusion process, while still achieving a sufficient degree of photosensitivity [3]. The moment of success came when I glimpsed a holographic image on a coated microscope slide of a polished penny.—One of those all-too-rare eureka moments!

a. The diagram of a microscope slide coated with a smart polymer layer, which has been impregnated with photosensitive silver bromide, being exposed in a trough of aqueous liquid. b. An actual transmission electron micrograph showing the very fine grain size achieved to make the fringe structure.

So the success had come by sequentially diffusing into the layer, a solution of silver nitrate and following it up by diffusing in a solution of lithium bromide. (I chose lithium rather than the common potassium salt because I thought I might need the extra leeway of its very large solubility in both water and alcohol [3].)

The reason this was such a breakthrough was that we could then use the principle on a whole range of pre-coated hydrophilic polymers which responded to various specific factors in the environment.

The vast majority of tests required in biotechnology are made in physiologically based aqueous liquids, so polymers need to be hydrophilic from this standpoint apart from the fact that the precipitation of AgBr is an ion exchange process which does not lend itself to taking place in a hydrophobic polymer. So an early question was just to see how far the scope of this diffusion process could be taken towards the hydrophobic end between the typical extremes of say polyacrylamide and polypropylene. Using as reactants the two most soluble salts of silver and bromine in organic liquids, namely silver perchlorate and lithium bromide, I tested out the sequential diffusion system on a few different polymers that were to hand. I found I could get reflection holograms in the cellulose based wrappers commonly on so many of our consumer products. But I did not manage to get any result on the backside of a piece of old Agfa holographic film. This meant that cellulose triacetate was too hydrophobic but with the help of a proportion of acetone I did manage to get an ‘OK’ result in cellulose diacetate polymer film.

I also managed to get a hologram in nylon (polyamide) but it was not brilliant. I must say at once for those who instantly jump as I did, to the idea of putting holographic gratings inside the fabric of say nylon tights, I had used nylon film and not mesh. I did, however, carry out a few experiments trying it with nylon mesh where the first requirement was to “index out” the mesh i.e. make it invisible by a suitable liquid to actually record the hologram. Although I managed to make the pure mesh almost disappear in an organic liquid (DMSO), the act of putting the AgBr salt inside the nylon mesh caused some unavoidable refractive index variation and therefore bad light scatter so I only got photographic images in silver, not holographic. A pulsed 532 nm YAG laser was used so movement did not cause the failure.

As for other common plastics, I found “Perspex” or PMMA (polymethylmethacrylate) to be too hydrophobic whereas poly(hydroxy)methylmethacrylate or poly-HEMA is excellent at forming bright holograms. A nice piece of work carried out in 1998 by my colleague Andrew Mayes (now at the University of East Anglia) used a small polyHEMA hologram to make very effective measurements of the alcohol content of various drinks [4]. I show the table of his results in table 1. Strong spirits are not included in the table because they shift the hologram replay wavelength into the infrared—beyond the range of the small reflection spectrometer available for us at the time.

The slide is sliced up and placed in a sample liquid in a cuvette. As the substance in the liquid alters then the replay colour of the hologram may change. In the 3 cuvettes shown, the same polymer material is contracting from left to right.

I remember we had a bit of trouble reclaiming the petty cash from the accounts department for this lot and had to convince them that it really was for a new scientific breakthrough. (Actually we only needed about 5 ml out of each bottle and as for the rest of each bottle… Well, we did not want to bother them with the finer details of the experiment!)

For several years we have been making polymers with certain chemical groups able to respond specifically to specific ions, enzymes and other analytes by swelling or contracting in saline solution, so when we make a reflection hologram of a mirror in these “smart” polymers we obtain a “smart hologram”mirror. (see: the OE magazine report – http://www.oemagazine.com/fromTheMagazine/mar03/diagnostics.html)

In a future article in the Holographer I hope to reveal more about the amazing possibilities opened up by this new holographic ball game.

References

[1]    J.  Blyth et al. 1996 Anal. Chem. 68 1089–94

[2]    H.  Thiry 1987 J. Phot. Sci. 35 150–4

[3]    J.  Blyth et al. 1999 Imaging Sci. J. 47 87–91

[4]    A.  Mayes et al. 1999 Anal. Chem. 71 3390–6