Results and Discussion
The grain structure resulting from the diffusion method can be seen in the electron micrograph of Fig.1. The good rounded structure compares well with that of the grains in the proprietary holographic film shown in Fig. 2.
The same materials on glass plates produced the two simple Denisyuk holograms shown in Fig.3. Both plates were given the same exposure time to a HeNe laser (633nm) in the same optical set-up. A spectral analysis of each hologram produced the data in Table 1. The three measured parameters of peak wavelength (with respect to the spectral response of the human eye), the bandwidth (expressed as full width half maximum, fwhm) and film thickness (taken as proportional to the number of contributing fringes) were combined crudely to give a figure of merit (fom) for brightness as:
brightness fom == fwhm*thickness2/(peak wavelength)
where fom(diffusion method) = 1.8 and fom(Slavich) = 1.6. Within the tolerance of this crude calculation, not accounting for in-film transmission loss and the true functional form of human eye response, the brightnesses can be expected to be similar, as was observed.
We had no knowledge of the old German work7,8 as we developed this process. Their work was based essentially on operating the system with the bath order reversed compared to our system. They first treated the gelatin layer with potassium bromide solution and then used the silver nitrate bath.. Liesegang8 explained that the relative concentrations should be in matched ionic proportion . We also through trial and error found that the ionic concentrations of Ag+ as nitrate solution seemed to require it to approximately match that of the Br- in the bromide bath. Liesegang also concluded that the optimum concentration of silver nitrate should be slightly greater than 5% w/v. This surprisingly corresponds with our figure which works out at 6%.
Liesegang emphasised particularly the importance of not allowing the layer to dry out before using the second bath (the Ag+ bath in his case). This is an interesting statement since it is the exact opposite of the procedure we found necessary with our second bath (Br - ). His argument was that through drying out, the effective concentration of bromide ions per unit volume of film was increased which then consequently caused a mismatch with the concentration in the Ag+ bath leading to a thick opaque layer depositing on the surface of the gelatin. This useless layer was described as "mit dem Finger abwischbar". Whereas for reasons we do not fully understand, we found that unless we dried our Ag+ laden layer first , most or all Ag+ ions apparently migrated faster out of the layer than the Br- ions could migrate in. Thus like Liesegang we also obtained a useless thick layer of AgBr which could be wiped off with the finger. When operating optimally our coatings come out of the bromide bath impressively clear without any surface deposit.
From the principles needed to obtain photosensitivity in AgBr films, it might be supposed that the bath order used by Liesegang should be preferable since the second bath would leave the AgBr grain in a rich excess of Ag+ ions and this is well known to enhance photosensitivity whereas in an environment which leaves the grain surrounded by an excess of Br- ions it would be expected to depress sensitivity13. However we seem to have obtained substantial photosensitivity by adding dye to our well agitated Br- bath. (We have had to have a high proportion of methanol in the Br- bath in order to have the dye in solution). If we instead put our plate in a separate dye bath at the end , the resulting photosensitivity is considerably less with the additional disadvantage that simple staining of the gelatin by the dye can occur strongly which causes needless absorption of laser light . In our method the dye is available at the start of the birth of the grain to complex with the Ag+ ions as opposed to having to activate all Ag+ ions in the completed grain.
We have found that it certainly worked to reverse the bath order and to initially saturate the gelatin with bromide ion plus dye, and then to immerse it in a silver nitrate bath. However, we were not able to gain any additional photosensitivity and that fact plus the inconvenience and expence of using a larger bath of highly spoilable silver nitrate has made us decide that our bath order was preferable for our needs. Liesegang s paper does mention briefly that reversing his bath order also worked.
We have substituted the silver nitrate with silver perchlorate dissolved in organic liquids in order to produce gratings in hydrophobic polymers as described elsewhere 6.
Conclusions
The diffusion method offers the following great advantages over the emulsion method.
1. Ability to produce silver-based holographic gratings in ready-made films of polymeric materials (even when somewhat hydrophobic ) which would otherwise have been impossible.
2. Ease and rapidity of operation.
3. A readily achievable, controlled degree of panchromatic sensitivity.
Acknowledgements
We would like to thank Dr. Hans Bjelkhagen and Prof. Nick Phillips at De Montfort University, Leicester for very helpful advice and testing.
References:
1 Bjelkhagen, H. Jeong, T. , Ro, R. "Old and modern Lippmann photography"
Sixth International Symposium on Display Holography (Lake Forest)
Vol. 3358, 72-83 (1997).
2 Phillips, N. , Heyworth H., Hare, T. "On Lippmann’s photography".
3 Alschuler, W. "On the physical and visual state of 100 -year-old Lippmann color photographs" Sixth Holography International Symposium on Display Holography (Lake Forest) Vol. 3358, 54-63 (1997).
4 Iwasaki, M. , Kuboto, T. "Ultra-fine-grain Silver Halide Emulsions for color", Sixth Holography International Symposium on Display Holography (Lake Forest) Vol. 3358, 54-63 (1997).
5 Fournier, J. , Alexander, B., Burnet, P. , Stamper, S. "Recent Developments in Lippmann photography", Sixth International Symposium on Display Holography . (Lake Forest) Vol. 3358, 95- 102 (1997).
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7 Aron , R. "Uber die Farbenwiedergabe mit der Lippmannschen Methode",Zeitschr. f. wiss. Phot. 15 , part 4, 97-125 (1915)
8 Liesegang, R.E. "Uber ein Badeferfahren zur Herstellung von Lippmann-Platten", . Phot. Rundschau 15 , 198-200 (1915)
9 Kaufman, J. "Update of Pseudo-color Reflection Techniques" , Proceedings of the International Symposium on Display Holography, 3, 367-379 (1988)
10 Blyth , J. Holosphere November , p 5 (1979 )
11 Saxby, G. Practical Holography Prentice Hall ISBN 0-13-097106-5 (1994)
12 Bjelkhagen, H. Silver halide recording materials for holography and their processing, Springer Series in Optical Sciences 66 , Springer-Verlag, Heidelberg, New York (1993)
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TABLE 1 Spectral Analysis of Sample Holograms
Diffusion method Slavich plate
wavelength of peak reflectivity 604nm 654nm
full width at half maximum 38nm 32nm
film thickness (from baseline modulation) 5.3+-0.1mm 5.7+-0.2mm
CAPTIONS
Fig. 1. A transmission electron micrograph from a Denisyuk hologram of a plain mirror made under 633nm laser light. This shows holographic fringes made from developed and fixed silver grains originally formed by the diffusion method. The thin silver-rich boundary on the right is in the hydrolysed sub-layer of the cellulose acetate base film.
Fig. 2. A fine grained Russian film , PFG-03M from Slavich produced by the emulsion method, in the same setup that produced Fig .1 but at a lower magnification.
Fig. 3. The simple Denisyuk hologram on the left is recorded in material using the diffusion method. It had been given the same exposure level to a HeNe laser as the proprietary material (PFG-03M from Slavich) used on the right.
