What happens during the development process is that metallic silver-halide crystals containing a latent-image speck (a few silver atoms) are converted into a silver grain by the reduction process triggered off by the reducing agent contained in the developer, and the reducing agent is then oxidised. The development in which the developing agent acts as a reducer is called chemical development. This is the most common way of developing photographic materials, which also applies to the majority of holographic silver-halide materials.
When light is allowed to fall on the emulsion its energy is absorbed by silver halide particles, causing local disruptions of some bonds that hold the crystalline structure together and release free silver atoms within the body of the crystal. Above a certain critical energy enough silver atoms are released to form a stable speck of metallic silver, or latent image. The developer is used to turn the latent image into a visible photographic image in metallic silver, a process called reduction. This solution containing a reducing agent is capable of reducing silver halide to silver, but only in the case of those crystals which bear a latent image.
Since a hologram is something very different from a conventional photographic picture, strict adherence to the processing recommendations just mentioned will not guarantee that a high-quality hologram will be obtained in each and every case.
Back in Lippmann's times one was faced with a similar problem, namely, that the processing parameters had to be adjusted to suit the development of colour photographs on silver-halide material which is quite similar to the holographic material of today. And thus, the processing techniques used for the development of transmission hologram will differ from those used for reflection holograms. The same goes for the production of amplitude holograms as opposed to phase holograms.
Besides considering the type of hologram being produced, one must also consider other parameters, such as the grain size of the holographic material used. The processing technique will be different for the ultra-fine grained emulsions and for materials with a somewhat larger grain structure. Another example of the importance of matching the processing methods with the type of hologram to be obtained comes from display holography. Here, a phase hologram is desired in which bleaching is done after development. This calls for a developing technique that will match the subsequent bleaching.
Oxidation: Devred Devox + e-
Reduction : Ag+ + e- Ag
The reaction will take place only if the equilibrium redox potential of the system developer / oxidised developer is more negative than that system Ag+ / Ag.
The latent image consist of specks residing in silver-halide crystals or grains. The specks have a catalytic effect in that they trigger off the process of chemical reduction in which each speck acts as a microelectrode which brings the developer molecules into electrical contact with the silver ions. A large number of silver atoms is thus created in all the silver-halide crystals with latent image specks. There is a tremendous amplification in this process.
Mix 1 part of solution A + 1 part of solution B + 2 parts of deionized water. Developing time is 2 minutes at 20oC.
Solution A Solution B
Metol 15 gram Sodium carbonate 60 gram
Pyrogallol 7 gram Deionize water 1 litre
Sodium sulfite 20 gram
Tetrasodium EDTA 2 gram
Potassium bromide 4 gram
Deionize water 1 litre
The induction period (the time necessary for the developer to induce the first signs of a visible image) is shorter for the PQ developer than for the MQ developer. PQ developers are also less sensitive to restrainer build-up because of halide release during development, which is why they give more uniform results when used repeatedly. One disadvantage of using phenidone is that it tends to produce fog, which is why a bromide restrainer is usually required in a phenidone developer.
Metol 2 gram Sodium sulfite (anhydrous) 90 gram Hydroquinone 8 gram Sodium carbonate (monohydrate) 52.5 gram Potassium bromide 5 gram Deionize water 1 litreDeveloping time is 4 to 5 minutes at 20oC
When the development has been completed a conventional photographic
material must be treated in an acid stop bath or it must be rinsed
in water, after which it is treated in a fixation bath. The fixation
solution will dissolve the unexposed silver-halide crystals leaving
only the silver grains in the gelatine. In principle, the fixation
step can be expressed by the formula
AgBr + 3 Na2S2O3 Ag(S2O3)3-5 + 6 Na+ + Br-
The silver thiosulfate ion in the formula is the most important one of the several possible silver thiosulfate complexes. It is easily soluble and will diffuse from the emulsion into the fixing bath. The remaining ions can be readily washed out from the emulsion in the subsequent wash. If the silver concentration in a fixing bath becomes excessive, less soluble complexes are formed which are difficult to wash out. If these remain in the emulsion they can cause yellow silver-sulphide stains. Therefore, the fixing bath should be replaced with a fresh one before the silver concentration becomes too high.
The fixer bath reduces the film emulsion's volume. For transmission holograms, this does not make any difference. For reflection holograms, where the interference fringes are nearly parallel to the emulsion, the reduction of the film emulsion is a problem. Therefore, we do not use fixer bath in the developing process of reflection holograms.
Bleaching is a particularly important technique for holograms recorded on Western holographic materials since it constitutes the only way of producing holograms with high diffraction efficiency on these materials.
When a holographic emulsion is developed, all crystals of silver halide which bear a latent image are converted into opaque grains of silver which replicate the pattern formed by the interference of the object and reference beam. If we can turn the silver into a transparent substance of high refractive index, no light will be absorbed. All the light will go to form the holographic image and we get a considerable improvement in diffraction efficiency.
Bleaching can be regarded as the reversed process of development. During the developing process, a silver ion is reduced to free silver and the developed film appears rather dark,
whereas during the bleaching process, metallic silver is oxidised
to silver ion. It is interesting to consider the chemical equation
for development being reversible. For example, the action of hydroquinone
on silver bromide is
C6H4(OH)2 + 2AgBr + 2OH C6H4O2
+2Ag +2Br +2HOH
The reversed action is the formation of hydroquinone and silver
bromide caused by the oxidising agent quinone acting on metallic
silver. In practice, the reversal (oxidation) can only take place
in an acid environment. The development (reduction) only takes
place in an alkaline solution.
There are various bleach solutions formulated on the basis of
the different oxidisers.
In the mixing of the bleacher deionized water is used. This is very useful since ordinary drinking water contains salts and fluorine.
Most often the theories of bleached, silver-halide holograms assume that the hologram is a pure phase hologram of the index or the relief type, or else a combination of the two (phase- only hologram). As a matter of fact, a bleached gelatine emulsion is actually a combination of both an amplitude and phase hologram.
Jeff Blyth bleacher consists of
Potassium dichromate (K2Cr2O7) 5 gram
Sodium hydrogen sulphate crystals 80 gram
Deionized water 1 litre
This bleach removes the silver and leaves the unexposed silver
bromide, and is very useful for developed reflection holograms.
The bleaching time is about 90 seconds.
Figure 6-1 Reversal bleaching
Ferric Nitrate consist of
Ferric Nitrate 100 gram
Kbr 30 gram
Phenosafranine 0.3 gram
Water 1 litre
The bleaching time is about 5 minutes.
Figure 6-2 Conventional (Rehalogenating) bleaching
Jeff Blyths bleacher is used in the bleaching process of reflection hologram. The composition is shown in section 6.4.3.
The developing process:
In the developer process for transmission holograms, a developer
from Kodak was used. This developer is a Black-and-White
developer called D-19 . The composition is shown in section
The fixer bath used in the process is AGEFIX, from AGFA.
Ferric nitrate was also used in the bleaching process of transmission hologram. The composition is shown in section 6.4.4.
The developing process: