Chapter 6 : Developing process

6.1 The developing process in holography ⁴⁴

Development is a processing technique by which the latent image recorded during the exposure of the material is converted into a silver image. In chemical development, this processing technique is called chemical reduction. From the chemical point of view, reduction is a process in which oxygen is removed from a chemical complex. The acting compound is called a reducer. Reduction is always accompanied by a reciprocal process called oxidation.

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.

6.1.1 The silver-halide process

Silver chloride, bromide and iodide are collectively known as silver halides. These are used in various proportions in photographic emulsions. In the manufacture of a photographic emulsion the silver halide is formed as a dispersion of exceedingly small particles (microcrystals) in gelatine. When coated on a glass or film base and allowed to dry, this becomes a thin, tough, transparent layer which is very sensitive to short wavelengths of light.

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.

6.1.2 Holographic Developers

Silver-halide materials for holography are all of the fine grained type. Such materials require the application of special processing techniques to achieve the best possible results. Therefore, a great deal of research has gone into the problem of processing methods of holographic silver-halide emulsions.

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.

6.1.3 Chemical Development

Chemical development is based on the reduction of silver-halide grains by a soluble developer substance. The deposition of silver atoms occurs in the next step. The following equations describe the process.

Oxidation: Dev_red Dev_ox + 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.

6.1.4 Agfa GP 62 developer

Agfa has formulated a developer based on the combination of metol and pyrogallol, which produces reflection holograms on their materials. The developer is the GP 62 and consists of

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

Mix 1 part of solution A + 1 part of solution B + 2 parts of deionized water. Developing time is 2 minutes at 20^oC.

6.2 Conventional Black and White developers

Today practically all standard Black-and-White photographic developers are of two main types. They are compounded of either metol and hydroquinone (MQ) or else of phenidone and hydroquinone (PQ). The reason for using a combination of two agents in a developer is the superadditivity effect that the two will have together. When combined, the two agents act in such a way that they will produce a higher density in the material than they would if they were used separately.

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.

6.2.1 Kodak D-19

Kodak D-19 Black-and-White developer is the most frequently used conventional developer for both transmission and reflection holograms. Kodak D-19 consists of

 
 Metol                           2   gram
 Sodium sulfite (anhydrous)     90   gram 
 Hydroquinone                    8   gram
 Sodium carbonate (monohydrate) 52.5 gram
 Potassium bromide               5   gram
 Deionize water                  1   litre

Developing time is 4 to 5 minutes at 20^oC
The MQ developer is a high-contrast, low fog developer that will produce clean amplitude transmission holograms on the majority of the existing materials.

6.3 Photographic fixation ⁴⁴

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.

6.4 Bleaching of holographic film emulsions ⁵ ⁴⁴

The conversion of amplitude holograms recorded on silver-halide materials into phase-contrast holograms, commonly referred to simply as phase holograms, was first performed by Rogers and Denisyuk. In the 70's a lot of research efforts went into the discovery of bleaching processes capable of producing both a high diffraction efficiency and a low scattering noise. In the mid 70's the holographic bleaching technique was better understood and became capable of giving more satisfactory results.

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.

6.4.1 General Bleaching theory

Bleaching is an important phase in the recording of holograms on silver-halide materials, ensuring a high diffraction efficiency, and so important for holographic images.

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.

6.4.2 General considerations

Bleaching of the interference fringes recorded in the emulsion will change the emulsion in such way that the light used for reconstruction of the image will be modulated by retardation of the wave front instead of by attenuation as in the case of amplitude holograms. Theoretically, this can lead to a substantial increase of the diffraction efficiency (100 %) of a hologram. Retardation of the wavefront is caused by variations of the refractive index within the emulsion (inner image) or by local variations of the emulsion thickness (relief image). At high spatial frequencies, the inner image predominates, whereas at low spatial frequencies the relief image is more distinct.

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.

6.4.3 Reversal bleaches for reflection holograms

In reversal bleaching, the developed silver image is converted into a soluble silver complex which is removed from the emulsion during bleaching, leaving the original, unexposed silver halide grains in the emulsion. These crystals modulate the light to reconstruct the holographic image. In reversal bleaching, the hologram is not fixed after the development, as otherwise all the silver halide grains would disappear after bleaching.

Jeff Blyth bleacher consists of

Potassium dichromate (K₂Cr₂O₇) 5 gram

Sodium hydrogen sulphate crystals 80 gram

(NaHSO₄. H₂O)

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

6.4.4 Rehalogenating bleaches for transmission and rainbow holograms

In conventional bleaching, the developed hologram containing the silver image is converted into a phase hologram by changing the silver image into a transparent silver halide. This is performed after fixation, after the unexposed silver halide crystals have been removed. We say that the developed silver grains have been rehalogenated.

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

6.5 The experimental developer process

6.5.1 Reflection holograms

In the developing process for reflection holograms, a developer from AGFA is used. This developer is based on the combination of metol and pyrogallol, which produces reflection holograms on their materials. This developer is the GP 62. The composition is shown in section 6.1.4

Jeff Blyths bleacher is used in the bleaching process of reflection hologram. The composition is shown in section 6.4.3.

The developing process:

Put the film in the developer GP 62 bath. The developing time is 2 minutes at 20^oC.
Wash the film in flowing water for 5 minutes, at temperature 20^oC.
Put the film in Jeff Blyths bleaching bath. The bleaching time is 1.5 minutes.
Wash the film in flowing water for 5 minutes, at temperature 20^oC.
Wash the film in technical spirit for 1.5 minutes.
Wash the film in isopropanol for 1.5.
Let the film stand still when the plate is drying.

6.5.2 Transmission and rainbow holograms

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 6.2.1.

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.