Photographic Development of Holographic Silver Halide Emulsion
During development process the exposed silver halides in the emulsion are chemically reduced to metallic silver. Developing agent is usually a benzene molecule containing ammonia (NH3), hydroxyl (OH), methyl (CH ) etc. groups. An alkali is added in the developer which accelerates the reduction and consumes the acid produced in the reaction. The developer will reduce the silver halide crystals only when there is a reaction latent image. Kodak developer D-19 (or Agfa-Gevaert developer G3p) has been the most popular developer since beginning. This developer has been optimized for black and white negative development. Different types of developers have since been evolved through a process of trial and error for holography.
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The unexposed silver halide crystals remain in emulsion after development. These are still photosensitive and limit the life of the developed emulsion. They can be removed by fixing with sodium thiosulphate (hypo) which forms a number of water-soluble silvery complexes along with a few water- insoluble complexes.
It is better to use two developing agents rather than one as their activity together is greater than individual one. Most developing agents work efficiently only in alkaline solution with pH>7. The developing agent together with an alkali may be sufficient to develop an emulsion but the solution will lose its reducing power on reaction with atmospheric oxygen. Therefore a preservative like sodium sulphite is added in large quantity. A restrainer is also added to prevent the developer from attacking unexposed crystals. KBr or an organic antifoggent may be used as a restrainer. The restrainer hinders the production of development fog.
A chemical developer contains five principal ingredients viz.
(a) the reducing agent such as amidol, glycin, hydroquinone, metol, paraaminophenol, pyro and pyrocatechol
(b) the accelerator, usually an alkali
(c) the preservative and stain preventer,
(d) the restrainer and
(e) water as solvent.
Without an accelerator the reducing agent would be ineffective as a developer. The preservative and stain preventers are sulphites, bisulphites and metasulphites. With amidol and metol, sodium sulphite plays a dual role as an accelerator and stain preventer. Neutral salt such as sodium sulphate in concentrated solutions check the swelling of gelatin.
The developer due to "direct" or "chemical" development converts grains of silver halide to filaments or worms of black silver. A significant amount of silver can be added to the grain by "solution-physical" development by transferring the silver atoms from neighbouring undeveloped micro crystals.
Silver halide solvents like sulphite,thio- sulphate or thiocyanate etch the microcrystal surfaces and produce mobile silver complexes. These diffuse to nearby already developed silver grain and produce spherical particles. The increase of silver in each grain can lead to substantial increase in the diffraction efficiency of the holograms if bleached. However, in the development process a certain amount of silver is precipitated at randomly located chunks. This gives a milky appearance to the bleached hologram. The random precipitation is pronounced in weak silver solvents such as sodium sulphite and weak in strong solvents such as ammonium thiocyanate. The strong solvents, if added in a minimum required quantity in the developer, will produce low noise holograms.
The requirements of development of reflection holograms are different than those of transmission holograms. This is so because the emulsion shrinkage has little effect on the fringe spacing in a transmission hologram. But in a reflecttion hologram because the fringe planes are parallel to the emulsion layer, shrinkage can reduce the fringe spacing and can rotate the fringe planes in slanted gratings. This will affect the efficiency and peak reflectivity of the hologram.
The drawbacks of solution-physical development can be removed if silver halide solvents are not used in the developers. Such developers are known as direct, chemical, surface or nonsolvent developers. Ascorbic acid is a developing agent with photographically inert oxidation products and has been used without any sulphite protection in MAA-3 developer. A more active developer is a PAAP developer which is vulnerable to aerial oxidation . These developers produce very clean holograms with high efficiency and low scatter noise. Addition of 0.05g ammonium thiocyanate to PAAP developer reduces the scatter noise considerably. Holograms developed in catechol and bleached in p-benzoquinone bleach gives high efficiencies.
Russian developers have been formulated to produce colloidal silver (particle diameters lie in the range of the order of 0.01-0.05 um) with a brown colour. Such developers permit the formation of holograms using only development without any fixing and bleaching stages. They are capable of creating both amplitude and phase modulation simultaneously. Since brown silver has lower opacity than black silver, very high efficiencies can be achieved by colloidal silver development. The efficiency depends on the size of the developed silver particles which in turn depends on the exposure and processing
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